Some New Applications and Techniques of Molecular Beams*

Some New Applications and Techniques of Molecular Beams* JOHN G. KING

AND

JERROLD R. ZACHARIAS

Department of Physics and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts Page 2 2 4 4

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. General Remarks on Beams.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Certain Applications ..................... ............ a. Maser ........... ....................................... b. Atomic Beam-Frequency Standard. . . . . . . . .............. c. Measurement of Magnetic Fields. . . . . . . . . d. Measurement of Acceleration.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

e. Length Standard.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f. Further Applications., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Sources of Neutral Molecular Beams.. . . . . . . . . . . . . . . 1. General Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . 2. Description of Sources and Associated Equipment. . . . . . . . . . . . . . . . . . . . a. Molecular-Beam Sources Using Gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Construction of Source Slits.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Note on Gas-Flow Control.. . . . . . . . . . . . . . . . . . . . .. d. Molecular-Beam Sourc,es for Operation to 500" C . . . . . . . . . . . . . . . . . . e. Atomic-Beam Sources for Operation a t Higher Temperatures.. . . . . . . f. Dissociating Sources.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g. Recirculating Sources. ..... .................... h. Miscellaneous. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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16 16 17 21 22 24 27 31

......... a. Principle of Operation. . . . . . . . . . . . . . . . . . . . 34 b. Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Deposition Detectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Ionizing Detectors .............. a. Surface Ionizers b. Surface Ionizers e. Comments on Ionizing Detectors

5. Mass Spectrometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. The Measurement of Sma.11 Currents.. . . . . . . . . . . . . . . . . . . . . . . . . . . a. Electrometers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* This work was supported in part by the Army (Signal Corps), t h e Air Force (Office of Scientific Research, Air Research and Development Command), and the Navy (Office of Naval Research). 1

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JOHN G. KING AND JERROLD R. ZACHARIAS

Page

b. Electron Multipliers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Electronic Equipment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Miscellaneous Detectors. . . . . . . . . . . . . . . . . . . . . .................. IV. Deflecting and Uniform Fields.. . . . . . . . . . . . . . . . . ................... 1. Deflecting Fields.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Magnet Excitation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Deflecting Magnet Designs.. . . . . . . . . . . . . . ......... 2. Uniform Fields., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. General.. ..... ..................................... gn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Deflections and Intensity.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Focusing of Atomic Beams., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b. Collision Alignment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Multiple Beams. .......................................... V. Radio-Frequency Equi ent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Requirements of Radio-Frequency Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Radio-Frequency Sources and Frequency Measurement. . . . . . . . . . . . . . . Fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Miscellany.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Beam-Control Devices.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Construction Details., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 61 61 61 63 65 67 67 68 70 70 72 72 73 73 73 76 77 77 78 79 83

I. INTRODUCTION 1. General Remarks on Beams

Ever since the experiments of Dunoyer (D8),beams of neutral molecules have been used t o study the properties of the molecules of which the beam is composed. I n most cases the molecules under study have been monatomic or diatomic, * which implies that the technique has not been applied to the study of chemical problems of great complexity, even though the molecular beam techniques are well suited t o such problems. Considerable attention has been given to the simpler problems of molecular atomic and nuclear physics with a n increasing emphasis in the postwar years on measurements of great precision or on substances of great rarity, such as the radioactive isotopes. Of this and earlier work, there are fine summaries ( F l , F 8 , H S , B l , E2, K l S , K 9 , R14) and even as this paper is written, two new summary volumes are about to appear-one by Ramsey (R2) and one by Smith (86).The emphasis in these volumes is naturally on the molecules for their own sake, whereas one purpose of the present paper is t o supplement them by presenting

* Except where the distinction is important, “atoms” and “molecules” will both be termed ‘Lmolecules.’’

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a molecular beam also as a device employed t o observe some other phenomenon-to use the molecular beam as a tool in some technical application. The techniques which are to be described in this article are naturally applicable t o laboratory measurements and in many cases would require considerable development in order to realize their use in a sealed-off vacuum tube for a commercial application. I n many of the applications and for many of the techniques to be discussed, however, the transition necessary t o go from a laboratory device of great complexity to a reliable device t ha t can be used easily is not very different from any usual commercial development. The full impact of this idea became evident only when it seemed attractive to use an atomic resonance frequency in the microwave region for a stable reference with which to determine a frequency t o high precision. Once having decided that a molecular beam is a reasonable tool in one application, it is then only a step t o consider some of the others. I n considering the applications which are described below, it will be well t o keep in mind the sources of strength and the sources of weakness of this technique. For a beam of molecules to proceed from a smallsourcewithout being affected by any other molecules, to a detector a t a distance of one meter, the density of ambient gas (or vacuum) should be less than 10-9 atmos. This is only a rough rule, because it naturally depends on the sizes of the molecules, but there are few cases where this value can be ten times worse and fewer where it need be ten times better. But this ambient density (usually expressed as a pressure) must be proportionately less for longer beams, or more correctly, for longer times of flight of the beam molecules. Since most molecules are moving a t the velocity of sound in a n equivalent gas, or a t about 300 m/sec, it is easy t o see th a t a free time of 10 msec is readily achievable with only modest vacuum precautions, but that for times of free flight which are substantially longer, more refined techniques are necessary. It is frequently tempting to neglect two important things which can affect an experiment but which are trivial in a simple measurement of the mean free path of a beam: (1) scattering by the molecules of the beam a t the moment of exit from the beam source has been well discussed by Estermann et al. (E4) in their work with cesium; (2) long-range interactions between molecules which have little effect on their motion of translation, but a serious effect on their state of quantization, have not to our knowledge been carefully studied. Evidence for effects of this latter sort sometimes appear in molecular-beam magnetic-resonance experimlents but in the past have been suppressed rather than examined. As will be discussed in detail later on, it is now possible t o produce

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a.

KING AND JERROLD R. ZACHARIAS

beams of almost all of the elements, with the possible exceptions of a few like carbon and tungsten, which evaporate with difficulty, and of a great many stable molecules. Any of these beams are detectable with more or less ease and efficiency. The heavy alkali metals, which can be converted to ions for detection, are the simplest to handle and can be easily observed a t the low rate of a few per second u p to a rate of 1016 per second, and in fact, this is achievable in the same apparatus. This large dynamic range can be accompanied by a signal-to-noise ratio (defined here as the number of molecules observed due to the desired effect to the number of molecules in the background) as high as 1000, or since this is a current ratio, 60 db. The authors see no reason that in special cases this ratio should not be increased, but since the important quantity is not the background current itself but the fluctuations in the background, it seems clear th a t high signal-to-noise has not been thoroughly exploited. Since many of the detectors to be described involve the conversion of neutral molecules into ions, it is obvious that mass spectrometry a t the detector can be very useful for the lowering of background noise and selection of isotope or molecular species. 2. Certain Applications

a. Maser. Generation of coherent radio-frequency oscillations by direct excitation of a high-Q cavity tuned t o the radiation frequency has been described by Gordon, Zeiger, and Townes (GS). Their device, which they have named “Maser,” can also be used as a very specialized form of radio-frequency amplifier and as a short-term control for stabilization of radio-frequency oscillations. Since their paper is readily available, no attempt will be made to repeat it here; however, a few remarks may be in order. Unlike the usual devices and the ones envisioned in the rest of this paper, it is the radiation emitted by the molecules th a t is observed. I n the usual experiments, one observes the effect of the radiation on the motion of the molecules in their path, and one observes this as a n increase or decrease in number of molecules arriving at a detector. T o make the radiation from the Maser observable, it is necessary for the number of molecules per second to be rather high, since a photon of microwave frequency is not energetic. One photon a t 24,000 Mc/sec has a n energy of 1.6 X joule. The Maser as described uses about 1 g of ammonia per day for its operation with a release of lo-* watt. Methods will be appearing in the literature soon that will reduce this amount considerably. b. Atomic Beam-Frequency Standard. Stabilization of radio-frequency oscillations by comparison with the 9192-Mc/sec resonance in cesium has been discussed by Lyons ( L l l ) ,Zacharias, Yates, and Haun (22) and by Essen (2%). Since as yet no complete paper has appeared which

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describes the detail of locking a signal generator t o a molecular-beam resonance, it seems appropriate here t o q u o t e Zacharias, Yates, and Haun f r o m t h e “Quarterly Progress R e p o r t ” of the Research Laboratory of Electronics (October 15, 1954), Massachusetts Institute of Technology. During the last quarter the cesium beam apparatus which has been under construction to provide a primary frequency standard for the Atomic Beam Laboratory has been in successful operation. Much useful information has been obtained showing that such a system can be expected to provide a long term stability approaching 1 part in 10’0. The system uses the field insensitive resonance line of C S * ~occurring ~, at approximately 9192.63197 Mc/sec, as the frequency standard. By exploiting the molecular beam techniques developed for measurements on scarce isotopes, which involve the use of a narrow beam and sensitive detector, a n apparatus has been designed to give a good signal-to-noise ratio using only one microgram of cesium per day. The expected linewidth was about 200 cps, making possible a stability of 1 part in 1010 by splitting the linewidth to 1 per cent. For those not familiar with the principles involved, the following brief description is provided. There are a number of magnetic resonance lines associated with a cesium (or other) atom, resulting from transitions between two quantum states. There exists a pair of states for which the energy is independent (to the first order) of the external magnetic field, so that the associated frequency is similarly field insensitive. (There is, however, a small term proportional to the square of the field.) The resonance is excited by applying a magnetic field a t the appropriate frequency, the width of the resonance curve being inversely proportional to the time the atom spends in the field. With a beam of atoms a long path without collision can be obtained with a high density of atoms, because of the ordered nature of their paths. The transition in question is associated with a reversal in the magnetic moment of the atom. If the beam of atoms is passed through a transverse inhomogeneous magnetic field, a deflection is produced in a direction determined by the sign of the magnetic moment. If a pair of such fields is placed on either side of the rf field (A and B magnets), a distinction can be made between atoms which have made the transition (or “flopped”) and the others, by observing whether the two deflections have added or canceled. A detector placed to measure the number of atoms on one or the other of the two paths will record a resonance curve as the frequency is varied. If the rf field is applied uniformly over a given length, a single peak is obtained. If two separated fields are used as in Ramsey’s method, a typical interference pattern is obtained whose spacing depends on the velocity of the atom. If a range of velocities is used, only the central peak is reinforced; the side peaks tend to average out. The use of a surface ionization detector (hot wire) serves to change neutral atoms into charged ions which may be detected electrically. With an electron multiplier as amplifier, the arrival of individual atoms can be recorded. The general arrangement of the beam apparatus is indicated in the simplified form of Fig. 1. The apparatus is housed in a stainless steel can 6 ft long and 10 in.

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JOHN G. KING AND JERROLD R. ZACHARIAS

in diameter into which it is lowered vertically. All connections are brought out through the top plate. The oven produces two narrow beams of cesium atoms, each of initial cross section 0.020 by 3.8 in. with a total divergence in angle of less than 1 deg. Details of the oven construction and performance are given in the following section, but with a total emission of g of cesium per day (5 X 1Olo atoms/sec), it was expected to provide lo7atoms/sec reaching the detector, of which one-eighth can provide the desired transition. The A and B magnets, which are identical, produce a transverse inhomogeneous magnetic field. With an Alnico core, and circular section pole pieces, a field strength of 6000 gauss and a gradient of more than 10,000 gauss/cm is A MAGNET CE

TOP LATE

(C)

FIG.1. Atomic-beam frequency standard.

obtained in a gap of 3.8 in. The present magnet length is 10 in., which produces a deflection of 0.025 radian on atoms of the most probable velocity. This is excessive; the magnet length will be reduced in the future to 355 in. and the gap increased to >d in. Magnetizing coils are incorporated so that the magnets can be set to the desired strength. The rf field system [see Fig. 34b] is designed to use the Ramsey method of separated fields. For each beam, two rectangular cavities, each a half-wavelength long and spaced 66 cm apart, produce a transverse rf field (parallel to the A and B fields), using the transverse component of the TEol field pattern. The cavities are fed symmetrically with a waveguide feed. To get a symmetrical resonance curve, it is essential that the phase of the fields in the two cavities should be the same. This is obtained by providing a second waveguide running between the cavities, which is loosely coupled to the cavities by the slots through which the cesium beam passes. If a probe is inserted at its middle point, a null signal is obtained if the phase and amplitude in the cavities are identical.

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The phase adjustment is made by a tuning screw in each cavity. This measurement can be checked during normal operation, but in general an initial adjustment is adequate. The steady field in the rf region is made up of the earth’s field and the leakage field from the A and B magnets, with provision for some control by small coils near the cavities. It has been found that the maximum field is below 1 gauss. I n the final form some magnetic screening will be provided to reduce the contribution of the earth’s field, and the desired field will be provided by a suitable coil system. The detector is a 0.040-in. tungsten ribbon. Cesium atoms falling on this are ionized and are then pulled off and deflected electrostatically to one or the other of two electron multipliers, each having a gain of about 3 X lo6. The apparatus is evacuated by a 235-in. oil diffusion pump; with liquid air traps a pressure of 5 X lop7mm is regularly maintained. A considerable amount of apparatus, which is described briefly below, has been constructed for use with the beam tube. The most important item is the signal source a t 9192 Mc/sec. This is produced in a silicon crystal tripler driven by a n S-band cavity oscillator. The oscillator, with a cavity with a Q of about 50,000 and a triode amplifier, has proved to have a short-term stability better than 1 part in 109. For convenience in the experimental operation of the beam tube, the long-term frequency stability has been improved by fitting a thermostat to the cavity of the oscillator. The thermostat uses a temperature sensitive resistance wound over the whole cylindrical surface of the cavity. This is connected in a sensitive bridge circuit that provides a continuous control of the power supplied to a heater winding outside the cavity, giving rapid and precise control of temperature. Changes of the thermostat setting provide a convenient means of frequency adjustment; they also provide a convenient method of producing a frequency changing linearly with time. Other facilities available include a n automatic frequency sweep circuit that sweeps the oscillator over a predetermined range by switching the oscillator cavity heating power between two levels. I n addition, provision is made for the time of the occurrence of the resonance curve (or any other event, such as a frequency marker from a crystal standard) in the sweep to control automatically the frequency limit desired so that it continues to cover the required range. The frequency of the S-band oscillator can be compared with the laboratory crystal standard, using the beat frequency between the S-band signal (3064 Mc/sec) and the 17th harmonic of 180 Mc/sec (3060 Mc/sec) derived from a 5-Mclsec crystal. An audio-frequency discriminator (with its peaks located a t 1000 and 1500 cps) has proved very convenient for recording small frequency variations. If a small amount of frequency modulation a t about 30 cps is applied to the S-band oscillator, the beam current has a 30-cps component which is proportional to the derivative of the resonance curve. When this is applied to a phase-sensitive detector, an output having the characteristic of a frequency discriminator is obtained with a zero value a t the resonant frequency. A Sanborn two-channel recorder has been used for recording the data and has proved of great value in a number of different types of measurement.

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JOHN G. KING A N D JERROLD R. ZACHARIAS

As soon as the apparatus was assembled with magnets of the quoted specifications, resonance curves of the Ramsey pattern were obtained. Detailed measurements are much too numerous to quote but the important conclusions are as follows. Resonance curves of the Ramsey pattern with good signal-to-noise ratios were obtained (Fig. 2). The oscillator proved to have more than adequate short-term stability for the purpose, and can produce more than enough power a t 9192 Mc/sec. The oscillator was successfully locked to the beam frequency, by using the beam output as an error signal for frequency control. With a very simple control system, a peak frequency deviation of + 1 part in lo9 was reached and the mean over longer periods would be much smaller.

FIG.2. Cesium resonance curve from atomic-beam frequency standard (upper curve). The lower curve is the discriminator output simultaneously recorded. Probably of greatest importance is the fact that the whole apparatus worked very much according to plan. The oven has run for two extended periods, one of 6 weeks, and in each instance the run was ended because the apparatus had to be opened to allow other unrelated changes to be made. The main feature requiring modification was that the linewidth obtained, of 360 cps, corresponds to using atoms of twice the most probable velocity. If the magnet strength was reduced to allow the use of lower velocity atoms, the background of “unflopped” atoms, already high, was further increased because of certain features of the geometry of the apparatus. For a number of reasons, a more effective arrangement is a layout in which the oven is offset to one side of the axis and the detector is offset to the other. This gives a loss of half the available atoms but can give a much lower background. An experiment with an excessive amount of displacement to suit the large magnet deflections available gave the expected results, shown in Fig. 3. This curve is an example of the discriminator type of characteristic obtained by applying a small amount of frequency

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modulation to the oscillator. A linewidth of 200 cps with negligible background and very good signal-to-noise ratio was obtained. The total available signal can be increased by using the optimum deflection angle. The large number of peaks was expected from the narrow velocity range used in this experiment. Information available from the present tests provides the essential information for the engineering design of molecular beam tubes of various sorts. The tube construction is simple and has proved very satisfactory. For the immediate future, the beam apparatus will be set up to provide a frequency standard by direct reference to the cesium resonance line with an accuracy of a single observation of about 1 in lo9. As soon as any greater precision is required, the necessary circuits for locking the oscillator continuously will be provided.

FIG.3. Cesium resonance curve from atomic-beam frequency standard using atoms in a narrow velocity range (lower curve). The upper curve is the discriminator output; both curves were obtained by frequency modulating the transition-inducing oscillator at 30 cps.

T h e question arises as t o how far this technique can be pushed. First of all, t h e beam can be made fairly long without great difficulty, for instance, a beam 30 or so meters long is completely feasible if one has a stairwell or some such height available. Second, t h e low-velocity tail of t h e Maxwellian distribution of velocities can be used without too great a sacrifice in intensity at a third t o a fifth of t h e most probable velocity. And last, one can use a signal-to-noise ratio of lo4.Without great effort it should then be possible t o control frequency, on a n absolute basis, t o one part in 10l2 and, for long averaging times, t o something better. An experiment is under way t o t r y t o achieve something still better, but n o results are yet available. T h e method involves using only those molecules which are moving slowly enough initially t o remain in t h e beam for

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1.4 sec, but this method may well turn out t o be too difficult, even if it is possible. c. Measurement of Magnetic Fields. Measurement of magnetic fields can be accomplished with the use of a beam of molecules with a magnetic moment. The method reduces simply to observing the Larmor frequency of the electron spin in the external field to be measured, just as in the experiment described above one observes the Larmor frequency of the electron spin in the magnetic field of the atomic nucleus. This Larmor frequency of the electron is 2.8 Mc/sec/gauss, but in most cases if one is trying t o observe the magnetic field of the earth or small variations of it, the full-frequency dependence is not available. The reason for this is that the angular momentum associated with the atomic nucleus is usually (D3) present t o reduce the Larmor frequency by a factor of 2 or 4, or more. Since the observation time of a molecule is easily made 10 msec and one can use a signal-to-noise ratio of lo4, it should be possible t o observe a magnetic resonance to cps out of, say, 1 Mc/sec corresponding t o a field of 10-8 gauss. An apparatus for achieving this might have t o be 3 m long, but it would not have to be heavy, since all of the radio frequency components would be a t the l-Mc/sec frequency. d . Measurement of Acceleration. Observation of small rates of rotation and of linear acceleration in inertial space is possible because as soon as the molecules leave the source and until they reach the detector, they are on their own. Some years ago Millman, Rabi, and Zacharias (MB) succeeded in aligning three slits a meter apart with a beam of indium atoms cm. With modern technique, observation could be improved to 2 X by a factor of about 100, and an alignment would be possible to 10-9 sec, a radian. Since the time of travel of the molecules is about rotation rate of lo-’ radian/sec would be observable. A linear acceleracm/sec2 would likewise tion transverse t o the beam path of 5 X produce a n observable effect. By the use of beams in opposite directions, it would be possible to sort out the effect of rotation from the effect of linear acceleration. e. Length Standard. It should be possible to use a molecular beam as an aid in making measurements of length to unusually high precision. The standard of length, usually thought of as the distance between two hazy regions on a platinum-iridium bar, must be thought of in terms of a wavelength in the visible or near-visible region of the spectrum. Unfortunately, microwaves, even the 0.5-mm waves now available, have such large diffraction patterns that length observations are always influenced by dimensions transverse to the direction of measurement. Waves in the visible or near visible are not coherent enough to permit their measurement by comparison with a frequency standard. We thus require a

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method of counting interference fringes out to a large number of fringes, and the limitations which are now imposed are caused by the overlapping of fringe orders due to the finite bandwidth of the spectral line used for illumination. If observations of the amount of light in the interference fringes were made with the aid of a filter narrower than the emitting source, the fringe count could be carried out to substantially greater path differences, and the overlapping of the orders for the light that the filter does not pass would be no hindrance. Now a beam of molecules passing a t right angles t o a beam of light is such a filter and has for many years been used almost in this way. Observations of atomic resonance lines in the visible and near-visible have been made by observing the effect of the molecular beam on the radiation. We are proposing here that the observations be made on the molecules of the beam so that when the interference fringes pass over the beam, the number of molecules which are affected by the light will undergo a periodic variation. Thcn only Doppler effect and the resonance line width of the molecules of the beam limit the path difference which can be tolerated in the interferometer. Since the molecular beam can be collimated to arbitrary accuracy and since it can be made to go at right angles to the light path t o a n accuracy of, say, or better, the Doppler shift can be reduced by a factor of 1000 or so. If it were necessary, it would be possible, by using two antiparallel beams, to reduce the Doppler shift to zero with only a residual Doppler broadening of a part in 1O'O. The signal-tonoise ratio in observations of this sort are inherently better than by observation of the light because it is possible t o use methods which respond only t o those molecules which have absorbed a photon. One such method depends on the fact that when a molecule absorbs a photon, it suffersthe recoil necessary to take up the linear momentum of the photon (light pressure, classically) and proceeds after absorption in a different direction from that of its initial motion. The detector is naturally placed at a position t o receive the deflected atoms. With mercury, preferably an isotope with no nuclear spin, it seems feasible t o measure to a precision of one part in 1O'O using its ultra-violet line a t 2537 A. Other substances might be more favorable, for instance, the 6573-A line of calcium might be pushed two orders of magnitude further. It seems t h at a redefinition of the standard of length should take such possibilities into consideration. f. Further Applications. There are two possible applications which have been discussed but which do not seem to be naturally suited to molecular beams. (1) Information Storage. I n principle it is possible to store information in a beam of molecules during its time of flight of, say, 10 msec and to

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JOHN G. KING AND JERROLD R . ZACHARIAS

use it like any other circulating information storage device. Consider a beam of monovelocity molecules which have been sorted for some particular angular momentum state. A pulse of these molecules can be flipped either totally or partially to another angular momentum state and the fraction of those flipped can then be determined. The information pulse t o be stored is fed into a radio-frequency field t o produce the flipping. Of interest is the fraction of storage time required to store a bit, and unfortunately, as far as the present authors know, this is still uncertain. (2) Isotope Separation. Since, for many of the elements, the isotopes have different magnetic properties owing to the angular momentum of the nuclei, i t has been suggested that beams of neutral particles be used for isotope separation. This sounds attractive a t first because the charge associated with an ion for electromagnetic isotope separators is so large, i.e., 96,500 coulombs/g-mol. This high electric charge is chiefly objectionable in such separators by reason of the space-charge effects. Therefore, if the particles are neutral, the densities can be higher. Unfortunately, the neutral particle magneton optics all suffer from velocity “chromatic aberration,’’ with the result that the isotopic yields are poor. There are discussions of the optics appropriate to this subject in papers by Korsunskii and Fogel ( K I I ) and by Friedburg and Paul (8’6) (see also Section IV,3). The subject of the motions of uncharged particles which carry a magnetic or electric moment in inhomogeneous fields has been inadequately treated in the classical literature. It can serve as a gold mine for final examination questions for doctoral candidates. Another purpose of the present article is to assist new users of molecular beams by providing descriptions of more or less conventional molecular-beam components with suggestions for future development. An experimental lore has grown up around the subject, which is not well documented. But worse than this, it has been the habit of workers in this field t o stop well short of trying to understand the phenomena underlying the techniques, for the simple reason that in general they were more interested in obtaining a measurement of a property of a molecule ot an atomic nucleus than in the technique. As will be obvious in the material which follows, there remain many gaps in our knowledge. I n general, those which are the most subtle involve the interaction of the molecules and ions with the surfaces that they touch.

11. SOURCES OF NEUTRAL MOLECULAR BEAMS 1. General Considerations

The rate of effusion of molecules from a thin slit when the pressure behind the slit is sufficiently low to insure molecular flow is given by F I :

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Pa

n = 3.5 X loz2-

d F T

where n is the number of molecules effusing per second, p is the pressure in millimeters of mercury within the source, a is the area of the slit in square centimeters, and M and T are, respectively, the molecular weight and the absolute temperature of the gas or vapor. It is assumed, as is usually true, that both the partial pressure of material forming the beam and the pressure of other gases in the source chamber of the apparatus is negligible. Attempts t o increase the intensity of the beam received a t a detector subtending a small solid angle a t the source by increasing the source pressure fail when the mean free path of the molecules in the source becomes comparable to the width of the slit (a vague criterion) because collisions of the molecules with one another near the slit result in the formation of a cloud of gas or vapor of larger area than the slit. The pressure outside the slit is now no longer negligible and the number of molecules effusing and the received beam intensity are both reduced. Since practicable source slit widths are on the order of 0.02 mm, the requirement of molecular flow dictate source pressures less than approximately 1 mm Hg. If it is necessary t o conserve a rare material or if the pumping speed of the source chamber is limited, long slits providing appreciable collimation may be used. The ratio of the number of molecules effusing from a long slit of infinite height t o the number of molecules entering the slit has been calculated by Clawing ((71) as a function of the ratio of the length of the slit t o its width, again assuming molecular flow, and is plotted in Fig. 4.Figure 4 also contains plots of a number of equations of different ranges of validity. The result of Clausing’s calculation (Cd) of the angular distribution of molecules effusing from a circular canal of length equal t o its diameter is shown in Fig. 5 ; it should be noted th a t the intensity in the forward direction is the same as th a t from a circular hole of the same area where the angular distribution follows a cosine law, but that the rate of effusion of atoms is approximately halved. These results are rarely exactly applicable but are very useful in making estimates of the performance to be expected from the source slits. For permanent gases the pressure in the source can be adjusted by throttling the flow with a suitably designed valve or leak and for condensible material by adjusting the temperature of the part of the source where the material is stored t o give the desired vapor pressure, all other parts of the source being kept a t a higher temperature. Of course, these parts should not be raised t o a gratuitously high temperature, since the natural line-width in resonance experiments will be proportional t o TH.

14 14

OH HN N G. G. KING KING AND AND JERROLD J E R R O L D R. R. ZACHARIAS ZACHARIAS JJ O

FIG.4. Transmission properties of rectangular slits of various shapes. The beam emerges parallel to L.

5 . Angular distribution distribution of molecules effusing from from a circular canal canal of of length length FIG.5. cosine distribution distribution to to equal to its diameter (lower curve). The upper curve shows the cosine be expected from a circular hole.

15

MOLECULAR BEAMS

TABLE I. Some Molecules of Which Beams Have Been Made Monatomic Molecules Ele-

Source type, material, and temperature (approx)

Z ment

1

2 3 5 7 8 9 10

11 13 17 18

19 24 26 27 28 29 30 21 33 35 36 37 47 48 49 50 51 53 55

H

Type of detector used*

c. (MOz03) P. P. E.E. metastable atoms I.G. P. E.E. metastable S.I. (W 0 2 ) U.I. C. AgN03 C. PbO S.I. (W) I.G. P. S.I. (W 02) S.I. (W) S.I. (W) I.G. P. S.I. (W) U.I. D. D. D. D.

Wood's tube Wood's tube Microwave arc Hot W tube (2500" K)

He Slit only Slit only Wood's tube Li Fe 750" C B Graphite oven 2100" C N Wood's tube (active N) 0 Wood's tube F Microwave arc (F,SF,), 600" C Ne Slit only Slit only Na Ni, Monel, 350" C A1 Tho2 in graphite, 1670" K C1 Microwave arc, 600" C A Slit only Slit only K Ni, 430-573" K ; Cu, 466-544" K Cr T h o 2 in graphite, 1500" C Fe Alz03, > 1535" c Co Ni A1203, 1400-1500" C Cu A1203, 1500-1700" K; MO n. Zn Ga C in Mo, 1600" K S.I. (W D. As 250" C (mostly Asc) S.I. (W Br Microwave arc, 600" C Kr Retherford & Kellogg (unpublished) S.I. (W) Rb 200" C (2RbC1 Ca-, 2Rb CaCM Ag A1203,1600-1700" K ; MoThOz in U.I. graphite D. Cd 440' C In Mo, 1500" K S.I. (W Sn A1203, 900-1000" c D. D. Sb A1203,400-500" C (much Sb,) Microwave arc, 600" C S.I. (W I S.I. (W) c s Fe, Monel, 350-450" K 500" C (2CsC1 Ca + 2Cs Clz) 200" C (CsC1 N a + Cs NaCI)

+

+

+ + Th)

+

0 2 )

+

+ +

+ +

References R6 K2, E9, N 5 , NZ, P2

J4

L6

24

K15, H4, W 3 H4, WS F9, M 8 w 9 , L14 J7 Kl6 J8

24

K15 R15, M9, DS L2 D5

z4

K15 M10, M11, M I 1 Bb G4 G5 G4 G4, G6, LY L15 Rl3 El0 KS K1S M9

w1

+

02)

+ Th)

L15, E i 0 M 6 , H13 (76, G4 G4 J5 C5, Eil, T1

* Detector types: C. = Chemical target, D. = Deposition, I.G. =_Ionization gauge, P. = Pirani gauge, S.I. = Surface ionizer, U.I. = Universal ionizer.

16

JOHN G. KING AND JERROLD R. ZACHARIAS

TABLE I. (Continued) ~

Ele-

2 ment

Source type, material, and temperature (approx)

56 Ba Fe, 950°C 59 P r T h o , in Mo, 2000" C 79 AU A1203; Mo, 1150-1300" C Tho2 in graphite 80 Hg 81 T1 Phosphor bronze; A1203, 720" C Fe 82 P b 800°C 83 Bi 770" C; A1203(much Biz); 60% Bi? a t 851" C

Type of detector used*

S.I. (W) S.I. (Mo) D. U.I. D., I.G. D. S.I. (W D. D.

+

References H6, Gd

L4 G4, CS

w1

0 2 )

L15, E l 0 G6, G4, L15 B10 G6, G4 G6, G I , L16, 25

Temperatures ranging from - 135" C for chlorine t o 1900" C for gold have been used. Vapor-pressure data for various elements can be found in many references, e.g., L1, D1, H 1 , S2, and A5, recent editions of the " Handbook of Chemistry and Physics" ( H l ) being particularly helpful in that extensive references are given. Should it be desired to produce a beam of one of the rarer elements, useful data on their chemical, physical, and metallurgical properties may be found in DW,H2, and H5. Sources may be needed that produce beams of atoms from molecular gases or vapors,* or that produce beams of atoms in metastable states, or that can recirculate small samples of unabundant elements ; in each case a different design is appropriate. Rather than discuss these many variations in general terms, we shall describe sources and associated components designed for specific applications but representative of their class in detail. Methods of construction and performance data where available will be given, but the caveat announced in the Introduction should be kept in mind. Table I gives data on the sources that have been used to produce beams of various materials. 2. Description of Sources and Associated Equipment

a. Molecular-Beam Sources Using Gases. An appropriately mounted tube terminated by a slit and provisions for controlling the flow of gas form this simplest of sources. If a cold beam of a permanent gas is required, the source can be mounted on a cold trap built with special care to prevent distortion when coolant is added and consequent loss of the close alignment needed in molecular experiments. Traps built of thin

* Conversely, there is sometimes difficulty in obtaining beams of molecules that dissociate at the temperature required in the source.

MOLECULAR BEAMS

17

(0.032411. wall) stainless-steel tubing, silver soldered, are satisfactory, although Invar has been used with additional lateral Invar clamping screws t o minimize lateral motions of the source ( K 1 ) . b. Construction of Source Slits. (1) Thin Slits. Slits as narrow as 0.005 in. may be cut in sources made of metal or other machinable material with standard slitting saws. Narrower slits are best made by

(a) FIQ.6. Slit-making apparatus. (a) A jig used to hold a strip c Kovar taut during sealing-in.

clamping knife edges (razor blades are sometimes suitable) in front of a larger aperture; lapping the mating surfaces is advisable to minimize leakage. Slits in glass sources have been made by waxing ground semicircular pieces of microscope cover glass to the ground end of a glass tube ( K d ) . Another method is to collapse a glass tube about a strip of Kovar of width and thickness equal to the dimensions of the desired slit and held taut and centered by a jig. The tube is then sawed off a t the constriction, the Kovar is dissolved out with acid, and the slit is ground inside and out t o the desired thickness. Slits 0.0005 in. wide, 0.125 in.

18

JOHN G . KING AND JERROLD R. ZACHARIAS

(4 FIG.6b-d. (b) Sawing a slit with an emery-charged wire. (c) Grinding the end of the slit tube. (d) Grinding the inside of the slit tube.

high, and 0.020 in. thick have been made in this manner. A simpler technique is t o saw the slit with an emery-charged copper wire drawn to and fro across the ground end of the source tube by a n obvious motordriven device. Kerosene is used to carry the emery. The results are not as clean as in the Kovar method, but a 0.003-in. slit in a 8-mm Vycor

19

MOLECULAR BEAMS

tube of 0.030-in. end-wall thickness can be made in approximately 4 hr. Thin diamond dust-charged wheels rotating a t high speed should be successful but have not been tried. Figure 6 shows some of the apparatus used in making slits. TABLE 1. (Continued) Polyatomic Molecules Molecule References Hz HD

Dz

Liz LiF LiCl LiBr LiI Nan NaF NaCl NaBr NaI Kz KF KCI KBr KI Rb z RbF RbCl RbBr RbI CSZ CsF CSCl

K1, K10 K1, E8 K1, K10 R17 R17, BY R l r , K17 B11, K20 K20 K18 R1 7, 01 N6,Ol N6 N6, 01, WlO K18 B9 R18 B9, R18 R18, W10 Kl7 K17 WlO K18 K18, H10 K18, W10

Molecule CsBr CSI NaCN KCN RbCN NaBOz KBOz Li2B40T NazB407 KzB407 NaFBeFz KFBeFz NaOH KOH NaCl A1CL KClAlCla CClzFs N2

sz

Hz0 HCl TI1 CHz0, CHaC1, CHOBr, CHJ C H , CzHs p-nitralin

References B11 R18 K18 K18 K18 M13 m15 MIS M13 M13 K19 K19

m14

M14 M15 M15 K1 K18 59

sr

e12 w10 J9 F10

wr

(2) Canals. The simplest directional slit system consists of a single Low source pressures must be used canal such as was used for NaZ2(DS). to keep the mean free path large, resulting in low beam intensities; many canals in parallel can be used to increase the beam intensity without loss of directionality. Several schemes have been tried; Stroke (XI), in work on radioactive cesium, used a vertical array of nine 28-gauge hypodermic needles, % in. long (see also G I ) , and Jaccarino ( J 1 ) has suggested as a gas source a similar arrangement of Vycor capillaries cemented into a slot in a Vycor tube with Sauereisen cement. Many parallel canals and maximum transmission can be obtained by stacking alternately strips of

20

JOHN G. KING AND JERROLD R. ZACHARIAS

corrugated and smooth nickel (or other malleable metal) ribbon (Fig. 7): The corrugated ribbon (“crinkly foil”) is easily prepared by rolling plain nickel ribbon between interlocking rollers (Fig. 8). The rollers were grooved longitudinally in a milling machine equipped with a fly cutter M

I-

FIG.7. End view of a directional slit, made by stacking strips of corrugated and smooth nickel ribbon alternately. The corrugations are 0.002 in. deep.

FIG.8. jllers use’ to prepare corrugated ribbon.

so ground that repeated sharpenings did not alter the groove profile (a 60-deg notch). A dividing head was used to space the grooves uniformly about the circumference. The rollers were made of bronze for easy machining and were hard chromed when,completed.

MOLECULAR BEAMS

21

Approximately 100 ft of corrugated nickel ribbon 0.001 in. thin and 0.5 in. wide has been prepared with 140 corrugations per inch of 0.002-in. depth. The use and performance of this ribbon will be discussed below. c. Note on Gas-Flow Control. (1) Valves.* Among the types of valves and leaks that have been used t o control the flow of permanent gases t o the source are Bourdon leaks, made by bending a flattened and nearly closed-off piece of tubing into a U; capillary leaks, sometimes made by inserting a closely fitting piece of platinum wire into a capillary tube

FIG.9. Needle valve for precise control at small gas-flow rates.

(heating the wire electrically can be made to control the flow) ; devices which release a metered amount of high-pressure gas into a large lowpressure reservoir at intervals; and needle valves of various types. Figure 9 shows a needle valve that has worked well. A 46 needle is used, and the hole is lapped beforehand with another identical needle which is then discarded, a seemingly necessary procedure if the valve is to have good gas-flow control. Besides the greaseless type with bellows, tapersealed valves with grease have given good service. For hydrogen, of course, palladium leaks may be used instead of valves; some precautions are suggested in J 2 . (2) Vapor-Pressure Control. Small amounts of dirt tend t o plug all these throttling valves, leading t o unsteady beams, and with corrosive

* See F 2 for much valuable

information.

22

JOHN G . KING AND JERROLD

R.

ZACHARIAS

gases like the halogens, cleaning and even complete replacement of the valve is frequently necessary. With condensible gases, such as the halogens, better beam steadiness over long periods has been attained by merely adjusting the temperature of the material t o give the desired vapor pressure in the source. For instance, some chlorine is frozen in a tube that passes through a block connected by a rod to a liquid Nz trap; current is supplied to heater windings around the block by a feedback circuit that maintains the block a t - 135’ C, the desired temperature. Similar schemes with wet and dry ice instead of liquid Nz are used for iodine and bromine, respectively. A crude valve, large enough not t o clog, is used in the source line as a shut-off. HEATING COIL

BAFF

FIG.10. Low-temperature oven. Note “anti-spritz ” baffles intended to keep the oven charge from boiling up and clogging the slit.

d . Molecular-Beam Sources for Operation to 500” C. (1) Thin-Slit Oven. A simple source or oven may be made by drilling an axial hole in a metal rod, cutting a slit near the top of the hole and providing a threaded plug with a copper gasket washer to close off the top. The oven is surrounded by a self-supporting or mica-spaced heating coil of 0.030411. molybdenum wire, with turns wound closest near the slit, and is mounted by thermally insulating spacers (e.g., Lavite). Such ovens may be made very small (1% X W S in.), as when used for calibration purposes when they are swung in front of the regular source, and have been made quite large (2% X 3d in.), as described in E l and shown in Fig. 10. Sixty hours of operation with potassium have been obtained with a 2-g load. Overfilling should be avoided, as well as rapid heating, or early clogging of the slits will result. Heating the slit first and keeping it hotter than the rest of the oven reduces condensation difficulties. These ovens have been made of different materials, often scraps of unknown composi-

MOLECULAR BEAMS

23

tion, but Davis (DS)finds K Monel relatively impermeable t o sodium, and this is presumably one of the better materials for alkali ovens. (2) Directional Oven. The ovens described above, although easy to build and suitable for everyday work with stable elements, are not efficient in conserving material or heater power. A very satisfactory directional oven (Fig. 11) can be constructed by sandwiching several layers of crinkly foil between two metal blocks with a n annealed copper

\'

' /

R

FIG.11. Directional oven1. The inset shows the copper wire gasket used both as a seal and a spacer. The lower cross-section view shows the arrangement used to break the ampoule containing the charge.

wire serving both as a gasket and a spacer. The blocks carry the heaters, which are made of 0.010-in. Mo wire, Alundum-coated (El)in preference to bulky quartz, ceramic, or mica insulation. An ampoule of alkali metal is placed in a separate chamber, which is connected by a tube to the space between the blocks which is sealed off by the copper wire. The threaded, copper-gasketed cap of the ampoule chamber is provided with a tapered hole for a locking taper plug, which rests lightly on the ampoule, leaving an annular opening through which the gas evolved during preliminary

24

JOHN G. G. KING KING AND AND JERROLD JERROLD R. R. ZACHARIAS ZACHARIAS JOHN

heating of the oven in vacuum can easily escape. A sharp blow struck on the tapered plug (e.g., by burning out a wire supporting a weight) breaks t h e ampoule and forces the plug down so that it seals the outgassing opening. The ampoule chamber is heated by conduction from the oven blocks, thus insuring t ha t it is heated last and least; a t temperatures of 80" C to 100" C (as used for cesium), attained with approximately 15 w of MAX

1.0

, 0.8

0.6

3'

1

-

OVEN ANGLE (RADIANS1

-

-0-

vs oven angle angle 8. 8. N N is is plotted plotted in in units units of of N,,, N,,, the the FIG.12. Plot of beam intensity N vs The slit canals were were >$-in. >$-in. long long and and the the stack stack of of intensity in the forward direction. The and 0.025-in. wide. At At an an oven oven temperature temperature of of 341' 341' K, K, some 500 canals was 1-in. high and away was was 1.1 1.1 X X lo7 lo7atoms/sec. atoms/sec. With With the the Nmax striking a detector 0.005-in. wide 12 in. away and although although the the central central peak peak was was not not oven a t 417" K, N,,, was 1.9 X lo7atoms/sec and at larger larger angles angles was was increased increased (by (by aa factor factor increased in width, the number of atoms at of 2 a t 0.08 radian).

the oven oven heater power, the ampoule chamber is some 15" C cooler than the slits clogging. blocks, and there has been no difficulty with the slits in Fig. 12 along along The angular distribution of emitted atoms is shown in given pertinent data (23).There appears to be no reason, given with other pertinent why adequate heating arrangements and a low-conductivity mounting, why temperatures, b u t this this has has these ovens should not be used at much higher temperatures, make didinot yet been done, although Pkter and Strandberg ( P I ) plan t o make rected beams of alkali halides for high-resolution microwave spectroscopy. Higher Temperatures. e. Atomic-Beam Sources for Operation at Higher temperatures the following following problems arise: supplysupply(1) General. At higher temperatures ing sufficient stable heating power, as much as several kilowatts, minimizminimiz-

MOLECULAR BEAMS

25

ing heat losses, controlling the temperature distribution throughout the oven, providing adequate radiation shielding to prevent heating of the source chamber walls and consequent outgassing, and preventing chemical reaction or alloying of the beam material with the oven walls or

FIG.13(a). Gallium oven assembly. The water-cooled outer box is mounted to the top plate of the source chamber. The inner chamber containing the oven block and radiation shields is separately mounted and pivoted to allow adjustment in a vertical plane of the beam angle.

clogging of the slits by excessive creep, effects all greatly accelerated by high temperatures. Simple techniques deal with the first three items, but the last are matters for empirical investigation. Thus, for instance, Lew (LZ,LS, L4) and Wessel and Lew (W1) have found small crucibles of thorium oxide effective in handling A l , Pr, and Au.

26

JOHN G. KING AND JERROLD R. ZACHARIAS

(2) Gallium Oven. Figure 13 shows two views of a n oven used by Daly ( 0 4 ) for gallium. The oven block is machined from graphite, a material that does not alloy with gallium ( R I S ) , and is provided with a slit 0.008 in. wide and 0.125 in. high, six Alundum-coated spiral molybdenum heaters and screw caps for access to the slit and loading. The oven block is located by three 0.040 tantalum rods and held down by a tantalum screw. The oven is surrounded by four 0.005411. tantalum foil radiation shields, * corrugated to minimize heat loss by conduction and to allow free escape of evolved gases. The entire assembly is mounted in

SECTION A-A

SECTION 8-8

FIG.13(b). Gallium oven block. Made of graphite. The arrangement of screw plugs and the slit may be seen in section A-A and the heater holes are shown in section B-B.

a water-cooled box. With 160 w supplied to the heaters temperatures of ca 1100" K were reached and a l-g gallium sample yielded a steady total beam of 1.4 X 1OI6 atoms/sec for approximately 100 hr. (3) Ovens t o 1900" C. Lew and others have used slender tubes of graphite or molybdenum directly heated by the passage of a large current t o obtain beams of refractory materials. A typical design is shown in Fig. 14. For aluminum (LZ, LS), gold and silver ( W l ) ,and chromium (BZ),graphite tubes with a thoria crucible containing the beam material to prevent it from creeping and clogging the slit or penetrating the graphite have been used; the crucible is isolated from the graphite by tantalum foil t o avoid reduction of the thoria. For praeseodymium (L4)a

* An interesting discussion of

multiple radiation shields may be found in L10.

27

MOLECULAR BEAMS

molybdenum tube and thoria crucible without tantalum foil were used.

A graphite tube reached a temperature of 1670" K when supplied with 800 w (Lg), and a molybdenum tube reached 2000" K with 2400 w (10 v a t 240 amp) (L4). Ovens may also be heated by electron bombardment. One design (L5), consisting of a molybdenum block lf/4X 1$/4X 1% in., equipped with a

I

I N S I D E DIAM. 0.63

cm

OUTSIDE D l A H . 0.83 c m

4.75

cm

I

Y : FIG.14. High-temperature oven as used by Lew. Heated by a heavy current passing through the tube-later designs have used T h o , crucibles.

single circular canal 0.026 in. in diam and 734 in. long, has been heated to 1900" C. One t o two amp of electron emission and 3000 v accelerating potential were available to supply the large heating power, which would have been difficult to obtain with conventional wire heaters. f. Dissociating Sources. (1) Wood's Tubes. Several methods have been used t o make atomic beams from molecular vapor or gases. Large Wood's tubes (W2) with aluminum electrodes remote from a central

28

JOHN G. KING AND JERROLD R. ZACHARIAS

water-cooled slit have been used t o obtain beams of atomic hydrogen (Kd, Pd) [and metastable helium atoms (H4, W S ) ] High . yields of atomic hydrogen (70 t o 90%) appear to require the use of hydrogen saturated with water vapor and the exclusion from the region near the slit of dirt, sputtered metal from the electrodes, and other impurities. The slit should be close t o the discharge. Typical operating conditions are l-mm Hg hydrogen pressure and 10,000 v a t 0.05 amp. Inspection of a correctly operating discharge with a spectroscope reveals the Balmer lines with only a very weak molecular background. Although the Wood's tube is an effective source of atoms and its low operating temperature ( 5300" K) leads t o narrow resonance lines, its large volume may be troublesome if only small amounts of a gas such as H 3 ( N d ) or He3 (WS) are available and if adsorption or contamination by the metal cathode is not tolerable. (2) Thermal Dissociator. Lamb and Retherford (LB) obtained about 64% atomic hydrogen with a thermal dissociator consisting of a tungsten tube (0.065 X in., 0.004-in. wall) heated to 2500" K by 160 w from a current of 80 amp passing through the tube from water-cooled molybdenum leads. Although the degree of dissociation is not so high at feasible temperatures as could be attained with a discharge, the absence of large ultraviolet light output was an advantage in eliminating background from photoelectrons emitted by the detector. A similar thermal dissociator was tried by Jaccarino and King ( J 3 ) for chlorine, and although 65% atoms was obtained a t 1900" C, the device was not reliable, the tungsten tubes failing because of chemical reaction and improper current regulation. Further development was stopped when successful results were achieved with microwave arcs. (3) Microwave Arcs. This type of atom source has been used for hydrogen ( N S ) and for the halogens ( 0 5 , J S , K S ) . The earlier design (Fig. 15) consisted of a cavity made from a length of S-band waveguide with a low-loss 707 glass discharge tube in which a slit was cut. The discharge tube was mounted a t a voltage maximum in the guide with a wax or O-ring vacuum seal as close to the slit end as possible. Approximately 50 w of C-W 10-cm power was supplied t o the cavity through a matched line by a QK61 magnetron operated from a current-regulated power supply. An air blast cooled the discharge tube. Although 90% atoms could be obtained,* the arc was often hard to start and frequently could not be persuaded to occur stably at the front of the tube near the slit, whereupon no atoms were seen in the beam. These difficulties led to the adoption of the source shown in Fig. 16 [originally devised as a light source by Davis (D6)]. The discharge tube passes through the drilled-out center of a standard ?&in. S-band coaxial tee and protrudes approxi* For the halogens, 30-60% atoms for hydrogen (J4).

MOLECULAR BEAMS

29

mately 94 in. Power is supplied to the arc through a coaxial Kovar seal from a magnetron as before. The arc is readily started with a Tesla coil and runs stably right behind the slit, presumably because the rf electric field is sufficiently strong near the slit, in contrast to the cavity source, where the slit is really outside the cavity. The discharge tube is cooled by radiation t o the water-cooled outer conductor of the coaxial tee. Because the discharge tube operates a t 800-1000° K, it is made of Vycor instead of 707 and the inner parts of the tee are silver soldered. Operating R F LEAD

,GAS

INLET

COOLING AIR

WINDOW

-

DETAIL OF DISCHARGE TUBE

FIG.15. Cavity microwave arc atom source.

pressures range from 0.05 to 0.5 mm Hg, so th a t if very small consumption is required, correspondingly small slits or a carrier gas must be used. When microwave arc atom sources are used, careful shielding is obviously desirable t o keep rf out of low-level circuits. (4)Sources of Atoms in Excited States. Many experiments have been performed with beams of atoms whose atomic ground state is a finestructure doublet (B, All Gal In, TI, C1, Br, I). For the lighter atoms, with fine-structure splittings 6 (cm-') comparable t o the thermal energies acquired in the source a t the temperature used to obtain a beam, th e Boltzmann factor, exp (- Ghc/kT), is large enough so th a t atoms in the

30

J O H N G. KING AND J E R R O L D R. ZACHARIAS

metastable state can be observed without further treatment. When this is not the case (as it is not for T1, Br, I), additional excitation must be supplied, either by electron bombardment or by optical excitation t o a higher state that decays to the metastable state. The first method has

FIG. 16. Coaxial microwave arc atom source. The numbered components are (1) sample whose vapor is t o be dissociated (in this case, iodine crystals) ( 2 ) shut-off valve (3) discharge tube (4)slit (5) Kovar vacuum seal in coaxial rf line (6, a, b, c) parts of a tuning arrangement later found superfluous. The slit should not protrude as shown.

been used t o obtain hydrogen atoms in the 22S4, metastable state, and a very complete discussion of the design requirements, construction, and performance of such a device may be found in L6. Work on thallium in the 2P, state optically excited is being done by Gould (R8), and reso-

MOLECULAR BEAMS

31

nances have been observed in the optically excited 2P46states of alkali atoms (P3).Alkali vapor lamps made by Philips or G.E.* were used, and because of the short lifetime of the P states, the atoms were excited in the transition region of the apparatus rather than a t the source. An advantage of optical excitation over excitation by cross bombardment of the atomic beam by electrons is that there is less recoil and therefore less broadening of the beam. Obviously relevant questions of light intensity and electroncurrent density, of cross-sections for the desired process, and of minimizing quenching of the metastable atoms must be considered in the choice and design of an exciter for a given application. g. Recirculating Sources. Unless very directional sources and a n apparatus with sufficiently broad geometry are used, so th a t a much larger fraction of the emitted atoms than the typical 4i07 reach the detector, it is necessary to recirculate the sample if large amounts are not available. Recirculation was early used with He and later by Estermann et al. (E8) with HD. More recently Nelson and Nafe ( N 2 ) used a mercury pump connected to the output of their main diffusion pumps to recirculate their tritium sample. A hot palladium thimble allowed only hydrogen t o pass into the discharge tube. Difficulties with loss of tritium and contamination of the H 3 by H1 were experienced. I n their work on metastable He3 atoms Weinrich and Hughes (WS) used a purifying and recirculating system (Fig. 17), since they found th a t -1 % ' impurity would appreciably reduce the concentration of metastable atoms. They used a mercury diffusion pump t o back the main diffusion pump and t o return the gas to the discharge tube; the purifying system consisted of a zirconium filament a t 1300" C to remove oxygen and nitrogen and a U-tube containing cupric oxide a t 550" C to oxidize hydrogen t o water which was frozen out together with other condensibles. A Toepler pump was used to return the gas to storage tanks. Various effects could be attributed to the aluminum discharge-tube cathode which cleaned up air but not hydrogen (which it in fact evolved) and absorbed helium when it began to sputter. This absorption was stopped a t first by scraping the cathode and eventually by replacing it. With this technique they were able to work with 3 cc N T P of helium. Work in progress a t M.I.T. with a 5-mg sample of IlZ9 has led to the design of a different recirculating system. The problem is different because iodine condenses readily and is chemically active, so th a t it is virtually impossible t o recover iodine that has left the source. It is planned to use a very directional Vycor source of small volume consisting of two chambers, one for the microwave arc and the other cooled t o collect the iodine which

* Phillips S060W or 93122, G.E. NA-1.

32

JOHN G. KING AND JERROLD R. ZACHARIAS

would ordinarily be lost. Recirculation takes place when the collecting chamber is closed off and warmed and the arc chamber cooled. h. Miscellaneous. An extensive body of technique on the use of beams of energetic neutral atoms (200 to 800 ev) produced from ions by charge exchange has resulted from the work of Amdur and others ( A l ) .Although the problems of line width and deflection discourage the use in resonance experiments of fast beams, the ease with which they can be detected might have been a n advantage had it not been for the recent success of universal ionization detectors. From vacuum system

From supply of He4

Copper oxide Zirconium purifier purifier

Auxiliary pump

FIG.17. Helium purifying, recirculating, and storage system as used by Weinrich and Hughes. Numbered circles are stopcocks.

Beams of extremely refractory metals might be made using the intense heat generated by a condensed spark occurring between vibrating electrodes made of the desired material. Kistiakowsky and Schlichter (K4) used a supersonic jet of ammonia suggested by Kantrowitz and Grey (K6) t o obtain beams higher in intensity by a factor of 20 than could be obtained from the usual lowpressure single-slit source. The large volumes of gas that must be handled are a marked disadvantage of this scheme.

111. DETECTORS OF BEAMSOF NEUTRALMOLECULES 1. General Considerations Detectors of neutral beams fall into two broad categories; those which give a direct measure of the beam intensity, for instance, by ionizing th e

MOLECULAR BEAMS

33

molecules and measuring the resultant ion current, and those in which the effect of the beam on some suitably sensitive device, such as a highly refined manometer, gives an indirect measure of the beam intensity. Detectors of the first type, which make it possible to identify and select a particular isotope, are advantageous when it is desired to work with beams of mixed isotopes, some of which may be present in great dilution. On the other hand, many elements are difficult to ionize with good efficiency and detectors of the second type must be used, if necessary with beams appropriately enriched in the desired isotope. Thus, the continuing search for effective detectors is divided into a search for better ionization schemes and a search for effects sensitive to small numbers of molecules. A satisfactory detector, besides being relatively easy to build and repair, should have the following properties : adequate signal-to-noise ratio (100 t o lOOO), linear response to beam intensity, relatively short time constant, and good long-term stability. It is instructive to consider the conditions under which the detector must operate. The number of molecules per second in a beam reaching a detector of area A located a t a distance 1 from a source is

nA

nb = R12

where n is the total number of molecules per second leaving the source, given by Eq. (1) of the preceding section. It is assumed that the source has a thin slit, so that the angular intensity distribution is approximately given by the cosine law, th at attentuation by scattering is negligible, and that the source and detector are "lined up" so th a t the normals to their effective areas coincide. The ratio of the number of beam molecules per second reaching the detector to the number n, of residual gas molecules per second striking it is

These quantities with representative values are as follows : p a

T M

p,

I

T,

M,

= = = = = = = =

source pressure, 0.01-1 mm Hg source area, 5 X 10-"5 X cm source temperature, 100-2000" K molecular weight of beam molecules, 1-200 residual gas pressure, 5 X lo-' mm Hg length of beam, 10-300 cm temperature of residual gas, 300" K molecular weight of residual gas, 30

34

JOHN G . KING A N D JERROLD R. ZACHARIAS

under The above ratio could therefore range from roughly lo2 to extreme conditions, so that if fluctuations in the residual gas pressure are not to mask the changes in beam intensity that are to be observed, it is often necessary t o use a detector that discriminates strongly in favor of the beam molecules. This is possible for two obvious reasons, the beam molecules have directed velocities, and they are often of a species not found in appreciable concentration in the residual gas. Detectors th a t measure the increase in pressure in a cavity due to the beam, which enters through a canal (Pirani gauge), or that measure the momentum flux of the beam (radiometer) depend mostly or entirely on its directed nature, whereas detectors in which the beam strikes a target and either condenses, making a deposit which is visible or whose radioactivity can be measured, or causes a visible chemical reaction depend on special properties of the beam molecules, as do detectors in which a current of ions formed from the beam is measured, often after mass-spectrometric analysis. Most of the recent work in molecular beams has been done with Pirani gauges for hydrogen and helium, deposition targets which can be removed for counting for short-lived radioactive isotopes, hot-wire detectors for a limited group of elements (alkalis, halogens, Al, Ga, In, and some others), and with universal ionizing detectors, which may ultimately make it possible to work with any element. Representative designs and the performance of these commonly used types of detector and their associated auxilliary equipment will be described in the following sections ; a final section will be devoted to brief descriptions of some of the many other types that have been tried or suggested. Table I shows a t a glance the detectors that have been used in different experiments. 2 . Pirani Gauges

a. Principle of Operation. If a molecular beam is allowed to enter a cavity through a channel, the pressure in the cavity will rise until the number of atoms entering the cavity per second and the number leaving per second are equal. For a rectangular channel of length 1, width w , and height h proportioned so that 1 >> h >> w, the ratio of the number of molecules entering the channel from all directions to the number leaving is given by (see Fig. 4) =

w[0.5

1

+ log, (2h/w)]

(4)

Thus, the equilibrium pressure in the cavity reaches a value K times the pressure that would have been re-zched had a thin slit instead of a channel been used and is

MOLECULAR BEAMS

35

where Td is the absolute temperature of the cavity and the other symbols are defined following Eqs. ( 2 ) and (3). Using typical values for hydrogen from an experiment by Julian ( J 4 ) K

=

60

n = 1.3 X lo1*molecules/sec

M = 2 Ta = 300°K

one finds p d = 3 X low6 mm Hg for the full beam. This pressure is measured with a refined Pirani gauge in which the cooling effect of the gas on the temperature, and hence the resistance, of an electricallyheated ribbon placed in the cavity is observed. I n practice, two separate cavities as nearly identical as possible are used, so that by measuring the difference in the pressure in the cavity to which the beam is admitted and that in the compensating cavity, effects of fluctuating residual gas pressure in the detector chamber are largely balanced out. Each cavity contains two ribbons connected in diagonally opposite arms of a Wheatstone bridge, thus doubling the sensitivity while providing the necessary compensation. Heat generated in the ribbons by the bridge current is dissipated by radiation, by conduction through the end supports of the ribbons, and by conduction through the gas surrounding the ribbons; since the heat conducted away by the gas is lo4 to l o 3 times smaller than the total heat dissipated, instabilities of the latter seriously limit the useable sensitivity. Various analyses of Pirani gauge performance have been worked out (J4, Rd), and the following is adapted from the work of Julian. The heat power removed from the ribbon by conduction through the gas under steady state conditions is

These symbols and typical values are as follows pd

A,

M

Td

T, a k f

gauge cavity pressure, 3 X mm Hg ribbon area, 0.092 cm2 = molecular weight of gas (H2), 2 = detector temperature, 300" K = ribbon temperature, 450" K = accommodation coefficient (Hz on Ni), 0.25 = Boltzmann's constant, 1.38 X 10-l6 erg/"C = number of degrees of freedom of molecules, 5 = =

36

JOHN G. KING AND JERROLD R. ZACHARIAS

Evidently, the first term represents the number of collisions per second with the ribbon and the second the net energy carried away per collision. For these values WGis about 0.5 pw. By a lengthy but straightforward analysis one can find the steady-state unbalanced current through a galvanometer connected in a bridge made up of four ribbons, the internal resistance of the galvanometer being equal to the resistance of one ribbon, t o be approximately

where I0 is the total gauge current, a is the temperature coefficient of resistance of the ribbon, R is the ribbon resistance, and Wo = Io2R/4 is the power dissipated in one ribbon. The measured values agree with the values predicted by this equation to within the rather large uncertainties present, particularly in the value of p d and hence WG. There are three time constants associated with the Pirani gauge, the fill-up time of the cavities when exposed to the beam, the thermal time constant of the ribbons, and the time constant of the galvanometer and associated circuits. The fill-up time constant is the number of molecules in the cavity a t equilibrium divided by the rate of arrival of molecules, or t, =

1.33 x 1O3p,v/kTa nb

(8)

where V is the volume of the cavity. Taking p d and n b from Eqs. (2) and ( 5 ) , Eq. (8) becomes KV t, = - 2.76 x 10-4 (9) A When a galvanometer whose resistance is equal t o the ribbon resistance is used, the thermal time constant of the ribbon is t -

-

C dWo/dT,

where C is the total heat capacity of the ribbon. Making K large by using a long narrow channel increases the equilibrium pressure for a given beam intensity and hence the sensitivity. Unless the cavity volume is made small, the fill-up time may become too long compared with the thermal time constant of the ribbon. Thus, ribbons with small heat capacity and large surface area give short thermal time constant and maximum sensitivity and should be of a material of small emissivity and large accommodation coefficient to make radiation relatively as small as possible. Heat conduction a t the ribbon ends can be

MOLECULAR BEAMS

37

made relatively small by using long ribbons. When it is considered that there are obvious practical limitations, such as the difficulty of mounting extremely thin ribbons in a cavity just large enough so that they do not touch the cavity walls when they sag on being heated, it is not surprising that the design of Pirani gauges is largely empirical, with approximate analyses serving as guides. b. Construction. Early successful designs of Pirani gauges for molecular beam experiments are described in K l and 21.Recent work has been done with gauges of improved form built as follows: A brass block (see Fig. 18) lapped t o optical flatness has two cavities cut in it in which the ribbons are mounted flat but without tension. The ends of the ribbons SMALL KOVAR METAL /TO GLASS SEAL

A 0 F

BRASS BLOCK

FIG.18. Pirani gauge block. Another similar block is clamped against the one shown, thus forming the ribbon cavities and entrance channels.

are soldered with Wood’s metal to small glass feed-through insulators which provide electrical connections. Platinum ribbons 1 mm wide and 3000 A thick made from rolled 0.001-in. Wollaston wire have been used, the job of preparing and mounting these ribbons being done by Baird Associates of Cambridge, Mass. Another flat block is used t o cover the block containing the ribbons and cavities, with a piece of aluminum or gold foil acting as a spacer and cut so as to provide the beam entrance and compensating channels. The whole sandwich is clamped by screws or springs and mounted to a trap if it should be desired t o cool the gauge. Cavity volumes of 0.05 cm3 and channel K of 400 are readily attainable with this method of construction. Various precautions must be observed in constructing the gauge: there must be no leaks around the foil, the ribbons must have adequate clearance, electrical connections must be

38

JOHN G. KING AND JERROLD R. ZACHARIAS

arranged to minimize stray thermoelectric emf and noise, and very careful shielding and bypassing are essential, since the gauge is also a sensitive square-law rf detector. Glass blocks have been used instead of brass in the hope of reducing the evolution or absorption of gas, the ribbons being attached to leads sealed where they enter the cavity with a drop of Glyptal. Data on the construction and performance of three Pirani gauges is listed in Table 11. TABLE 11. Pirani Gauge Data Julian

(JC)

Ribbon; Length, cm Width, cm Thickness, cm Resistance a t 300" K, ohms Material Cavity: Length, cm Depth, cm Width, cm Material Channel; Length, cm Height, cm Width, cm

Prodell and Kusch (P2)

Kolsky et al. (K10)

3.7 0.013 5 x 10-4 13 Ni

3.8 0.038 1 . 3 x 10-4

3.7 0.05 0.2 Glass

3.8 0,025 0.2 Glass

7.9 0.1 0.48 Brass

Operating data: Ribbon temperature (" K) Total bridge current, amp Time constant, sec

4.4 3 0.0025 -60

3.12 0.33 0.0025 -200

2.5 0.79 0.0015 300

450 0.046 3.5

0.030 7-8 for max response

Sensitivity for Hz, molecules/sec-pv

300 0.024 12 for 90% of max response

1.2

K

*

x

... Pt

1012 -4.3

7.6 0.051 10-4 19

Pt

x

10'0 (est.)

1 . 1 X 1O'O

* NOTE:All three Pirani gauges were used with galvanometers yielding sensitivities of approximately 7pv/cm. The sensitivity of Pirani gauges is not high compared with th a t of detectors which count individual particles, it is not fast, and it is often unexplainably erratic and noisy in its operation. Bederson has suggested applying the pressure-amplifying feature of a McLeod gauge t o a Pirani by providing movable pistons, which in effect vary the cavity volume and compressed the accumulated gas; a thousandfold increase in sensitivity should be attainable, although, of course, continuous indication must be abandoned. Although there are numerous difficulties t o be

MOLECULAR BEAMS

39

overcome, such a device should be feasible, and one was built a t M.I.T. but never tried carefully enough to yield reliable data. The use of crinkly foil t o produce channels of high K and of sufficient width to make it possible to use broader beam geometry effectively, at least for atomic experiments, should not be overlooked. Perhaps the worst feature of the Pirani gauge as far as future developments are concerned is the difficulty of controlling absorption of gas by the cavity walls, and it is our belief that this may be the reason that the performance of recent highly refined designs, though representing a n improvement over that of earlier designs, has not come up t o design expectations. Unfortunately, it is difficult to reduce the amount of surface exposed t o the gas to be detected by a large factor. 3. Deposition Detectors

I n the earliest molecular beam experiments, such as Dunoyer’s 1911 experiment with sodium (D8), the presence of a beam was observed by allowing i t t o condense on a cooled surface until a visible deposit was formed. The surface material, the temperature a t which it must be maintained, and the minimum beam intensity which will form a permanent deposit all depend on the kind of molecule of which the beam is composed. Invisible deposits can often be developed chemically, and rough quantitative measurements of beam intensity can sometimes be made either from densitometer measurements or by measuring the time required for a visible trace of the beam to appear. Such beam detection techniques are quite effective when it is desired to observe the spatial intensity dislribution of the beam, as in the Stern-Gerlach experiment, and much important early work was done with them. They are, however, ill suited to modern refocusing resonance experiments in which a continuous indication of beam intensity with a reasonably short time constant is desired. Beams of atomic hydrogen, atomic oxygen, and active nitrogen have been detected by allowing them to fall on targets coated with Moos, PbO, and AgNO3, respectively, where they produced visible traces by chemical reaction; these methods have many of the difficulties of the condensation methods. Fraser ( P I ) discusses these detection techniques a t some length, describes experiments in which they have been used, and gives extensive references. If the beam trace is to be visible, relatively long exposure times may be required (24 hr in some cases) even with chemical development, SO that deposition methods would seem to be largely of historical interest; but recently they have been used effectively to detect beams of radioactive isotopes of moderately short lifetimes. Bellamy and Smith (B3) used deposition methods in experiments on 14.8-hr NaZ4and 12.4-hr K42.

40

JOHN G. KING A N D JERROLD R. ZACHARIAS

A 1-g sodium sample of 200-mc activity in which t h e Na24/Na23 ratio was about 2 x 10-8 was used in a source similar t o Davis’ (DS). Their description of their detection method states

. . . the beam falls on a 0.025411.

wide hot oxidized tungsten strip, one end of which is grounded. The positive ions are attracted to a 1-cm diam brass target mounted on an insulated support which can be removed from the vacuum system through a gate valve. The target, which is 100 v negative with respect to the ground, is connected to the input of a dc amplifier and current integrator. A second similarly situated, normally grounded collector is mounted in the detector chamber; it can be switched to the dc amplifier input in place of the target, which is then grounded. This collector is used when setting the C field and making other adjustments, to avoid depositing unwanted activity on the target. The ratio of the activity collected on the removable target during a run a t a particular frequency to the total beam current integrated over the same period is used as a measure of the active isotope beam intensity. Since the time taken to plot out a resonance curve by this method is comparable with the half-lives of 24Kaand 42K, allowance for decay in the source is made by monitoring the counter with a standard made from the material in the oven. T h e target activity after collecting t h e beam for 20 t o 30 min was about 50 counts/min and rose t o 150 counts/min when a resonance was observed. Comparable results were obtained with K42,except t h a t t h e sample activity was 20 me, t h e K42/K39ratio was 2 X and the counting rates were one-tenth of those observed in t h e NaZ4experiment. It will be noted t h a t Bellamy and Smith combined t h e surface ionization detector (see next section), which is 100% efficient for alkali metals, with deposition techniques. With samples in which t h e radioactive isotope is highly diluted and of relatively short half-life, these techniques are probably simpler t h a n those in which t h e current of ions of t h e desired isotope is selected with a mass spectrometer of necessarily high resolution and measured with a n electron multiplier. It is probable that t h e accelerated ions were driven into t h e target, which was not cooled, a n d that a negligible amount of t h e collected beam was lost b y re-evaporation. Cooled targets were used b y Goodman and Wexler ( G I ) in their work with 3.1-hr C S to ~ collect ~ th~ e neutral ~ beam atoms. Their description of their technique follows.

Scomponent ~ ~ was~ measured ~ by condensing the beam on 1-in. The active C diameter copper disks (Ms-in. thick), which were held by spring clamps on the faces of a brass octagon attached to the bottom of a liquid nitrogen trap. These disks were washed with water, alcohol, and ether before being used. If a disk showed radioactive contamination, it was first etched in dilute HC1. No other surface treatment was necessary. The long tube containing the collector was rotated in O-ring gaskets so as to position each of eight disks in turn behind the

41

MOLECULAR B E A M S

collector slit. Vertical movement of the tube was effected by hydraulic lifts. A drybox attached around the tube above the apparatus made it possible to bring the cold octagon and trap through vacuum locks into a dry helium atmosphere for the changing of disks. Measurement of the activity was made by placing each disk in a windowless G-M flow counter containing a 98% He-2 % Isobutane gas mixture.

Wexler has reported (W4) some experimental work on the sticking of various beam atoms t o various surfaces on which the above technique is presumably based. TABLE 111. Radioactive Beams Detected by Deposition

Isotope 14.8-hr Na2' 12.4-hr K42

Approx. Approx. Sample counts/ counts/ Collection min, at time Collector Type of Referactivity min, off (me) resonance resonance (min) material counter ence 200 20

12.8-hr Cue4 4.7-hr Rb81

Brass Brass

...

50

150 15

20-30

47

75

5

Copper Sulfur

5

Copper

Tungsten Copper Windowless G-M Copper Scintillator Copper Scintillator

15 15

70 70

5 5

2.7 d AulgS

7.5

50-100

200

5

3.15-d Au'"

9

50-100

150

5

Scintillator 2~ G-M Scintillator 2~ G-M

BJ B3 L7 Hl4

L7

G7

G7 C6

GI

CS CJ

The deposition method is particularly valuable in work with beams of radioactive elements that cannot be ionized efficiently so th a t the methods described in the next section are inapplicable a t present. Radioactive isotopes of copper, silver, and gold have been investigated in this way. Table I11 summarizes the recent work with radioactive beams detected by deposition. 4. Ionizing Detectors Ionizing the neutral atoms or molecules of the beam makes it possible to select ions of the desired isotope with a mass spectrometer and t o measure the ion currents with an electron multiplier; since the background counting rate of a multiplier can be reduced to a few counts per

42

J O H N G. KING AND JERROLD R. ZACHARIAS

minute, it is possible, depending on the efficiency of the ionizer, its background ion output, and the resolution of the mass-spectrometer, to work with extraordinarily low beam intensities or with greatly diluted isotopes. a. Surface Ionizers (Positive Ions). This type of ionizer has been used in many investigations (see references given in N 4 ) but was first applied as a detector of atomic and molecular beams by Taylor (2’2) in 1929. The beam atoms or molecules fall on a hot wire, and some fraction leave the surface as positive ions. The theory of this process will not be reviewed here, but has been discussed by many authors (see, for instance, V l ) . The ratio n+/n of the number of positive ions to the number of neutral atoms leaving a hot metal surface per second is ( M I )

where I is the ionization potential of the atom, 4 is the work function of the surface, and T is its absolute temperature. For the alkali metals Cs, Rb, and K, whose ionization potentials are less than the work function of the commonly used tungsten wire (4.5 ev) this ratio ranges, for a wire temperature of 1200” K from 600 to 6; such numbers are not t o be taken any more seriously than as an indication of the fact that no difficulty is experienced in detecting these elements. * The lighter alkali metals, Na and Li, whose ionization potentials exceed the work function of tungsten, can be detected by oxidizing the tungsten, thus raising its work function to approximately 5.9 ev ( W 5 ) . It is found that alkali halide molecules dissociate a t the hot wire, so th at beams of these molecules can be detected by observing the emitted positive alkali ions (RS). Various elements of group I11 can be detected on an oxidized tungsten wire with varying efficiency. Table I V gives some data on ionizing detectors as applied t o molecular beams. Values of ionization potentials and work functions, besides being given in ( H I ) , are plotted in ( M 2 ) in a way that is convenient for estimating detector performance. It is difficult t o discuss the performance of surface ionizing detectors because of the lack of reliable experimental results under controlled conditions, but it is not hard to see why such peripheral research is rarely thoroughly carried out by atomic beam workers. Too low a hot-wire temperature results in low ionizing efficiency and slow response. Daly ( 0 4 ) has compared the response of a hot wire t o steady and modulated K beams and found that the dc response rose to a maximum from approxi-

* Since all new wires are “flashed” to drive off surface alkali contamination, a thoriated tungsten wire inadvertently installed in a n ionizer became activated (+ = 2.6 ev), whereupon no alkalis could be detected (see also L9).

43

MOLECULAR BEAMS

TABLE IV. Beams Detected with Surface Ionizers Wire

W W+OzW+Th

MO

Max temperature 2200 K): 2900 1600 work function (ev): 4.5 5.87 2.63 ( M 2 ) (W6) ( K W

2160 4.27

(0

Beam molecules forming Ionization potential positive ions (ev) ( H I ) n+/n n+/n

Li Na Li & Na Halides K Rb

CS

K, Rb, Cs Halides Ga In T1 A1 Ba Pr Beam molecules forming negative ions

F

c1 Br I CSI CsBr CSCl CsF

5.97 5.76 6.07 5.96 5.19 5.8

Electron affinity (ev)

-4 ( M l ) 3.74 ( Y l ) 3.64 ( Y I ) 3.31 ( Y 1 )

n+/n

n+/n

References for efficiency data*

10 100 1c-100

5.36 5.12 4.32 4.16 3.87

(Ma

10 10 100 10-100

D4

0.17 1 0.1 100.4 2

n-/n n-/n 10-3 10-4 10-6

n-/n

x

12 H6 10-4

~4

n-/n

10-2 10-3 10-4

0.04-0.02 0.2-0.11 0.6-0.3 <0.01

TS TS TS TS

* Where no reference is given, data is questionable. mately 690 to 770" K and then fell off to 60% of its maximum values a t 900" K. The response to a beam modulated a t 93 cps rose to its maximum value (60% of the maximum dc response) between 770 and 900" K. This is difficult t o interpret in detail but certainly indicates that the time constant of the ionizer for K (and for Ga at a higher temperature, as Daly later found) is less than 10 msec. A more complete investigation of the

44

J O H N G. KING AND JERROLD R. ZACHARIAS

response time of surface ionizers for alkali has been carried out by Knauer ( K 7 ) with a rotating shutter. He finds times ranging from sec, depending on the temperature of the hot wire. Lew (L4) to 3 x found the delay in the ionization of Pr atoms on Mo wires to vary between 1 and 2 sec a t 2160" K and -30 sec a t 1870" K. The ratio given by in this case, so that it would appear that the Eq. (11) is about 2 X long time constant goes with the low efficiency of ionization. If a surface capable of being heated t o high temperatures while preserving a stable high work function could be found, many more elements might be ionizable in this way. Unfortunately, oxide coatings evaporate a t high temperatures, as seen from the fact that the efficiency of detection for Li becomes extremely small if a n oxide-coated wire is flashed above 1600" K (F1). I n many instances ( 0 4 , KB) oxygen has been supplied continuously, and Kingdon (KB) reports having ionized Cu, Bi, and Ca but estimates the work function of his oxidized wire to be 9.2 ev. Since he used no mass spectrometer, his results may Ge questionable. Recently, however, Lurio (L8) has found it possible to detect Ca with a W wire a t 2000" K, but no details are available. Sometimes an atom with a high ionization potential, which requires an oxidized wire, may form a compound that reduces the work function and prevents ionization. This effect may account for the difficulties in detecting Ba (GZ, H 6 ) and may be expected for Sr. Fluorine raises the work function of tungsten to 5.6 ev ( M 3 ) up t o 2650" K when the fluorine is driven off; below 2600" K the surface is found to be very stable, a considerable advantage over oxidized surfaces, but one that has not yet been exploited. SFa might also work and be more convenient to handle. Title has recently informed us that while working in Smith's laboratory he found th a t Bi could be ionized with a W wire treated with sulfur. Hot tungsten wires emit a considerable background of ions, largely potassium, and, if oxidized, sodium. The magnitude of this background varies widely from one roll of wire to the next, and it is possible that the contaminating alkalis are introduced in the processing of the ores which sometimes involves fusion with alkali compounds ( H 5 ) . Flashing the wire or operating i t a t high temperatures for long periods is often effective in reducing the background if it arises from surface contamination, but i t is easily seen that a contamination which is negligible chemically speaking and distributed throughout the metal would be virtually impossible t o remove. It would be especially bad to use any of the so-called "nonsag" wires which have N a or K deliberately added. These backgrounds are often unsteady, being emitted in bursts, so that anyone contemplating an experiment with Ca4I, for instance, would be well advised to consult with the wire manufacturers. When the tungsten wire is oxidized, other

MOLECULAR BEAMS

45

unidentified background ions appear (DQ). The problem is not too serious when a mass spectrometer is used, except for unfortunate coincidences (e.g., K41-Ca41),since isotopes which cannot be made abundant in the beam are not likely to be abundant in the hot wire background. Preliminary experiments by Zacharias and Weiss (26) indicate that a Pt wire operated at the low temperature of 700" K detects Cs efficiently and emits a smaller and steadier K background than a W wire. b. Surface Ionizers (Negative I o n s ) . Neutral halogen atoms can attach a n electron a t or near the surface of a hot wire to form negative ions. Corresponding t o Eq. (ll),the ratio n-/n of the number of negative ions to the number of neutral atoms leaving the hot wire per second is

where A is the electron affinity of the atom and C is a constant. Observation of these ion currents has been used to determine electron affinities ( M S , M.4, Y1) and to detect atomic and molecular beams of C1, Br, and I (D5, KS, J 5 ) . Jaccarino and King have also detected F. Halogen molecules evidently dissociate a t the wire and no molecular ions are observed. Cesium halides, and presumably other alkali halides, are also dissociated to give negative ions (7'3). Table IV gives pertinent data. I n contrast t o the case of positive ion formation what is needed here is a surface of low work function which must be operated a t a rather high temperature t o prevent the formation of surface layers of halogens which presumably increases the work function of the surface. Ba-SrO surfaces have been found unsatisfactory, possibly because of their poor resistance to poisoning (83); thoriated tungsten is very satisfactory except for C1, where the background is often disproportionately large and for which pure tungsten is not too inefficient. The experimental results with thoriated tungsten do not agree with the predictions of Eq. (12) , the yield of negative ions being generally lower than expected, perhaps because of unrealistic assumptions for the value of 4; conjectures concerning this and other possible sources of the discrepancy are advanced in T S and 83. The relations between wire temperature and speed of response, ionizing efficiency, and background output are essentially similar for positive and negative ions. Chlorine, bromine, and iodine appear in the ion background, chlorine being by far the most prominent, and the background is not greatly reduced by flashing or prolonged heating of the wire. Electrons are also emitted, of course, the best ionizing efficiency being obtained when the electron current density is approximately 5 ma/cm2 for thoriated tungsten, and perhaps 100 times smaller for pure tungsten. It is found that the electron emission is reduced as much as

46

JOHN G. KING AND JERROLD R. ZACHARIAS

10% when a halogen beam of typical intensity falls on the wire. This suggests t ha t the response of such a wire may be far from linear, especially a t high beam intensity and low wire temperatures, but no data are available on this effect. The electrons emitted in such relatively copious amounts must of course be deflected away from the current measuring equipment with a mass spectrometer, which can be very crude if isotope selection is not necessary. c. Construction and Operation of Surface Ionizers. The simplest form of ionizing detector, as used to detect beams of molecules or atoms forming positive ions, consists of a tungsten wire mounted under spring tension between a pair of terminals insulated from ground, a n insulated ion collector surrounding the wire, and a grounded electrostatic shield, which is often cooled by a liquid-air trap. Slits are provided in the collector and shield t o allow the beam to impinge on the wire and to enable the operator t o see the wire for temperature measurements and rough alignment. The beam entrance slit should be wide enough so that the diameter of the wire is the effective width of the detector but not so wide as to allow ions t o escape; a knitted wire mesh of high transmission might be used over the slit t o collect ions that would otherwise be lost. To minimize background from the wire, the length of both wire and collector electrode should not be much greater than the beam height, and a small advantage has been gained by using a semicircular cylindrical collector th a t does not receive background ions from the side of the wire not exposed to the beam. Material and construction details are not critical, but since the collector can become quite hot, some precautions to reduce outgassing or even vaporization are in order; nickel or stainless steel with spotwelded leads is suitable. Any refractory high-grade insulating material can be used for supports, and the small stand-off insulators used in radio equipment, though suspect on account of their porosity, are convenient. The heating current may be derived from storage batteries with switchable fixed resistors and a rheostat for coarse and fine current adjustment. Rectifier supplies or even ac may be used, but in all cases suitable by-passing and shielding should be applied t o keep noise and hum out of the electrometer. A switch for momentarily increasing the hot-wire current to flash the wire should be provided. Many workers use the extensive data on the properties of tungsten filaments th a t have been compiled (JB) to estimate the temperature of the wire in terms of the heating current. The center of the hot wire is maintained a t a small positive potential (10-45 volts) with respect to ground by a battery or other low-impedance power supply connected to a tapped resistor across the heater current supply. The ions are thus accelerated towards the collector, which is

MOLECULAR BEAMS

47

connected t o the electrometer grid and held near ground potential by a high nonpolarizing resistance. For usual input circuit capacitances of the order of 20 ppf, 101o-lO1l ohms are customary. Various types of surface ionizer have been devised for use with mass spectrometers. A simple design which has been extensively used a t M.I.T. is shown in Fig. 19. Neutral atoms arrive and ions leave through the wire mesh-covered opening in the grounded flat front plate. Regular slits have been used as well, sometimes a single one for both atoms and ions and sometimes two, but the alignment problems are then considerably greater, especially if maximum mass spectrometer resolution is required ; obviously, the ions emerge normal to the front plate, whereas the atoms can

0

FRONT PLATE

FIG.19. Surface ionizer for use with mass spectrometer. The front plate has a n opening covered with wire mesh through which the atoms enter and the ions leave. The middle view shows the ribbon clamped in its groove under spring tension, and the right-hand view shows the mounting of the ribbon carrying plate on insulated supports and the current leads.

enter a t any angle. The hot wire, now really a ribbon, lies in a groove cut in the rear plate which is held parallel to the front plate by insulated spacers. The ribbon is either spot-welded or clamped to insulated terminals through which the heating current passes and is held taut by a spring. A fine hypodermic needle aimed along the ribbon and connected by tubing to an external reservoir and needle valve has been found a convenient way of introducing oxygen when an oxidized wire must be used. An adjustable accelerating potential difference is maintained between the front and rear plates, so that the ions emerge from the ionizer homogeneous in energy to within the thermal energy with which they were emitted from the hot ribbon and in a beam which, on account of the uniformity of t,he electric field between the plates, diverges only because of the components of thermal velocity perpendicular to the field. The ion

48

JOHN G . KING AND JERROLD R. ZACHARIAS

beam is then deflected by a magnetic field so that only ions of the desired e / M reach a current measuring device. The good geometry of this ionizer has made high mass-spectrometer resolution readily attainable, ordinarily with negligible loss of ions. When the beam height exceeds a reasonable mass spectrometer magnet gap width, it is possible to bring the ions t o a focus a t the center of the gap by using a curved front plate on the ionizer. I n this way Davis (D9)was able t o gain a factor of 3 in intensity with some sacrifice in resolution. Braunstein and Trischka (B7) used a modified form of a n ionizer described by Thorp ( T 4) ,in which the ion beam emerges normal to the molecular beam and an electrostatic lens is used to produce a parallel beam of ions; 60% of the ions formed a t the hot wire arrive a t the collector of the mass spectrometer. Stable noise-free voltage supplies must be used when high resolution is sought. This presents little difficulty except for negative ions, with which low-impedance voltage supplies must be used so that fluctuations in the large accompanying electron current do not change the accelerating voltage sufficiently t o defocus the mass-spectrometer. A more serious problem is the defocusing of the ion beam by charged u p dielectric surfaces, which persists after all visible dielectrics, insulating supports, etc., have been hidden by shields. Heating the ionizer box t o approximately 150" C and enclosing the ion beam in a heated tube of appropriate shape eliminates the effect, presumably by preventing the condensation of diffusion pump oil. If the heat is turned off, the resolution sometimes drops by a factor of 10 and various slow time constant effects are noted, with either sign of ion. The large electron current accompanying negative ions also forms brown dielectric deposits wherever the electrons strike a metal surface. A small permanent magnet mounted right behind the ribbon deflects the electrons and their deposits to a harmless region remote from the ion beam and eliminates the need for frequent cleaning. Substituting mercury or ion pumps for oil pumps and removing other sources of oil and grease vapors is the logical solution t o these problems. Even when all dielectric effects have been minimized, however, the electron beam, now deflected t o one side, increases the divergence of the negative ion beam so that it has so far been impossible to achieve th e same mass-spectrometer resolution that was had with positive ions. d. Universal Ionizers. The limited applicability of the surface ionizer detector had led t o a number of attempts t o construct a truly universal detector, but no notable success was achieved until the independent work of Wessel and Lew ( W I ) and Fricke ( F S ) in applying the mass-spectrometer source developed by Heil ( H 7 ) and Paul (P6)(see also B5 for this and other types of ionizers). Figure 20 shows the ionizer constructed by

MOLECULAR B E A M S

49

Wessel a n d Lew and the following description is t a k e n from their paper ( W l ): Ionization of the atoms in the beam is accomplished by means of electrons oscillating back and forth transversely across the beam. The electrons are guided in their oscillations by a magnetic field of a few hundred gauss directed parallel to the oscillations. An exploded view of the device is shown in Fig. 2 [our Fig. 201. The permanent magnet which supplies the guide field is not shown. The ionizer assembly is mounted on the end of a tubular liquid air trap which extends into

FIG.20. Universal ionizer, as used by Wessel and Lew.

the vacuum envelope of the apparatus and the seal is made vacuum tight by means of Apiezon “ Q ” wax. All electrical leads go through the plate. Ionization of the atoms takes place in the chamber through which the atomic beam passes. This chamber is a rectangular box which is open a t both ends and which has a vertical slit in each of the broad faces. Tungsten filaments, measuring 0.005 x 0.038 cm in cross section, are mounted opposite these slits. One of these filaments b is shown stretched between two stainless steel clamps c. The lower of these two clamps can slide up and down in a groove. A coil spring d attached to the lower clamp keeps the tungsten filament taut. All these parts are mounted on a fired lava piece. Electrons emitted by the heated tungsten filaments are accelerated into the chamber a to an energy of from 30 to 100 ev. The total current is usually between 3 and 15 ma. The ions that are formed are pulled out of the ionization region by an electrode e and further accelerated by electrodes g and h. The potential of e

50

JOHN G . KING A N D JERROLD R . ZACHARIAS

relative to the chamber a is from 0 to 20 volts. The potential of the chamber a relative to h, which is necessarily at ground potential, depends on the mass of the ion and the strength of the magnetic field in the mass spectrometer magnet. For the silver isotopes it is usually around 1300 v. The electrode g is at some intermediate potential adjusted experimentally for maximum ion intensity. The exit slit in h as well as the entrance slit in m measures 0.038 X 1.27 cm. The electrostatic deflecting plates i permit the ion beam to be directed more accurately a t the mass spectrometer magnet. The copper tube k is merely a shield between the ions and the stray fields of lead-in wires. Located between the electrodes e and g and at the same potential as e is a tungsten grid f which can be heated electrically. Its purpose is to serve as a surface ionization detector for the alkalis. Although it intercepts only about 8 % of the incident beam, its efficiency of ionization for alkali atoms which do strike it is so high (substantially 100%) that the net efficiency for alkalis is much higher than that of the electron bombardment ionizer. In fact, on ionizing a beam of potassium atoms first with the tungsten grid and then with electron bombardment ionizer and assuming that the tungsten grid is 100 % efficient, we have estimated that the electron bombardment ionizer ionizes 1 out of every 3000 K39 atoms. In the present experiment, the tungsten grid is used mainly for the detection of the Cs atoms used in the calibration of the C field. The entire ionizer assembly is easily demountable for such repairs as the replacement of the filaments. In operation the pole pieces of the external magnet supplying the guide field are adjusted for the best signal-to-noise ratio.

+

Fricke’s ionizer is essentially similar except t h a t t h e filaments parallel t h e atomic beam so t h a t t h e atoms spend a longer time in the ionizing region. Fewer details are given t h a n were given i n W1, b u t it is reported that 1 out of 20 of t h e beam atoms is ionized a n d t h a t t h e background fluctuations (at mass 106) with a n amplifier bandwidth of 0.1 cps correspond t o a beam density of lo6 atoms/cm3 a t t h e detector, or a beamcurrent density of roughly 3 X lo9 atoms/sec/cm2. e. Comments on Ionizing Detectors. If fi is t h e average particle current from a n ionizer and Sf is t h e bandwidth of t h e current measuring device, t h e rms shot noise current ns will be

n, = (2fi&f)55

(13)

and the importance of minimizing t h e background ion output is obvious if low-intensity beams are t o be detected. Fluctuations in emission, due among other things t o fluctuations in t h e vacuum, will give rise t o a type of flicker noise. The neutral atom beam will also carry shot noise. There is not enough reliable information concerning t h e spectrum, origin, a n d methods of reducing t h e noise output from ionizing detectors. Further investigation of surface ionizer detectors will probably prove fruitful, b u t except for trying very pure wires a t high temperatures in

MOLECULAR BEAMS

51

good vacuum, or fluorinated wires, it is difficult to know how t o proceed. Similarly, improved vacuum may reduce the troublesome background from the universal ionizers, and, if satisfactory mass-spectrometer resolution with good efficiency can be obtained, one of the thorny problems of the molecular beam technique may finally be solved. A good idea of superior design and construction methods for ionizers and allied devices can be had from Hagstrum’s paper ( H 8 ) . See also KWI . 5 . Mass Spectrometers

Since the surface ionizer produces a ribbon-shaped beam of small divergence and of nearly monoenergetic ions, only the mass spectrometer magnet need be considered here. For relatively high resolution work with beams of mixed isotopes of a single element, conventional 60-deg types (B5, B6, 11) have been used, usually with first-order focusing and symmetrical placement of ion source and collector. The magnet is usually entirely within the apparatus and its U-shaped core is excited by a winding of insulated water-cooled copper tubing. A particular magnet made of Armco iron has a 6-in. radius ion path, 0.5%. gap, and a winding giving 33 gauss/amp. The ions pass through heated tubes to reduce electrostatic effects, and the source and collector slits are 0.010 and 0.015 in., respectively. Figure 21 shows some mass-spectrometer curves for cesium isotopes (S1). A similar 60-deg mass spectrometer with a n ion path radius of 8 cm and source and collector slits 0.015 in. wide but with the curved front plate previously mentioned gave resolution such th a t only 1 in lo4 ions a t the NaZ2position was due t o NaZ3(D9). Recently, mass spectrometers of low resolution have been used to eliminate troublesome background. A design used by Daly ( D 4 ) and shown in Fig. 22 has an enclosed cylindrical return yoke to reduce external stray field, two Alnico V rods 136 in. in diameter by 2% in. long, magnetizing coils, and circular pole pieces with a %-in. gap. With 18,000 amp-turns applied briefly, a permanent field of 4800 gauss is obtained in the gap. Daly reports a mass resolution M I A M =: 35 for gallium. Zacharias and Weiss, in working with a cesium beam of 4-in. height, have passed the ions through a sector field edgewise, that is, so that the magnetic field is parallel to the width of the beam, and have obtained sufficient resolution to eliminate most of the potassium background. For some of these low-resolution applications, rf mass spectrometers might be convenient, but they have not as yet been applied to atomic-beam work. A double mass spectrometer can be used a t the detector end of the apparatus, say, for the purpose of improving the abundance resolution for K40 (present naturally to one part in lo4),since the resolutions of the two mass spectrometers will multiply. If the resolution (by which we

52 52

JOHN G. G. KING KING AND AND JERROLD JERROLD R. R. ZACHARIAS ZACHARIAS JOHN

133

loo 7

r .aw

,

JW

o w

MASS SPECTROMETER-VOLTS

FIG.21. Mass-spectrometer curves for cesium isotopes. This is a composite curve, curve, since all three radioactive isotopes were not present in the beam at one time.

FIG.22. 22. Low-resolution Low-resolution mass mass spectrometer, spectrometer, for for background background reduction. reduction. FIG.

MOLECULAR BEAMS

53

mean the fractional number of atoms of the abundant isotopes which stray over into the position for a rare one) for each one is lop4, then the resolution for the two in tandem will be Hahn ( H 1 1 ) has used two simple 60-deg mass spectrometers in this way and found the background of K39a t the K40 position to be less than one in lo7 (Fig. 23). Double mass spectrometry has also been used recently by Zacharias and Weiss in connection with a detector having a large area (4 X 34 in.). For heavy

FIG.23. Intensity of double mass-spectrometer beam as a function of mass number.

molecules, like cesium, a time-of-flight mass spectrometer is rather simple t o superimpose on a magnetic spectrometer. A pulsed voltage is used to accelerate the ions from the hot-wire detector, and the appropriate ions are observed after the proper time a t the electron multiplier. Thus, it is possible t o obtain excellent mass resolution despite rather coarse geometry. Another method of using double mass spectroscopy is t o use one spectrometer a t the source and another a t the detector. Naturally, if the spectrometer a t the source involves selection of ions rather than neutral particles, it will be necessary to neutralize them before sending them into the beam. This neutralizing can be accomplished b y passing

54

JOHN G. KING AND JERROLD R. ZACHARIAS

the ions through a gassy region or by allowing them t o neutralize on a solid surface with subsequent re-evaporation. This procedure may sound extraordinarily clumsy, but there are cases where It is very attractive to obtain the necessary signal-to-noise ratio. 6. The Measurement of Small Currents

I n a n experiment with a chlorine beam 100 cm long, the considerations previously discussed indicate that a typical figure for the number of atoms reaching the detector per second would be 10l2. If the surface ionizer produces one ion for every lo4atoms and a transition between two 1) = 16 states (1 = J = 95) is t o be observed, of the (21 l ) ( 2 J currents of the order of 5 X 10-l2 amp must be measured. This is readily done with an electrometer tube and galvanometer. On the other hand, Davis in his experiment with 2.6-yr Na22(D3, D9) had to observe currents of the order of lo-” amp, for which only electron multipliers are satisfactory. Since the sensitivity of an electrometer is limited by the thermal noise voltage in its grid resistor,

+

+

el = &iZEFf (14) which for a 10-sec time constant and a grid resistor of 10” ohms corresponds t o a current of ca 10-l6 amp, it is seen th a t for currents less than amp the use of multipliers is mandatory if inconveniently long time constants are to be avoided. a. Electrometers. The G.E. FP54 (GL5740) electrometer tube has been extensively used in circuits which balance out variations in the storage-battery voltages (D10,P4, S4, AB). Since it has a n output impedance of about los ohms and the sensitive galvanometers ordinarily used (for instance, Leeds and Northrup type R, resistance 516 ohms, period 7 sec, sensitivity a t 1 m 0.00041 pa/mm, C.D.R.X. 9000 ohms) requires a critical damping resistor of ca lo4 ohms, it is feasible to obtain an extra gain of 100 by using a simple triode amplifier with a gain of 10 and a n output impedance of lo4 ohms (K3).An R-C equalizer may then be used t o shorten the effective time constant with some sacrifice of gain. A homemade T-pad with a factor-of-3 attenuator operative on all ranges is used instead of an Ayrton shunt, which requires a high circuit impedance; the factor-of-3 attenuator is found remarkably convenient. With this arrangement and the galvanometer used, the most sensitive range corresponds t o 5 X 10-l8 amp/mm and is, in agreement with Eq. (14), quite unusable. Without the amplifier, however, the electrometer was too steady. Other electrometer tubes, in particular the “split FP54” GL5674 ( U S ) or a vibrating reed electrometer (P?‘)might be advantageous, and for lower sensitivities, receiving tubes such as the 954 or 959 operated as

MOLECULAR BEAMS

55

space-charge tetrodes with low electrode voltages are satisfactory ( N 1 ) . Automobile storage batteries continuously charged a t a low rate (150 ma) are satisfactory for these low-voltage circuits. A bad cell, as revealed by a high-current cell-tester, introduces much noise. Heavy-duty B batteries with power supplies th at balance out the drain from them may be used to operate the amplifiers. Careful shielding and bypassing of all circuits is important, especially when high-level rf equipment is used nearby. Attention t o grounding is also vital, particularly a s the number of cables between the apparatus and control desk grows. The electrometer tube is usually placed close to the collector, ordinarily a shielded Faraday cup, and mounted to minimize vibration and influence from stray magnetic fields. Operating the tube in an evacuated container or actually placing its grid end in the apparatus by using an O-ring seal around the tubular bulb is advisable, but arrangements t o get a t both ends of the grid resistor for calibration purposes should be provided. Tube sockets, cable connectors, rheostats, t a p switches, and meters should all be investigated as sources of noise. I n spite of the long time constant of the usual galvanometer, 60-cps voltages should be carefully excluded from the circuits, since they can readily be demodulated and give spurious indications, particularly if the ion beam is modulated a t 60 cps for any reason. b. Electron Multipliers.Electron multipliers have been used t o detect ions from atomic beam detectors in two forms: one is a copy of the RCA 931A .photomultiplier with 4% beryllium-96 % ' copper alloy plates (see Fig. 24), and the other is built of the same material according to the description of Allen (42). Although the 931A type was used effectively (Ld, DS,Dg),the limited number (9) of stages of multiplication th a t could be used, the relative difficulty of assembly, and the difficulty of eliminating leakage and corona in such a small device led to the adoption of the larger Allen type. With 10 to 16 stages and a secondary emission ratio of 3.5, gains of 3 X lo5 to 5 X lo8 can be obtained witjh a background c.ounting rate of less than one count per second. Although variable results have been obtained, comparisons of the multiplier output with the indications of a calibrated electrometer tube for K+ ion currents within the range of both instruments show th at each 4-kev K+ ion striking the first plate of the multiplier produces a t least one electron and th a t the device is therefore able to count individual particles. The 0.007-in. thick Be-Cu multiplier plates, of dimensions given in A2, are cut so that the edges can be bent over for stiffness and to crimp in the 0.025411. nickel mounting wires and are then formed t o the correct shape, all operations being performed with a set of dies of appropriate design. Each plate is then polished with $6 emery cloth, washed in water and alcohol and heated to 700" C in a vacuum of lop6mm Hg for 20 min.

56

JOHN G. KING AND JERROLD R. ZACHARIAS

Other temperatures and times and heating in an oxygen atmosphere have been suggested (Ad, AS) and would doubtless repay adoption, since secondary emission ratios of 10 have been reported following such treatment (see references in AS). Since it is a layer of B e 0 that is thought to be desired, it would seem illogical t o use hydrogen-fired electrodes, but plates treated in this way have also worked well ( E l ) . After the plates have been fired, they are promptly assembled between mica plates provided with appropriate holes for the mounting wires, the voltage-divider PULSE -- - - A - -M- - I

7-

r -------1

T O H M S I

-

L

0 - C AMPLIFIER

,

1

a GALVANOMETER R= lo6 OHMS

ENTRANCE SLIT -PROBE COPPER BAFFLE COOLED BY LIQUID AIR

;

Fz

;

SCHEMATIC

FIG.24. Electron multiplier of 931A type. On the left is shown a view of the multiplier with the front plate, which carriers the ion entrance slit, removed. Beneath it is a cross section showing the plate arrangement, and on the right is a schematic.

resistors are individually attached with small setscrew connectors (for easy removal when the multiplier needs refiring), and the entire assembly is stored under vacuum, since prolonged exposure to air results in a loss of gain. Allen ( A 3 ) reports that the maximum multiplication of a surface decayed from over 6 to 5.3 in 2-hr exposure. Figures 25, 26, and 27 show an assembled multiplier, the plates, and the plate-forming dies. Higher secondary yields and greater resistance to exposure to air may be achieved with Ag-Mg alloy plates (R4, X8) (see also B4 for a wide compilation of secondary emission information). To avoid the need for a number of dies, the venetian-blind and grid design of the E.M.I. Vx 5031

57

MOLECULAR BEAMS

FIG.25. Assembled Allen-type multiplier. Six of the voltage-divider resistors are shown clamped in place above the upper mica mounting plate.

64

SPOT-WELDED COLLECTOR

*I MAKE I DEVELOPED SIZE 1.875 I 0 . 5 0 0

*2

*3

MAKE I0 DEVELOPED S I Z E 1.875 ~ 1 . 2 7 3

MAKE I DEVELOPED S I Z E 1.075 r 1.580

*4 MAKE I DEVELOPED SIZE

1.875

I

1.14

5

MATERIAL 0 0 0 5 THICK BERVLLIVMCOPPER

FIG.26. Plates for a ten-stage Allen-type multiplier.

58

J O H N G. K I N G AND J E R R O L D R. ZACHARIAS

photomultiplier might be adapted, or alternatively a method of mounting that forms the plates to the correct shape can readily be devised. Breaking open a commercial photomultiplier is another solution, and since photosensitivity is undesirable in atomic-beam work, the destruction of its photosensitive cathode would not be disadvantageous. For work with positive ions it has been customary to ground the last plate of the multiplier, thus using the voltage across the multiplier to accelerate the ions and reducing the insulation requirements of the output coupling circuits; care should then be taken that the ions are not deflected t o the first plate a t a point where the secondaries produced cannot reach the second plate and that there are no sharp points t o cause field emission. For

FIG.27. Dies for forming multiplier plates. From left to right: (1) cutting the corners to allow folding, (2) folding over the edges, (3) forming the plate (the front of the die has been removed), (4)crimping-in the mounting wires.

negative ions, the first plate must be made positive by several kilovolts and every precaution must be taken t o prevent leakage or corona noise in the output circuit. Liquid-Nz-cooled baffles with appropriate ion entrance slits have been found t o reduce the background count, prcsumably from ions coming from a nearby diffusion pump (L2). The use of such a baffle may lead to electrostatic troubles, and the better solution is t o place the pump further away. It is advisable t o place the multiplier in a shield, which may be made of mu-metal if stray magnetic fields are present; such fields should not exceed 10 gauss. For reasons that are obscure, small improvements in multiplier gain are sometimes obtainable by judicious external placement of a magnet. High voltages are conveniently derived from rf power supplies thoroughly filtered by large capacitors. If modulation of a transition is used, R-C parallel-?' filters tuned t o the modulation frequency are useful

MOLECULAR BEAMS

59

in the power supply leads. The voltage-divider resistors are chosen t o draw currents that are large compared with the signal current; for 300 v per stage, 10 meg is usual. Note that the output signal current is frequently several microamperes. c. Electronic Equipment. Electron multipliers have been used with AEC type 101 and 501 preamplifiers, pulse amplifiers, and scalers. Their circuits may be found in ES. For high counting rates scale-0f-2~~ scalers or counting-rate meters of conventional design are used. Listening to a mechanical register affords the least fatiguing way of observing increases in low counting rates. Daly (D4) has recently used a negative-feedback parallel-T tuned amplifier (Vb) and phase-sensitive detector t o observe modulated resonances in gallium. This amplifier has a pass-band of 10 cps a t 150 cps, l-meg input impedance, and produces full-scale deflection on a 200-pa output meter for 1-mv peak input, with a noise voltage referred to the input of 50-pv peak. When used with an electron multiplier with a gain of 5 X los, 2 X 10-l6 amp or lo3 ions/sec gives full-scale deflection. Such a n arrangement is convenient for use with a CRO or graphical recorder for automatic data presentation, the value of which in establishing line shapes and making fine alignment of the apparatus possible is well known. A straightforward circuit for producing indexing marks on the record a t each 60, 300, and 3000 cps of the beat note between the oscillator inducing the transition and the laboratory frequency standard is under construction. The transition modulation frequency should be made as high as possible t o avoid low-frequency fluctuations of large amplitude, including conjectured l/f noise from the ionizer. On the other hand, the response time of the detector (in some instances), the need for avoiding side-band effects in the rf inducing the transition, and the fact th a t the velocity spread of the beam atoms smears out the modulation during their flight from the transition region to the detector, all limit the modulation frequency. Thus, Daly found 93 cps suitable for Ga, but Holloway has had to go t o 30 cps with C1. Lower frequencies such as 10 cps have a minor advantage in the ease with which anything can be modulated with relays, including rf with a coaxial relay, without the usual ground and groundloop problems. If the detector were gated in synchronism with the modulation but with adjustable phase, a certain degree of velocity selection could be attained. (See K.22.)

7 . Miscellaneous Detectors A number of detectors which have been used with varying success and some as yet untried ideas for detectors are discussed briefly below. More

60

JOHN G. KING AND JERROLD R. ZACHARIAS

details and references may be found in the books on beams and the various review articles previously mentioned. 1. Lamb and Retherford (L6) have detected metastable hydrogen atoms on a cold tungsten ribbon with a yield of ca go electron per atom. Hughes et al. (H4) have done likewise with metastable helium atoms and obtain a yield of 0.24 electron per atom (see references in H 4 ) . 2. Paul and Wessel (P5) have used a sensitive balance to measure the momentum carried by a beam of Ag atoms and from the mass deposited have determined the mean velocity of the atoms. 3. Wohlwill (W7) has detected beams of organic molecules by measuring the rise in temperature of a thin nickel ribbon due to the heat of condensation. Atomic hydrogen and oxygen could be detected from the effect of their heats of recombination. 4. Fricke and Friedburg (F4) have observed the effect of a n impinging beam of iodine molecules on the conductivity of a thin C u 2 0 film. It has a time constant of 60 see a t 200” C. The search for semiconductor effects should be continued. 5. Kingdon’s “cage” ( K 8 ) has been used occasionally ( E 6 ) ;it consists of a space-charge-limited cylindrical diode whose space charge is disturbed by ions formed from the beam. With better vacuum techniques its lack of stability, the major complaint, might be remedied. 6. Occasionally the sensitivity of hot-wire surface ionizers can be improved by allowing the atoms to collect on a cold wire and then flashing it (R15) or using a retarding potential which is then reversed ( W 6 ) ,the measurement being made ballistically in either case. It is stated in R15 that a multiplication of 20 per minute of deposition is achieved. 7. Following a suggestion of Jaccarino and Bederson, Holloway ( H 9 ) constructed a radioactive deposition detector with a continuous clean copper tape which could be reeled in vacuum from contact with a liquid-N2cooled surface to an end-window counter. Background from the oven might be troublesome, but phosphorous (it was planned to do P32)appeared t o adhere well to the copper. 8. A detector in which an oxygen beam strikes a thoriated tungsten filament and reduces the electron emission, which is compared with th a t from a neighboring part of the wire, was devised by King and tried by Remler (R5).Preliminary results showed an unbalanced electron current of 25 pa when ca 4 x 1013 oxygen molecules per second impinged on the wire (compare L17). 9. Another detector for oxygen has been suggested by Jaccarino in which a n atomic beam of Li atoms is directed a t a pure tungsten wire on which the oxygen beam also impinges. One might expect to observe Li+ due t o oxidation of the wire, but no conclusive tests have been made.

MOLECULAR BEAMS

61

Note, however, that the electron affinity of oxygen [2.2 ev ( M 3 ) ] might make it detectable as a negative ion if a surface of stable low work function under the experimental conditions were used. 10. Following a suggestion of S. C. Brown, King looked for H- ions emitted when H atoms strike a field-emitting wire. Under the conditions of the experiment (none too clean) the yield of H- was less than ion per atom. 11. It would be interesting to see what ions come out of a tungsten ribbon on which beams of various neutral atoms impinge while the wire is simultaneously bombarded with electrons. 12. Although not a beam detection method in the usual sense, recent work by Marton et al. ( M 1 6 ) in which photographs of beams of Cd atoms are made by using Schlieren techniques with a crossed electron beam should be noted.

IV. DEFLECTING AND UNIFORMFIELDS The neutral atoms of a beam are acted on by a force F , = p ( a H / a z ) while in a magnetic field H with a gradient component aH/az, where p is their effective magnetic moment and z is a coordinate perpendicular t o the broad side (or height) of the ribbon-like beam. Direct observation of the deflected beam makes it possible to determine p. The size of the deflection depends on the velocity of the atoms. I n radio-frequency magnetic resonance experiments, two deflecting fields are arranged either so that the deflection produced by the first (A) field is compensated by th a t produced by the second (B) field, and a transition taking place in the intervening uniform (C) field which changes the effective moment of the atom is observed as a decrease in beam intensity a t the detector; or so that the deflection produced by both fields add, and a transition is ohserved as an increase in the detected beam. Ideally, the refocusing conditions are independent of the velocity of the atoms. These techniques are also effective with beams of molecules. Electrical properties of atoms and molecules may also be investigated by analogous methods, which will not be discussed here, in which electric fields are substituted for magnetic fields (H10). 1. DeJEerting Fields

a. General. Stern and Gerlach (S5) used a magnet with a wedgeshaped pole (later opposite one with a rectangular trough) t o obtain their deflecting field (Fig. 28a). The difficulty of measuring the gradient in such a n arrangement led to its eventual replacement by the two-wire field described by Rabi, Kellogg, and Zacharias (R6),in which the beam passes near two wires carrying current in opposite directions (see Fig.

62

JOHN G. KING AND JERROLD R. ZACHARIAS

28b). The field and gradient are given in (R6) as

where I is 0.1 times the current in amperes, 2a is the distance between centers of the wires, and rl and r2 are the distances from the centers of

BEAM BEAM

FIG.28. (a) Upper left: Early deflecting magnet pole piece cross section. (b) Upper right: Two-wire field. (c) Lower left: Cross section of iron pole pieces producing a field equivalent to the two wire field. (d) Lower right: Cross section of a strayless one-turn coaxial deflecting magnet.

the wires t o the point under consideration, all in centimeters. The gradient is most nearly constant over the height of the beam for z = 1.2a, as can be seen from the graph of Eq. (16) in R6;* for z = a and r1 = r 2 , the ratio of gradient to field, a useful parameter, is ( d H / a z ) / H = -l/u. Kusch and Prodell (PW)have recently used two-wire fields within a large uniform field, for which arrangement better uniformity of the gradient over the beam height is achieved by running the beam a t z = 0 . 9 ~ . Though the two-wire field produces a calculable and fairly uniform gradient, its utility is restricted because of the large currents needed for

* Figures

5 and 6 in R6.

MOLECULAR B E A M S

63

high fields and the difficulty of cooling the necessarily small wires or tubes needed for high gradient-to-field ratios. Iron magnets with appropriately shaped pole pieces have been extensively used where higher fields than could be conveniently attained with wires were necessary. Since the magnetic equipotentials of the two-wire field are circles passing through the wire centers, pole faces yielding an equivalent field can readily be milled wit'h cutters, available in many sizes, that cut grooves and beads of semicircular cross section. Figure 28c shows a typical arrangement. The radius of the bead is usually made equal to a and th a t of the groove as small as will accommodate the deflection of the beam. A magnet for deflecting molecules ( p = 1 N.M.) which is described by Kusch ( K 9 ) has Permendur pole pieces (which saturate a t 21 kilogauss) of radii 1.25 mm = a and 1.47 mm, giving a 1-mm gap and ( a H / & ) / H = 8 cm-I. Details of magnets for deflecting atoms ( p = 1 B.M.) are given in a later section. By elementary though tedious calculations, it is possible to arrive a t configurations of semicircular beads and grooves yielding a larger region of uniform gradient in the z direction (or other special property), but although this might be useful in an apparatus with very large deflections, it is not ordinarily worth the trouble, particularly since the beam location is not very critical in most resonance experiments. Departures from ideal conditions are usually apparent in variations in the velocity distribution of refocussed atoms with different effective moments and the fact that transitions th at should not be refocused appear weakly. Besides the two-wire field, four- and six-wire fields and their iron equivalents have recently been used to focus beams of neutral atoms in an effort t o increase the effective solid angle subtended by the detector a t the source. b. Magnet Excitation. Large storage batteries have frequently been used t o supply high currents to magnet windings consisting of relatively few turns of water-cooled copper tubing or of copper bus bar to which water-cooled tubing is soft-soldered. Electrical insulation of the winding is provided by Teflon strips or tubing (perforated for easy evacuation), suitably baked-out Fiberglas sleeving, or mica inserted a t every point where a short circuit might occur. The heavy currents are controlled by rheostats made from water-cooled thin-wall brass or steel tubes equipped with sliders, and small adjustments are made either with tapable Nichrome strips or conventional rheostats. Large commercial dpdt knife switches, manganin shunts, and flat copper busbars either greased and bolted or soldered a t the joints complete the circuit. Surplus submarine storage cells with a 10-hr rating close to 900 amp are an effective though bulky source of power. They may be conveniently charged with a weld-

64

JOHN G. KING AND JERROLD R. ZACHARIAS

ing generator. When these batteries are operated a t 100 amp, they exhibit a reasonably steady drift of 0.1% per hour (after an initial discharge of several hundred ampere-hours) corresponding to the drop in terminal voltage of the batteries on discharge. Infrequent discontinuous jumps of 0.05% of unknown origin have also been observed (81).The effect of the drift, even of the uniform field, is often not serious if the transitions are not very field dependent, since data can be taken over a period of time and the drift corrected for; if necessary, an automatic control supplying current t o a trimming coil or operating a motor-driven rheostat so as t o keep the magnet current constant, as measured with a shunt and potentiometer, may be used ( K 1 0 ) .The field rather than the current could be kept constant if the control were operated by a proton probe or a “flux lock,” that is, a coil through which the flux is kept constant. With large batteries, currents of 1000 amp have been frequently used, but if batteries or the space for them is not available, dc generators may be used; higher resistance windings may then be necessary. Lemonick, Pipkin, and Hamilton (L5) built six-pole magnets, one of which had 40 turns of %-in. o.d., 0.030-in.-wall copper tubing on each of the six poles, each coil being 2880 cm long. A current of 60 amp was found sufficient t o saturate the magnet. The six parts of the winding were in series electrically, giving a total resistance of 0.6 ohm, but they were in parallel for cooling. On the basis of 150 amp in the winding (ca 1 w/cm) and a 50” C rise in water temperature, Lemonick et al. (L5) computed from energy considerations and Poiseulle’s equation that 1.2 gal/min of cooling water a t 540 psi was necessary. The effect of poor heat transfer from copper t o water was estimated by an empirical equation given by McAdams ( M 5 ) , and was found negligible. Another feasible approach when the windings are of simple form is t o enclose them in a vacuum-tight box that can be slipped over the magnet yoke. There is then no difficulty with outgassing of insulation or pumping out of multilayer coils. Lemonick et al. built such a magnet with two 580-turn windings, which dissipated a maximum of 32 w and was adequately cooled by water-cooling of the coil forms. Provision was made to evacuate the boxes t o test for leaks. If it were necessary t o dissipate higher power, i t would be feasible t o space the layers of the winding and circulate cooling oil through it, but the construction of the coil box would have t o be executed with great care. I n this connection i t is recommended that, wherever possible, tubing containing coolant be in a single piece, since accidental overheating or the stress on the windings from magnetic forces may cause leaks a t the joints in the tubing. Because these joints are usually inaccessible, a repair which is neither permanent nor likely

MOLECULAR B E AM S

65

t o improve cooling is usually effected by circulating a thin Glyptal solution through the tubing. It is also possible by carrying the magnetic circuit outside the vacuum envelope of the apparatus t o use coils of high enough resistance t o be operated from electronically regulated supplies, as has been done by Goodman and Wexler ( G I ) . The control of leakage flux and stray fields may become difficult with such arrangements, nor are those in which the beam passes through the magnct gap in a tube of appropriate cross-section nor those in which the pole pieces are in the vacuum and the rest of the yoke outside free from criticism. The design of magnetic circuits and windings for electromagnets is discussed briefly in ( F 5 ) and exhaustively in (B8, R 7 ) , as well as in the literature supplied by manufacturers of magnetic materials (A4).A useful rule of thumb for magnets with gaps of rectangular cross section is

B = -0.495Ni W where B is the induction in the gap in gauss, N is the number of turns, i the current in amperes in the winding, and W is the gap width in inches. Recently, Alnico V permanent magnets have been used in deflecting magnets. Magnetizing windings are provided through which momentary high currents from batteries may be passed, as well as smaller steady currents for fine adjustment. The advantages of these magnets are obvious and the major disadvantage, lack of flexibility and resettability, should be readily overcome by providing each magnet with a Hall-effect probe to monitor the field and a sufficiently flexible control system for the magnetizing current so that the permanent magnet can be moved around its demagnetization characteristic in a controlled fashion. The design of permanent magnets is discussed in F5, B8, and 12. Here the rule of thumb, derived from the demagnetization curve of Alnico V, suggests t h a t the length of the magnet be 18 times the width of the gap. Their relative areas depends, of course, on the leakage flux. Fears t h a t the Alnico V would hurt the vacuum are unfounded. c. Deflecting Magnet Designs. The stray fields from the deflecting magnets can produce sufficient inhomogeneity in the uniform field t o cause broadening and distortion of the line shape but can be reduced within limits by shielding and by separation of the three fields. I n a further effort to reduce the stray field from the obvious deflecting magnet design with a U-shaped yoke, magnets topologically equivalent t o a one-turn winding on a toroidal core with a two-wire field gap have been used. One such magnet, shown in cross-section in Fig. 28d, had a = 0.635 cm, a maximum gap width of 0.157 em, and a length of 66 em and produced

66

JOHN G . KING AND JERROLD R. ZACHARIAS

7 gauss/amp. The single turn was water-cooled. With closely spaced cur-

rent leads, the stray field outside the magnet was negligible, but the narrow gap and high currents needed were marked disadvantages. The deflecting magnet shown cut away in Fig. 29 has a return yoke of rectangular cross section (6 X 7 in.) made of two %-in. hot-rolled steel plates bent into a U-shape and bolted together by means of ears that may also serve for mountings. The Armco magnet iron pole pieces are of standard design, with a = >/4-in. and a gap of % in., except th a t provision for t w o beams has been made. They are clamped by J-bolts against 2 X

/ FIG.29. Deflecting magnet using Alnico V. The windings are for magnetizing the Alnico.

2 X 10-in. ground blocks of Alnico V and are located laterally by brass spacer blocks held by screws passing through the yoke. A ten-turn magnetizing coil bent from 3i X >i-in. copper bar, suitably insulated with Teflon and clamped in position, is provided. The pole pieces and magnets extend t o within lti-in . of the end of the yoke, so th a t a %-inch plate of cold-rolled steel with a hole for the beam can be screwed over the opening to act as a shield. After magnetization with a current of approximately 3000 amp, a stable field of 8000 gauss is obtained a t the bead; 1 in. outside the shield the stray field is of the order of 1 gauss and could easily be reduced. Without the shield, the field a t that point is ca 100 gauss. A number of satisfactory magnets of varying dimensions to suit the application have been built to this design. No attempt has yet been

MOLECULAR B E A M S

67

made to obtain high fields, but with the small gap volumes of atomicbeam magnets, this should not be difficult. A rough computation for the magnet described above shows that 11,000 gauss should be attainable. 2. Uniform Fields

a. General. Suppose a transition with a field dependence of 0.35 mc/ gauss ( F = 4, mF = -1, mF = -2 in Cs) were t o be induced in a uniform field by a n oscillating field of length 1. If the natural linewidth -u/l, where v is the most probable velocity of the atoms in the beam, were to be observed, the field inhomogeneity A H / H over the transition region should be less than v/0.35 X lo6 HI. For a field of lo3 gauss, 1 = 5 cm and u = 3 X lo4 cm/sec, A H / H is 2 X so that if the magnet gap width is 0.25 in., the pole faces must be flat and parallel to 5 pin., neglecting the effects of fringing and magnetic inhomogeneities in the polepiece material. As a matter of fact, it is difficult to reduce A H / H over any reasonable distance to less than lob4, but it is not usual t o demand high precision of field-dependent transitions unless the Ramsey separated oscillating field method (Rg), which places much less stringent demands on field homogeneity, is used. Since experiments are often performed a t low fields the fringing fields of the deflecting magnets and the stray fields from high-current leads should be kept low by appropriate design as previously discussed. Unfortunately, a t low fields the effect of inhomogeneities in magnetization of the pole pieces becomes serious, but could perhaps be remedied by the brute-force method of providing a large number of small individually adjustable trimming fields. Alternatively, a magnet with a very large gap and pole faces may be used, as was done by Prodell and Kusch (P2). It is not known whether repeated grinding and annealing of the pole pieces, special annealing procedures, or the use of special high-purity iron would give better results. Taub and Kusch ( T I ) found that Armco iron was superior t o Permendur where maximum homogeneity was required. Pole pieces made of sintered powdered magnetic material might exhibit some advantage. An alternative is to use two accurately parallel broad copper bars as a one-turn electromagnet. Additional conductors mounted a t the edge of the bars and carrying separately adjustable currents make it poesible t o improve the uniformity of the field over the beam height. Such a magnet 150 cm long designed by Nagle has been used a t M.I.T., but, not having the magnetic shielding provided by an iron magnet, proved so sensitive t o stray fields that it could only be used a t times when the magnetic disturbances from nearby subways were absent. The earth’s field [horizontal intensity, 0.17 gauss; dip, 70 deg in Massachusetts ( H I ) ]is often a satisfactory though rather inflexible source of magnetic field and

68

JOHN G . KING AND JERROLD R. ZACHARIAS

has provided the sole field in some experiments (2%). It may be supplemented by large Helmholtz coils (E5, H I 2 ) . The remarks in the section on magnetic excitation apply here, but with added emphasis on the need for steady currents. I n observing a field dependent transition a t increasingly large fields in an iron magnet, it has been found that narrower linewidths could often be obtained by applying momentarily a large current to the magnet winding than could be obtained by increasing the current continuously. Permanent magnets have not been used t o obtain uniform fields [except by Corngold as mentioned by Ramsey (R2)]because it is felt t h a t such a magnetically hard material as Alnico would exhibit large inhomogeneities. The investigation of the properties of such a magnet might be rewarding, nonetheless. By making the width of the pole faces six times the gap width, the inhomogeneity due t o fringing is sufficiently reduced over the beam height if it does not exceed one-half the gap width. The use of a pole-face profile with ridges a t the edge as in cyclotron magnets might be advantageous (RIO).I n any case, thin steel shims are usually inserted between the pole pieces and the yoke to which they are bolted until the field is sufficiently homogeneous, as determined with a small flip coil and sensitive galvanometer. The absolute calibration of the field can be carried out with a proton probe ( K I 4 ) or by measuring a transition of known field dependence with an auxilliary beam of, for instance, I( or Cs. Flip coils, rotating-coil magnetometers, Hall-effect probes, etc., and the field-current ratio of the magnet are of obvious utility in obtaining varyingly reliable values of the field. b. Uniform Field-Magnet Design. Armco magnet iron is usually used for these magnets, annealed according to the procedure given in A 4 . The need for high precision in maintaining the correct gap spacing has already been indicated, and this is usually achieved by grinding all mating surfaces, using locating pins and providing appropriate nonmagnetic spacers in the gap. If the Ramsey separated oscillating field method is t o be used, trimming coils a t the position of the two oscillating fields should be provided. The following description of a large uniform magnet (see Fig. 30) constructed by Eisinger is taken from E l : The magnet is made of Armco magnetic iron and is copperplated to prevent it

from rusting. It is 26 in. long and the cross section of its rectangular core is 26 by 136 in. I t has a gap of 0.245 in. The main windings consist of twelve turns of 36- by >$in. copper bars. These copper bars are water-cooled by means of >/4-in.copper tubing soldered along grooves which are milled along the copper bars. The magnet has an auxiliary winding of twelve turns of number 12 copper

MOLECULAR BEAMS

69

wire and two trimming fields, one a t each end, \\ hich are 6 in. long and consist of two turns of number 1.2 copper wire each. I n order to insulate the core from the windings and the turns from each other, all surfaces were painted nith several coats of glyptal which had to be baked repeatedly to forestall outgassing inside the vacuum. Sheets of mica were inserted wherever possible. The auxiliary and trimming coils were insulated by glass tubing. I n spite of great care exercised in the machining of the pole pieces, the homogeneity of the C field left much to be desired when the magnet was first assembled. The pole faces are 1-in. thick and are bolted to the core proper by means of seven machine studs. The field in the gap was improved by inserting small strips of iron

FIG.30. Uniform field (C) magnet. A Gin. scale may be seen standing vertically in the foreground.

shim stock between the core and the pole pieces. The homogeneity of the field was measured by moving a small flip coil along the beam direction in small steps and observing the current induced in the flip coil by means of a sensitive galvananieter. A similar technique was used to improve the field in the vertical direction, the flip coil being raised and lowered a t various points along themagnet. Needless to say, this technique is an extremely laborious one, but it finally met with success. The homogeneity attained mas such that the field varies less than 0.05 % over the entire length of the magnet. The current for the main winding is supplied by four U.S. Navy submarine storage batteries connected in series-parallel. The magnet delivers approximately 30 gauss/amp. Stroke (XI) h a s found that the field of this m a g n e t is uniform t o 0.01% over a distance of 1.6 em. In summary, t h e s t a t e of uniform-field magnet design is n o t entirely satisfactory a n d a n ingenious invention is needed.

70

J O H N G. K I N G AND JERROLD R. ZACHARIAS

3. Deflections and Intensity

a. Focusing of Atomic Beams. Although the use of directional sources with apparatus of conventionally small solid angle reduces the waste of beam material by a large factor (e.g., 100) compared with a source with a single slit, increased intensity is obtained by increasing the width of the beam, which is limited by the feasible deflecting power. Magnetic lenses for focusing beams of neutral atoms have been discussed (V3, K l l ) and used (F7, L5) recently as a means of overcoming the intensity difficulties of the atomic-beam method. Essentially, the idea is to produce a magnetic field in which atoms with a given effective magnetic moment are acted on or by a linear radially-directed restoring force, so that regardless of the angle with respect to the axis of the apparatus a t which they emerged from the source (within limits set by the aperture of the magnet, its deflecting power, and the velocity of the atoms), they will return to the axis after executing one half-period of a simple harmonic motion. Such a magnet is therefore a converging lens for some atoms but a diverging lens for atoms with effective magnetic moments of opposite sign. Two magnets and a suitable arrangement of circular stops corresponding to the collimator slits of a conventional apparatus to limit the possible orbits form a n apparatus that focuses onto a detector those atoms which, emerging from the source in a relatively large hollow cone, undergo a transition that changes the sign of their effective moment. Various magnet configurations are discussed in the references given, but Lemonick, Pipkin, and Hamilton (L5,L7) used magnets with six radially disposed alternately north and south poles (Fig. 31), tapered longitudinally t o increase the aperture for the deflected atoms. Detailed design considerations are given in L5. Figure 32 shows the arrangement of stops and magnets used, with the magnets shown as lenses, and the following description is taken from L7: The A magnet acts as a converging lens for atoms having a negative strong field moment; the B magnet acts as a diverging lens for atoms having a positive strong field moment. By arranging the stops as shown schematically in Fig. 1 [our Fig. 321, only atoms which have undergone a transition in the C field resulting in a change in the sign of the strong field moment can reach the detector. Each atom must cross the dotted line twice. Calculation showed that this arrangement of optics enabled us to compensate sufficiently for the chromatic aberration caused by the Maxwellfan distribution, so that we were able to obtain an effective 4n steradian for flopped atoms reaching the detector solid angle of (5 X from the oven and having a kinetic energy between 300 and 1200' K.

Lemonick et al. (L5) point out that a gain of a factor of 50 in solid angle over a conventional apparatus should ultimately be obtainable.

71

MOLECULAR BEAMS

Actually, a gain of 5.6 was obtained, but, as they state, “Since the ratio 5.6 compares the first practical focusing apparatus with a rather idealized conventional apparatus, it is hardly in contradiction t o Eq. (21).” Their Eq. (21) gives the factor of 50 mentioned above.

END VIEW A M A G N E l

FIG.31. Six-pole focusing magnet yoke. The poles marked “long tip” extend further along the axis of the magnet than the others to provide a transition t o the horizontal uniform field that reduces Majorana flop. DETECTOR BUTTON

7

FIG.32. Arrangement of stops and magnetic lenses in a focussing apparatus.

Friedburg (8’7) has used a six-wire field and has simultaneously pulsed the beam and the magnet current; with a suitable drift space the fast atoms arrive a t the magnet first when the field is high and are focused a t the same point as the later slow atoms that arrive when the field is lower. In this way a notable reduction in the chromatic aberration th a t afflicts

72

JOHN G. KING AND JERROLD R. ZACHARIAS

these lenses has been achieved, b u t no resonance experiments have as y e t been reported using this technique. Zacharias is currently working with quadrupole magnets designed t o focus broad beams of slow Cs atoms at low fields where their effective moment varies approximately linearly with field. No data are as yet available. Bennewitz, Paul, and Schlier (BQ)have been successful in using fourpole electric fields t o focus beams of polar molecules, and report a potential gain of 300 in effective solid angle over a conventional apparatus. b. Collision Alignment. Ramsey (R11) has recently suggested t h a t a beam of mole'cules or atoms of spin >31 attenuated b y passage through a monatomic scattering gas should exhibit collision alignment. H e states, In experiments with either atoms or molecules, the existence of collision alignment should be observable in many ways. For example, in molecular-beam magnetic-resonance experiments with molecular hydrogen, separate nuclear resonances occur for different values of M iso the relative intensity of these resonances should be modified by attenuation with a scattering gas. Alternatively, observations of the apparent cross section in successive scattering attenuators should indicate the occurrence of alignment by a diminution in the apparent cross section in the last attenuator in comparison with the first. I n such an experiment, however, care must be taken to distinguish this effect from those corresponding to the changes in molecular velocity distribution by the attenuation. In addition to its value as a tool for the study of collision phenomena and molecular shapes, collision alignment should be useful in some cases as a replacement for the inhomogeneous deflecting fields in molecular- and atomic-beam magnetic-resonance experiments. For example, if an atomic beam passes through two successive attenuation regions, the total attenuation will be less if the atom retains its orientation than if it is reoriented between the two attenuation regions. Consequently, a diminution in transmitted beam intensity indicates the presence of a resonance reorientation of the atom. An atomic-beam magnetic-resonance experiment with collision alignment of the atoms possesses the advantage that much broader and more intense beams can be used than with the conventional molecular-beam resonance method. c. Multiple Beams. Stern ( 8 7 ) suggested t h e use of a deflecting magnet with many grooves (100) in a vertical array (named " Multiplikator"), whereby a gain in intensity could be achieved after magnet gaps had been greatly reduced in obtaining high gradients. I n determining t h e magnetic moment of the water molecule, Knauer a n d Stern ( K I 2 ) mention t h a t one difficulty in the method arose from t h e vertical divergence of t h e beam. This scheme should be re-examined now t h a t directional beams can readily be produced. Note also t h a t there will be no Majorana flop (transitions induced b y the inability of t h e precessing atom t o keep u p

73

MOLECULAR BEAMS

with the changing direction of the field it sees) arising from cross fire as there would have been had the Multiplikator been applied to resonance experiments without directional sources.

V.

RADIO-FREQUENCY

EQUIPMENT

1. Requirements of Radio-Frequency Sources

A transition is to be induced a t low fields between magnetic substates of a given hyperfine level in an atom. The magnitude of the oscillating magnetic field H’ for optimum transition probability should be (R2, SS)

H’

+

=

~

0.6

gFpOt

+

wheregF = gJ[F(F 4- 1) 4- J ( J 1 ) - I ( I 4- 1 ) ] / 2 F ( F 1) is theg-factor for the given hyperfine level, g J is the Lande g-factor for the atom, po = 1.4 Mc/gauss, and t is the time spent by the atom in the oscillating field. For chlorine atoms in the ground state I = J = 34 and, 9.7 = 4/, so g F = 3.5;if t = lop4sec, the magnitude of the oscillating field required is about 6 milligauss, which is typical, within an order of magnitude, of the fields required in atomic experiments. With the rf wires customarily used, currents of the order of 5 ma will produce such fields; the rf power required, of course, depends on the tuning of and losses in the circuit, a few milliwatts usually being adequate. More power is convenient to make it possible t o saturate the transition (overflopping) and to determine empirically the rf level for optimum transition probability. Much more rf power is required to produce oscillating fields of a few gauss corresponding to nuclear and molecular g-factors when working with beams of molecules, and it is often found more practical t o leave the frequency constant and vary the dc transition field than to build relatively high-power tunable oscillators. Other properties of the source of rf besides the power requirement crudely outlined above are that it should emit a narrow spectrum, be readily tunable, and be sufficiently stable so that i t contributes negligibly to the width of the observed transitions. 2. Radio-Frequency Sources and Frequency Measuremen.t *

Commercial signal-generators and oscillators, which are available over a large frequency range, have been extensively used, often with batteries or improved power supplies. Surplus airborne jamming transmitters using

* A review of the techniques of rf generation and measurement (with references) as applied to microwave spectroscopy may be found in the recent book of Townes and Schawlow ( T 5 ) .

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JOHN G. KING AND JERROLD R. ZACHARIAS

various planar triodes are convenient on account of their high output and broad tuning range (e.g., 300-2100 Mc/sec with one change of the feedback coupling device). At frequencies above 1000 Mc a variety of reflex klystrons with outputs of 10-200 mw are available and can be made satisfactorily stable for many purposes by brute-force methods of mechanical, thermal, and electrical isolation. For klystrons not built integrally with a cavity, i t is easy to build rectangular cavities with adjustable plungers or cylindrical cavities made by clamping rings of various diameters between plates connected to the klystron grids; fine tuning adjustments can be effected with tuning screws to supplement the electronic tuning of the klystron. Ordinary precautions in matching [isolating a klystron from a reflecting load with a one-way Faraday-effect element (“Uniline,” Cascade Research Corp., Los Gatos, Calif.) improves its performance (XI)] and using minimum coupling to the oscillator assure adequate performance except when the need for high precision dictates one of the following schemes. Starting from a stable quartz crystal at, say, 5 Mc/sec, one can synthesize the desired frequency with a network of electron-tube and crystal-frequency multipliers, mixing in a variable frequency from a suitable stable interpolation oscillator a t a convenient point. By means of tubes only, frequencies up t o 767 Mc/sec have been synthesized in this way (D4), but higher frequencies could be readily obtained, particularly if little power were needed. Since the stable crystal and many of the multipliers exist as part of the laboratory frequency standard, such systems are convenient, their relative inflexibility being no disadvantage if the transition frequency has already been roughly measured so that searching over a large range is not necessary. The system should be tested to make certain that only one frequency is present in, the output and th a t one of the frequency multipliers does not start oscillating as the interpolation frequency is varied. An extremely stable cavity oscillator shown in Fig. 33 has been devised by Yates and Zacharias (22, Rl2). A Western Electric 416B planar triode is used as a n amplifier t o produce 12-db power gain at 3000 Mc/sec. The lower rectangular cavity is % wavelength long in the waveguide dominant mode, and the upper cylindrical anode cavity forms a gi-wavelength radial line. The tube is located off center in the cathode cavity a t a point that matches its input admittance. Coupling probes in the cavities are connected t o loops in the large T E M Ifeedback cavity by coaxial lines 4/4 wavelengths long. Tuning screws on the cavity can be adjusted to insure oscillation. The feedback cavity length equals its diameter and its measured unloaded Q of 5 X lo4is in reasonable agreement with the Cornputed &. A circular plate a t the bottom of the cavity spaced by a ring can

MOLECULAR BEAMS

75

be distorted by the central clamping screw to provide fine tuning; at the same time this arrangement shifts the frequency of the otherwise degenerate 1ow-Q TMlll mode. Heavy and rigid construction results in negligible microphonic response. Tuning is effected both by changing the tube cathode bias slightly and, as previously mentioned, by adjusting the automatically controlled temperature of the cavity, which is equipped with two windings, one a heater and the other a resistance thermometer. The short-term stability of these oscillators, which have so far been used only PLATE LEAD

MAIN C A V I T Y

FIG.33. Stable cavity oscillator (3000Mc/sec).

with the cesium frequency standard, is 1 part in lo9.I n that application they provide 5-10 mw of power at 3064 Mc/sec, which when fed t o a silicon diode tripler, yields 0.1 mw at 9192 Mc/sec. Numerous circuits have been used to lock klystrons to a stable frequency. Very recently Searle and McRae at M.I.T. have stabilized a 2K41 klystron operating a t 3000 Mc/sec with a harmonic of the multiplied output of a crystal. The scheme, simple in design but sophisticated in execution, uses a 200-Kc/sec i-f amplifier with a bandwidth of 300 Kc/sec and a discriminator to supply a d-c control signal to the repeller to such effect that two of the oscillators can be made to produce a reason-

76

JOHN G. KING A N D JERROLD R . ZACHARIAS

ably stable 1-cps beat, even though they are operated entirely from ac power supplies. We hope t o adopt such stabilized oscillators for every laboratory application requiring high precision ; a t present they appear t o be the solution t o a vexatious problem of long standing which the Yates oscillator, despite its effectiveness, was not, on account of the nuisance of supplying enough cavities for a broad frequency range. Techniques of frequency measurement are well known and only a few matters will be discussed here. Difficulties in receiving WWV on account of noise and the presence of nearby interfering carriers of unknown origin seem t o indicate that future precision measurements be performed relative t o the (4,O ++ 3,O) cesium transition a t 9,192,631,830 f 10 cps ( E 5 ) . An apparatus currently used for other research may be adapted for this purpose. At the same time that precision increases, the experimenter should be increasingly vigilant in guarding against the gross errors of beat identification that can arise with heterodyning systems. Frequent checks should be made with auxilliary independent instruments of less precision, such as coaxial, cavity, or portable heterodyne wavemeters. Lastly, digital counting frequency meters which are most convenient for the measurement of interpolation frequencies up t o 10 Mc/sec should be carefully watched lest they develop counting errors while preserving internal consistency; it is not apparent, however, t o these authors how this might happen, but it is a disturbing suspicion. I n summary, although the amount of electronic elaboration associated with a given experiment reflects to some extent the background of the experimenter, i t is our feeling that the part of molecular-beam research dealing with precision and small effects will increasingly require such elaborate techniques. 3. Radio-Frequency Fields

The oscillating fields a t low frequencies are usually produced by simple loops t o which connections are made by coaxial lines with a center conductor passing through a small Kovar feed-through vacuum seal. Depending on whether it is desired to have Millman effect ( M y ) or not, a hairpin-shaped loop parallel t o the beam and bent up a t its ends t o allow the beam t o pass or a symmetrical current sheet of U cross section may be used (see Fig. 31 in X6). The loop shown in Fig. 34a is designed t o produce oscillating components parallel to the d-c transition field t o induce Am = 0 transitions. It and similar wires have rather unwisely been used a t frequencies as high as lo4 Mc/sec, where the distribution of rf and consequently the line shape become difficult t o predict. Shielding the loop to restrict the rf field, or best of all, if the width of the uniform field magnet gap permits, the use of a waveguide cavity, controls these

MOLECULAR BEAMS

77

effects. Lurio and Prodell (LlW) have used a K-band waveguide tapered to a square cross section (to fit a given magnet gap) whose cut-off frequency was raised by filling it with Teflon (dielectric constant ca 2 a t 2 X lo4 Mc/sec).

(a FIG. 34. (a) Radio-frequency field loop. The loop fits in the C-magnet gap, the holes a t the end being for the beam. An additional small pickup loop is often provided for monitoring purposes. (b) Two-cavity rf field as used in the atomic beam frequency standard. The beam passes through the lower waveguide, which is actually coupled weakly to the cavities by small slots (not as shown) similar to the one visible in the outer wall of one cavity.

A two-cavity rf field used with the Ramsey separated oscillating field method (R9) can be seen in Figs. l a and 34b and is described in Section I,2,b. VI. MISCELLANY I . Beam-Control Devices

Figure 35a shows a much used type of collimator slit designed to be placed in the uniform field magnet gap, whose effective width is adjustable by rotating the shaft on the greased fine (40 per in.) thread providing the vacuum seal. Better seals than this should be used, such as the

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double-0 ring with pump-out mentioned below. Rotating the taper in which the collimator is eccentrically mounted translates it, and a long removable arm attached to the collimator shaft and held fixed reduces interaction between the position and width adjustments. Figure 35b shows a stopping ribbon whose effective width and position are similarly

FIG.35. (a) Collimator slit. (b) Stopping ribbon.

adjustable. The edge of the C-frame is convenient in stopping off deflections on one side or the other of the apparatus center line, It is placed a t the exit of the second deflecting (B) magnet. Figure 35c is a beam-height limiter. The mask with the notch may be rotated t o limit the beam height; a collimator slit is also incorporated. 2 . Construction Details

Molecular-beam apparatus has been built in many ways to satisfy the need for component alignment under vacuum, stable alignment, flexibil-

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ity in feed-through of leads, etc. Recently, two apparatus have been built a t M.I.T., one of which is entirely assembled between two I-beams mounted t o a top plate and inserted in a stainless-steel vacuum envelope. The ease with which it may be opened up for any sort of work compensates largely for the limited number of adjustments that can be made from outside. The other apparatus, which uses a large cast can for a vacuum envelope, has had all its magnets mounted on a tray 6 ft long th a t can be

FIG.35 (c). Beam-height limiter.

easily slid out for alteration and visual alignment of the components. * The detector and source are separately adjustable. Because permanent magnets are used, there are no stiff, heavy current leads of critical placement. We envisage a combination of these features together with careful attention tjo vacuum in our future apparatus. 3. Vacuum

It might seem unnecessary for the present authors t o include any discussion of a technique which has been dealt with so extensively in trea-

* As in any device requiring alignment, kinematic design, or at least an awareness of the constraints present, is important (see 84, E7, W8).

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JOHN G. KING AND JE R R OL D R. ZACHARIAS

tises (D7, R16, A4), journals (EL?),and conferences (C4). Neither of us, however, has yet seen a vacuum system so simple, flexible, and troublefree that a novice could complette a small experiment of the molecularbeam type without wasting a large fraction of his time in obtaining the

FIG.36. Test apparatus vacuum system.

necessary vacuum. It may be useful t o describe the nth but not final attempt a t such a device (see Fig. 36). It may well turn out that the so-called Evapor-ion P u m p ” may so supplement the available devices that the present discussion will be obsolete by the time it appears in print.

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1. The test space, shown on the right, consists of a 10-in. diam stainless-steel tube (made of rolled $is-in. sheet, seam-welded). The diameter was chosen to be somewhat larger than 9 x -in . diam stainless steel kitchen bowls, which serve to close the top and bottom. They are soft-soldered in place in such a way that gravity aids the soldering process. The sketch shows how t'his is accomplished but hides the fact that the seal for the bottom bowl should be just enough different so that gravity aids the

0

Id--_il OlFF PUMP

FIG.37. Six-inch vacuum valve.

soldering process there also. It is probable that metal O-rings pressed between heavy metal flanges would be as good. The Medusa-like Kovar seal-throughs in the bottom are chosen as being relatively rugged and capable of carrying considerable current or a reasonably high voltage. 2. The large valve shown in detail in Fig. 36 and the one shown in Fig. 37 are included here for the simple reason that valves of this size are easy to make but expensive and heavy when bought commercially. Their value in isolating different parts of the system for leak accumulation tests and alterations makes providing them well worth the trouble. The only feature t o be observed carefully is the double-chevron ring seal which allows the long motion necessary for complete opening of the valve. I n

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a double seal of this sort, it is to be noticed that the fore-pressure of less than a thousandth of atmosphere keeps such a seal operable at all pressures up t o 10-8 mm Hg or better. Teflon is so far the only material which needs no lubricant and will stand some outgassing. 3. The ionization gauge shown is one of an experimental series which have large connecting tubes to the vacuum to be measured and a thin wire ion collector plate. Ionization gauges of the Bayard-Alpert type have the remarkable property that they require only a short outgasshg time and read well t o very low pressures. Some laboratories have ionization-gauge circuits which were built for older types of tubes, and it is reasonable t o make a gauge which has the properties of the Bayard-Alpert type but which fits the old circuits. 4. The trapping system consists of a 2-1 liquid air trap which is surrounded by a space which holds dry ice. A layer of insulation (not shown) protects the dry-ice container, which in turn refrigerates the baffle shown a bit too low in the sketch. This combination really keeps the oil out of the test system and provides a fair pumping speed, since the basic connecting tube diameter is 6 in. even though the pump diameter is only 4 in. The liquid air-dry ice combination has the advantage that it holds the liquid air trapping for a t least 60 hr which covers the modern long weekend from Friday to Monday. 5. The pump system is obvious except that it is quite inexpensive t o overdesign the diffusion booster to permit a reasonably sized storage volume for the fore-system. The mechanical pump is held on a simple cradle so that it can be easily lifted to permit pushing the whole system around and allowed t o rest on the floor when in operation to prevent shaking the apparatus. 6. The storage system in the fore-line must use as good a vacuum technique as the main low-pressure system if it is to be used for long-time running without the mechanical pump. The only rubber tubing therefore appears beyond the double valve. The reason for making the storage system in the foreline small is that it pumps out more quickly; otherwise, larger ones work better. 7. A useful modification of this system is to run a fore pump lead with a valve between the test can and the space between the two foreline valves. They are not shown on this apparatus, because it was built to experiment with devices in the pressure region between 10-9 and mm Hg. 8. Materials for vacuum vessels and for construction of the devices wholly within the container can be of a much wider variety than is generally supposed, provided there are no internal surfaces which contaminate or poison permanently as do activated cathodes. Stainless steel has

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many fine properties for the outer shell, which arise chiefly from the fact that its strength holds up after the heating required for Heliarc welding and soft soldering. The poor thermal conductivity of thin stainless steel is of considerable advantage in such joining operations. The wall thickness does not have t o be great, as is evidenced by the use of kitchen bowls of Sfi-in. diam. The chief restrictions on the choice of materials for internal parts is that they be able to stand heating t o about 100” C during a n outgassing. Naturally, for permanently sealed-off tubes, the outgassing temperatures are much higher. The efficacy of this outgassing a t low temperature is evidenced by the following observations. We have a t present a large vacuum envelope which maintains a pressure of about 3 X lops mm Hg. It contains, among other things, large iron magnets and their copper windings, Teflon insulation (about 40 ftz), enamel-covered wire, commercial resistors for the electron-multiplier voltage divider, soldering flux, solder, pipe ashes, Cambridge dust, and other more usual vacuum materials. Within this vacuum system, there is a section which is isolated from it by a multiplicity of long canals. This internal region is separately trapped with liquid air and has reached a pressure of 5 X mm Hg. These low pressures are measured with ionization gauges without any tubulation, so that the natural pumping of a n ion gauge does not falsify the readings. The excellence of the vacuum attained, despite the rather casual use of materials, can be attributed to the low-temperature outgassing and t o the presence of cesium vapor condensed on the walls or on the liquid air traps. The behavior of surfaces in a n evacuated vessel still remains a mystery to the present authors.

ACKNOWLEDGMENT Our sincere thanks are due to Mr. Frank O’Brien for his assistance in preparing the figures. REFERENCES

A l . Amdur, I., and Pearlman, H., J. Chem. Phys. 8,7 (1940); 9,503 (1941); 11, 157 (1943). Ab. Allen, J. S., Rev. Sci. Instr. 18,739 (1947). AS. Allen, J. S., Proc. Inst. Radio Engrs. 38, 346 (1950). A4. American Rolling Mill Co., Armco Magnetic Ingot Iron. Middletown, Ohio, 1945. A6. American Chemical Society, “Physical Properties of Chemical Compounds.” Washington, D.C., 1955. A6. Armistead, F. C., Rev. Sci. Instr. 20, 747 (1949). B1. Bessey, W. H., and Simpson, 0. C., Chem. Revs. 30,239 (1942). BZ. Brix, P., Eisinger, J. T., Lew, H., and Wessel, G., Phys. Rev. 92, 647 (1953). BS. Bellamy, E. H., and Smith, K., Phil. Mag. [7] 44, 33 (1953).

84

JOHN G . KING AND JERROLD R . ZACHARIAS

B4. Bruining, H., “Physics and Application of Secondary Electron Emission.” McGraw-Hill, New York, 1954. B5. Barnard, G. P., “Modern Mass Spectrometry.” Institute of Physics, London, 1953. B6‘. Bainbridge, K. T., i n “Experimental Nuclear Physics” (E. SegrB, ed.), Vol. 1, Part V, p. 559. Wiley, New York, 1953. B7. Braunstein, R., and Trischka, J. W., Phys. Rev. 98, 1092 (1955). B8. Bozorth, R. M., “Ferromagnetism.” Van Nostrand, New York, 1951. B9. Bennewitz, H. G., Paul, W., and Schlier, C., Z . Physik 141, 6 (1955). B10. Berman, A., Kusch, P., and Mann, A. K., Phys. Rev. 77, 140 (1950). B11. Brody, S. B., Nierenberg, W. A., and Ramsey, N. F., Phys. Rev. 72,258 (1947). C1. Clausing, P., Physica 9, 65 (1929) CB. Clausing, P., 2. Physik 66,471 (1930). CS. Christensen, R. L., Hamilton, D. R., Lemonick, A., Pipkin, F. M., Reynolds, J. B., and Stroke, H. H., to be published in Phys. Rev. C4. Committee on Vacuum Techniques, 1954 Vacuum Symposium Transactions, Boston (1955). C5. Cohen, V. W., Phys. Rev. 46, 713 (1934). C6. Cohen, V. W., Phys. Rev. 96, 569 (1954). D l . Ditchburn, R. W., and Gilmour, J. C., Revs. Mod. Phys. 13, 310 (1941). Dd. DeMent, J., and Dake, H. C., “Rarer Metals.” Chemical Publishing, New York, 1946. DS. Davis, L., Nagle, D. E., and Zacharias, J. R., Phys. Rev. 76, 1068 (1949). 0 4 . Daly, R. T., Ph.D. Thesis, Physics Dept., M.I.T., Cambridge, 1954 (unpublished). 0 5 . Davis, L., Feld, B. T., Zabel, C. W., and Zacharias, J. R., Phys. Rev. 76, 1076 (1949). D6. Davis, L., private communication (1953). D7. Dushman, S., “Vacuum Technique.” Wiley, New York, 1949. D8. Dunoyer, L., Compt. rend. 162,594 (1911). D9. Davis, L., Jr., Mass. Inst. Technol. Research Lab. Electronics Tech. Rept. No. 88 (1948) (unpublished). DlO. Dubridge, L. A,, and Brown, H., Rev. Sci. Instr. 4, 532 (1933). E l . Eisinger, J. T., and Bederson, B., Mass. Inst. Technol. Research Lab. Electronics Tech. Rept. No. 212 (1952) (unpublished). EB. Estermann, I., Revs. Mod. Phys. 18,300 (1946). ES. Elmore, W. C., and Sands, M., Electronics: Experimental Techniques. McGrawHill, New York, 1949. E4. Estermann, I., Foner, S. N., and Stern, O., Phys. Rev. 71, 250 (1947). E5. Essen, L., and Parry, J. V. L., Nature 176, 280 (1955). E6. Estermann, I., and Stern, O., 2. Physik 86, 135 (1933). E7. Elliott, A., and Dickson, J. H., “Laboratory Instruments.” Chemical Publishing, New York, 1953. E8. Estermann, I., Simpson, 0. C., and Stern, O., Phys. Rev. 62,535 (1937). E9. Estermann, I., Simpson, 0. C., and Stern, O., Phys. Rev. 62, 535 (1937). E10. Ellett, A,, Olson, H. F., and Zahl, H. A.,Phys. Rev. 34, 493 (1929). E l l . Estermann, I., Simpson, 0. C., and Stern, O., Phys. Rev. 71, 238 (1947). El2. Estermann, I., and Fraser, R. G. J., J. Chem. Phys. 1, 390 (1933). F 1 . Fraser, R. G. J., “Molecular Rays.” Cambridge U.P., London, 1931. FZ. Farkas, A., and Melville, H. W., “Experimental Methods in Gas Reactions.” Macmillan, London, 1939.

MOLECULAR BEAMS

85

FS. Fricke, G., 2. Physik 141,166 (1955). F4. Fricke, G., and Friedburg, H., 2. Physik 141, 171 (1955). F5. Fretter, W. B., “Introduction to Experimental Physics.” Prentice-Hall, New York, 1954. F6. Friedburg, H., and Paul, W., Nuturwissenschuftm 38, 159 (1951). F7. Friedburg, H., 2. Physik 130, 493 (1951). F8. Fraser, R. G. J., “Molecular Beams.” Methuen, London, 1937. F9. Fox, M., and Rabi, I. I., Phys. Rev. 48, 746 (1935). F10. Fraser, R. G. J., and Jewitt, T. N., Phys. Rev. 60, 1091 (1936). G1. Goodman, L. S., and Wexler, S., Phys. Rev. 99, 192 (1955). G2. Guthrie, A. N., Phys. Rev. 49, 168 (1936). GS. Gordon, J. P., Zeiger, H., and Townes, C. H., Phys. Rev. 99, 1264 (1955). G I . Gerlach, W., Ann. Physik [4] 76, 163 (1925). G5. Gerlach, W., J . phys. radium [61 10, 273 (1929). G6. Gerlach, W., and Albers, A. C., Z . Physik 26, 106 (1924). G7. Goodman, L. S., and Wexler, S., Phys. Rev. to be published. H 1 . Hodgmm, C. D., “Handbook of Chemistry and Physics,” 36th ed. Chemical Rubber Publishing, Cleveland, 1954. H 2 . Hampel, C. A., “Rare Metals Handbook.” Rheinhold, New York, 1954. H S . Hamilton, D. R., Am. J . Phys. 9, 319 (1941). H4. Hughes, V., Tucker, G., Rhoderick, E., and Weinrich, G., Phys. Rev. 91, 828 (1953). H 5 . Hopkins, B. S., “Chapters in the Chemistry of the Less Familiar Elements.” Stipes Publishing, Champaign, Ill., 1939. H 6 . Hay, R. H., Phys. Rev. 60, 75 (1941). H 7 . Heil, H., 2. Physik 120, 212 (1943). H8. Hagstrum, H. D., Rev. Sci. Insfr. 24, 1122 (1953). H9. Holloway, J. H., S.B. Thesis, Physics Dept., M.I.T., Cambridge, 1952. H10. Hughes, H. K., Phys. Rev. 72, 614 (1947). H11. Hahn, T. M., Jr., Ph.D. Thesis, Physics Dept., M.I.T., Cambridge, 1950 (unpublished). HIW. Haynes, S. K., and Wedding, J. W., Rev. Sci. Instr. 22, 97 (1951). H1S. Hamilton, D. R., Phys. Rev. 66,30 (1939). H l 4 . Hobson, J. P., Hubbs, J. C., Nierenberg, W. A., and Silsbee, H. B., Phys. Rev. 96, 1450 (1954). Ii‘. Inghram, M. G., Advances in Electronics 1, 219 (1948). 18. Indiana Steel Products Co., Indiana Permanent Magnet Manual #4. Valparaiso, Indiana, 1948. J1. Jaccarino, V., private communication (1954). J2. Juenker, D. W., van Swaay, M., and Birchenall, C. E., Rev. Sci. Inslr. 26, 888 (1955). JS. Jaccarino, V., Ph.D. Thesis, Physics Dept., M.I.T., Cambridge, 1951 (unpublished). J4. Julian, R., Ph.D. Thesis, Physics Dept., M.I.T., Cambridge, 1947 (unpublished). J6. Jaccarino, V., King, J. G., Satten, R. A., and Stroke, H. H., Phys. Rev. 94, 1798 (1954). J6. Jones, H. A,, and Langmuir, I., Gen Elec. Rev. 30, 310,354,408 (1927). J 7 . Jackson, D. A., and Broadway, I,. F., Proc. Roy. Soc. Al27, 678 (1930). J8. Jaccarino, V., and King, J. G., private communication (1953). J9. Jewitt, T. N., Phys. Rev. 46,616 (1934).

86

JOHN G. KING AND JERROLD R . ZACHARIAS

K1. Kellogg, J. M. B., Rabi, I. I., Ramsey, N. F., and Zacharias, J. R., Phys. Rev. 66,728 (1939). KB. Kellogg, J. M. B., Rabi, I. I., and Zacharias, J. R., Phys. Rev. 60, 472 (1936). K9. King, J. G., and Jaccarino, V., Phys. Rev. 94, 1610 (1954). K4. Kantrowitz, A., and Grey, J., Rev. Sci. Instr. 22, 328 (1951). K6. Kistiakowsky, G. B., and Schlichter, W. P., Rev. Sci. Znstr. 22, 333 (1951). K6. Kingdon, K. H., Phys. Rev. 23, 778 (1924). K7. Knauer, F., Z . Physik 126,279 (1949). K8. Kingdon, K. H., Phys. Rev. 21, 408 (1923). K9. Kusch, P., “Lecture Notes in Molecular Beams.” Columbia University, 1950 (unpublished). KlO. Kolsky, H. G., Phipps, T. E., Jr., Ramsey, N. F., and Silsbee, H. B., Phys. Rev. 87, 395 (1952). K11. Korsunskii, M. I., and Fogel, Y. M., J . Exptl. Theoret. Phys. (USSR) 21, 25 (1951) (and references therein). K12. Knauer, F., and Stern, O., Z . Physik 39, 780 (1926). K1S. Kellogg, J. M. B., and Millman, S., Revs. Mod. Phys. 18, 323 (1946). Kl4. Koenig, S. H., Prodell, A. G., and Kusch, P., Phys. Rev. 88, 191 (1952). K16. Kellogg, J. M. B., and Ramsey, N. F., Phys. Rev. 63, 331 (1938). K16. Kurt, 0. E., and Phipps, T. E., Phys. Rev. 34, 1357 (1929). Kl7. Kusch, P., and Millman, S., Phys. Rev. 66, 527 (1939). K18. Kusch, P., Millman, S., and Rabi, I. I., Phys. Rev. 66, 1176 (1939). K19. Kusch, P., Millman, S., and Rabi, I . I., Phys. Rev. 66, 666 (1939). KBO. Kusch, P., and Mann, A. K., Phys. Rev. 76, 707 (1949). K21. Kohl, W. H., “Materials Technology for Electron Tubes.” Rheinhold, New York, 1951. K82. Kofosky, I. L., and Levinstein, H., Phys. Rev. 74, 500 (1948). L1. Law, R. R., Rev. Sci. Znstr. 19,920 (1948). LB. Lew, H., Phys. Rev. 76, 1086 (1949). LS. Lew, H., and Wessel, G., Phys. Rev. 90, 1 (1953). L4. Lew, H., Phys. Rev. 91,619 (1953). L5. Lemonick, A., Pipkin, F. M., and Hamilton, D. R., Report No. NYO-7317, Princeton University, 1955. (unpublished) L6. Lamb, W. E., Jr., and Retherford, R. C., Phys. Rev. 79, 549 (1950). L7. Lemonick, A., and Pipkin, F. M., Phys. Rev. 96, 1356 (1954). L8. Lurio, A,, private communication (1955). L9. Langmuir, I., and Kingdon, K. H., Proc. Roy. SOC.A107, 61 (1925). LiO. Lawson, A. W., and Fano, R., Rev. Sci. Znstr. 18, 727 (1947). L l i . Lyons, H., Ann. N . Y . Acad. Sci. 66, 831 (1952). L18. Lurio, A., and Prodell, A. G., Phys. Rev. to be puhlished. L13. Lafferty, J. M., and Kingdon, K. H., J . Appl. Phys. 17, 894 (1946). Ll4. Lew, H., private communication (1953). L16. Leu, A., Z . Physik 41, 551 (1927). L16. Leu, A,, Z . Physik 49, 498 (1928). Ll7. Langmuir, I., and Villars, D. S., J . Am. Chew Soc. 63,486 (1931). M 1 . Massey, H. S. W., and Burhop, E. H. S., “Electronic and Ionic Impact Phenomena.” Oxford U.P., New York, 1952. MB. Michaelson, H. B., J . A p p l . Phys. 21, 536 (1950). MS. Metlay, M., and Kimball, G. E., J . Chem. Phys. 16, 774 (1948). M4. Massey, H. S. W., “Negative Ions.” Cambridge U.P., London, 1938. M5. McAdams, W. H., “Heat Transmission.” McGraw-Hill, New York, 1942.

MOLECULAR BEAMS

87

Millman, S., Rabi, I. I., and Zacharias, J. R., Phys. Rev. 63, 384 (1938). Millman, S., Phys. Rev. 66, 628 (1939). Manley, J. H., and Millman, S., Phys. Rev. 61, 19 (1937). Millman, S., and Fox, M., Phys. Rev. 60, 220 (1936). Millman, S., Phys. Rev. 47, 739 (1935). Mais, W. H., Phys. Rev. 46, 773 (1934). M l d . Miller, R. C., and Kusch, P., Phys. Rev. 99, 1319 (1955). M1S. Millman, S., Kusch, P., and Rabi, I. I., Phys. Rev. 66, 165 (1939). M14. Millman, S., and Kusch, P., Phys. Rev. 60, 91 (1941). M15. Millman, S., and Kusch, P., Phys. Rev. 66, 303 (1939). M16. Marton, L., Schubert, D. C., and Mielczarek, S. R., J . A p p l . Phys. to be published. N l . Nielsen, C. E., Rev. Sci.Znstr. 18, 18 (1947). NB. Nelson, E. B., and Nafe, J. E., Phys. Rev. 76, 1194 (1949). NS. Nagle, D. E., Julian, R. E., and Zacharias, J. R., Phys. Rev. 72, 971 (1947). N4. Nottingham, W. B., “Bibliography on Physical Electronics.” Cambridge, Mass. Addison-Wesley, Cambridge, 1954. N6. Nafe, J . E., and Nelson, E. B., Phys. Rev. 73, 718 (1948). N6. Nierenberg, W . A., and Ramsey, N. F., Phys. Rev. 72, 1075 (1947). 0 1 . Ochs, S. A., Cote, R. E., and Kusch, P., J . Chern. Phys. 21, 459 (1953). P1. Peter, M., and Strandberg, M. W. P., private communication (1955). PZ. Prodell, A. G., and Kusch, P., Phys. Rev. 88, 184 (1952). PS. Perl, M. L., Rabi, I. I., and Senitaky, B., Phys. Rev. 98, 611 (1955). P4. Penick, D . B., Rev. Sci. Znstr. 6, 115 (1935). P5. Paul, W., and Wessell, G., 2. Physik 124, 691 (1948). P6. Paul, W., Z. Physik 124, 244 (1948). P7. Palensky, H., Swank, R. K., and Grenchik, R., Rev. Sci. Znstr. 18, 298 (1947). R1. Rosebury, F., “Tube Laboratory Manual.” Research Laboratory of Electronics, M.I.T., Cambridge, 1951 (unpublished). R8. Ramsey, N. F., “Molecular Beam.” Oxford U.P., New York, in press. RS. Rodebusch, W. H., and Henry, W. F., Phys. Rev. 39, 386 (1932). R4. Rappaport, P., J . A p p l . Phys. 26, 288 (1954). R5. Remler, E. A., S.B. Thesis, Physics Dept., M.I.T., Cambridge, 1955 (unpublished). R6. Rabi, I. I., Kellogg, J. M. B., and Zacharias, J. R., Phys. Rev. 46, 157 (1934). R7. Roters, H. C., “Electromagnetic Devices.” Wiley, New York, 1947. R8. Rabi, I. I., Morris Loeb Lecture, Harvard University, 1955. R9. Ramsey, N. F., Phys. Rev. 78, 695 (1950); N. F. Ramsey and H. B. Silsbee, Phys. Rev. 84, 406 (1951). R10. Rose, M. E., Phys. Rev. 63, 715 (1938). R11. Ramsey, N. F., Phys. Rev. 98, 1853 (1955). RIB. Rearwin, R. H., S.M. Thesis, Dept. of Electrical Engineering, M.I.T., Cambridge, 1954 (unpublished). R1S. Renzetti, N., Phys. Rev. 67, 753 (1940). Rl4. Ramsey, N. F., in “Experimental Nuclear Physics” (E. SegrB, ed.), Vol. I, Part 111, p. 358. Wiley, New York, 1953. R15. Rabi, I. I., and Cohen, V. W., Phys. Rev. 46, 707 (1934). R16. Reimann, A. C., “Vacuum Technique.” Chapman and Hall, London, 1952. Rl7. Rabi, I. I., Millman, S., and Kusch, P., Phys. Rev. 66, 526 (1939). R18. Rodebush, W. H., Murray, L. A,, and Rixler, M. E., J . Chem. Phys. 4, 730 M6. M7. M8. M9. M10. Mll.

(1936).

88

JOHN G. KING AND JERROLD R. ZACHARIAS

Sl. Stroke, H. H., Ph.D. Thesis, Physics Dept., M.I.T., Cambridge, 1954 (unpublished). S2. Sanderson, R. T., “Vacuum Manipulation of Volatile Compounds.’’ Wiley, New York, 1948. SS. Stroke, H. H., S.M. Thesis, Physics Dept., M.I.T., Cambridge, 1952 (unpublished). S4. Strong, J., “Procedures in Experimental Physics.” Prentice-Hall, New York,

1938. S6. Stern, O., 2.Physik 7, 249 (1921). S6. Smith, K. F., “Molecular Beams.” Methuen, London, 1955. S7. Stern, O., 2. Physik 39, 751 (1926). 58. Stone, R. P., Rev. Sci. Znstr. 20, 935 (1949). S9. Shaw, E. J., Phipps, T. E., and Rodebush, W. H., Phys. Rev. 36, 1126(1930). T1. Taub, H., and Kusch, P., Phys. Rev. 76, 1481 (1949). 2’2. Taylor, J. B., 2.Physik 67, 242 (1929). TS. Trischka, J. W., Marple, D. T. F., and White, A., Phys. Rev. 86, 136 (1952). T4. Thorp, J. S., J . Sci. Instr. 26, 201 (1949). 2‘5. Townes, C. H., and Schawlow, A. L., “Microwave Spectroscopy.” McGrawHill, New York, 1955. V1. Varnerin, L. J., Jr., Phys. Rev. 91, 859 (1953). V2. Valley, G. E., and Wallman, H., “Vacuum Tube Amplifiers.” McGraw-Hill, New York, 1948. VS. Vauthier, R., Compt. rend. 228, 1113 (1949). W1. Wessel, G., and Lew, H., Phys. Rev. 92, 641 (1953). W.2. Wood, R. W., Proc. Roy. SOC.97, 455 (1920). WS. Weinrich, G., and Hughes, V. W., Phys. Rev. 96, 1541 (1954). W4. Wexler, S., Conference on Atomic Beam, Brookhaven National Labs., Upton, New York, 1954. W6. Weissler, G. L., and Wilson, T. N., J . A p p l . Phys. 24, 472 (1953). W6. White, W. C., Proc. Inst. Radio Engrs. 38, 852 (1950). W7. Wohwill, H., 2. Physik 80, 67 (1933). W8. Whitehead, T. N., “The Design and Use of Instruments and Accurate Mechanism.” Dover Publications, New York, 1954. W9. Wessel, G., Phys. Rev. 92, 1581 (1953). W10. Wrede, E., Z . Physik 44, 261 (1927). Y1. Yonov, N. E., Compt. rend. acad. Sci. U.S.S.R. 28, 512 (1940). 21. Zabel, R. M., Phys. Rev. 46, 411 (1934). 2 2 . Zacharias, J. R., Yates, J. G., and Haun, R. D., 35th Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., Cambridge, 1954 (unpublished). 23. Zacharias, J. R., and Haun, R. D., 35th Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., Cambridge, 1954 (unpublished). 2.4. Zabel, R. M., Phys. Rev. 42, 218 (1932). 26. Zartman, I. F., Phys. Rev. 37, 382 (1931). 26. Zacharias, J. R., and Weiss, R., privatc communication (1956).