A polarized 3He+ ion source

NUCLEAR

INSTRUMENTS

AND METHODS

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NORTH-HOLLAND

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A P O L A R I Z E D aHe+ I O N S O U R C E * D. O. F I N D L E Y ~, S. D. B A K E R b, E. B. C A R T E R ~ and N. D. S T O C K W E L L a

Rice University, Houston, Texas, U.S.A. Received 3 February 1969 A source of polarized 3He + ions has been constructed and tested. A n optical p u m p i n g technique is used to polarize the :nuclear spins of the ions, which are extracted in the m a n n e r of an rf ion source of conventional design. In a nuclear double s c a t -

tering experiment, a polarization of 0.05 was observed in the ion beam with an intensity of 4 ~ A and an emittance of l cm • r a d . eV½.

1. Introduction

metastable atoms undergo many collisions with the far more numerous ground-state atoms and, by metastability exchange collisions, transfer their polarization to the nuclear spin system of the ground state atoms. 3He+ ions are also created in the weak discharge. The mechanism for production of 3He ions in a weak discharge is not well understood, and an expression for the polarization of the ions at the time of their formation cannot be written down. However, the large cross-section ( g 10-1Scm2) for electron exchange in 3He+-3He ion-atom collisions 7) offers a mechanism for bringing the ions to an equilibrium nuclear polarization equal to that of the ground state atoms. The problem, then, is to extract ions in useful number from the polarized gas. In practice this requires a compromise between the requirements for high ion beam currents and high polarization.

In this paper we describe a source of polarized 3He ÷ ions, which is especially recommended by its simplicity of design, construction, and operation. These advantages stem from the use of an optical pumping technique a-a) to polarize the ions and from the use of conventional rf ion source geometry 4's) to extract the ions. An announcement on the attainment of a polarized beam has already appeared6); this paper is intended as a more complete report. In section 2 we shall give a brief description of optical pumping of 3He to show how the requirements of the optical pumping process influence the design of the polarized ion source. This section will be followed by a description of the components of the ]polarized ion source. Section 4 of this paper deals with operation of the polarized ion source: the clean-up procedure, some operating parameters, and an experiment performed to measure the ion beam polarization.

2.2. GAS PRESSURE 2. Design considerations

2.1.

OPTICAL PUMPING OF 3 H e

In the optical pumping method of polarization1), a ,;ample of very pure 3He gas at a pressure of the order of I Torr is placed in an optically transparent container in a uniform magnetic field. A weak rf discharge excites a small fraction of the 3He atoms from the 11S0 (IF--½) ground state to the 2381 ( F = 3 or ½) metastable state. Circularly polarized resonance light (23S ~ 23P) pumps angular momentum into the triplet spin system, and, in this manner, the metastable 23S, atoms become nuclear spin polarized. The * W o r k supported in part by the U. S. Atomic Energy Commission. A E C Fellow in Nuclear Science. b Alfred P. Sloan F o u n d a t i o n Fellow. c N o w at Trinity University, San Antonio, Texas. a N o w at Aerospace Corporation, San Bernardino, California, 92404. JUNE 1969

125

An rf ion source of conventional design produces a maximum ion beam current output when the pressure of the gas is of the order of 0.02 TorrS). Unfortunately the optical pumping process is not at all efficient at such low pressures because of the rapid diffusion of metastables to the container walls, where they are deexcited before transferring their polarization to the ground state spin system. It has been found that the attainable nuclear polarization in a sample of 3 H e gas is a steeply rising function of pressure, reaching a maximum between 1 and 3 Torr, where it begins to decrease~,2,8). 2.3. DISCHARGEINTENSITY When optical pumping is being done on a sample of 3He gas in a closed container, it is usually found that the maximum polarization is obtained when the rf intensity is such that the weakest self-sustaining discharge is maintained. The polarization falls off

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D.O. FINDLEY et al.

rapidly as the intensity of the discharge is increasedl'8). This is to be contrasted with the strong discharge considered desirable in usual rf ion sources. However, in the case of optical pumping in the flowing gas of an ion source, the situation is somewhat different from that in a closed cell. For the flowing system one must consider the effect of the dwell time, the mean time that the 3He atoms spend in the optical pumping cell before leaving the cell through the exit canal. It has been found that the time characterizing the approach to equilibrium of the ground state polarization, the optical pumping time, also decreases sharply with an increase in discharge intensity in much the same way as does the polarizationS). Ideally the optical pumping time should be much less than the dwell time, so that the maximum polarization is achieved before the atom is lost through the exit canal. Hence, if because of other design demands, the dwell time is made short compared with the pumping time with the weakest self-sustaining discharge, it is advantageous to shorten the optical pumping time by increasing the intensity of the discharge. In such a case, the beam polarization may actually be higher with the brighter discharge than with the weakest self-sustaining discharge.

compared to the optical pumping time and the dwell time. An expression for this relaxation time has been derived 9) which can be used to establish a limit on the magnetic field gradients which can be tolerated in the neighborhood of the ion source bottle. To reduce magnetic field gradients we avoided the use of ferromagnetic material in the construction of the ion source and placed auxiliary equipment containing ferromagnetic materials as far from the ion source bottle as practical, typically more than 50 cm. This requirement of field homogeneity is to be contrasted with the usual field configuration in an rf ion source, where the field reaches a maximum at the position of the exit canal.

2.4.

3. Components of the polarized SHe ion source

MAGNETIC FIELDS

Another design consideration is the requirement that magnetic field gradients over the volume of the optical pumping cell be sufficiently small. The relaxation time due to a magnetic field gradient must be made long

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2.5. GAS PURITY The ion source bottle atmosphere must be free of impurity atoms during the optical pumping process. It has been estimated that the partial pressure of all contaminants must be less than 10-4 Torrl); otherwise, collisions between metastable 3He atoms and impurity atoms or molecules will tend to destroy the metastables. Therefore, all surfaces of the ion source over which 3He gas flows before extraction must be capable of being cleaned and maintained clean during the use of the ion source.

The polarized 3He ion source is in design basically an rf ion source of the type developed at Oak Ridge 4) with a few differences which were required by the optical pumping process. The major part of the ion

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A POLARIZED

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source components were either taken directly from a commercially available rf ion source a°) or were modeled after those of the commercial unit. A schematic view of the polarized 3He ion source is shown approximately to scale in fig. 1. 3.1. ION SOURCE BOTTLE The ion source bottle consists of a thin pyrex glass spherical bulb which is joined to the cylindrical base end of a standard ion bottle. A tungsten positive electrode is mounted in a narrow pyrex pipe extending from the upper portion of the bulb. To provide a high pumping speed for cleaning the bottle of adsorbed gases, a glass vacuum line (not shown in fig. 1) of impedance much smaller than that of the exit canal is attached to the spherical bulb of the ion bottle. A right-angle high-vacuum glass stopcock is placed in the vacuum line as close as is possible (3 cm) to the ion source bottle to restrict the gas volume optically pumped to a small region ( ~ 86 cm3), thus reducing the volume over which magnetic field homogeneity must be maintained.

ION SOURCE

127

Fig. lb is an enlarged view of a section of fig. la, showing the arrangement for mounting the ion bottle to the base plate. The gasket used in mounting the ion bottle is made of Viton instead of the usual butyl rubber, because of the high temperatures (up to 110 °C) sustained in this region during the system, clean-up procedure to be described in section 4.1. We suspect that the outgassing of this Viton gasket was the prime source of vacuum system impurities for the first several months of its use. In the future, if this construction is retained, the Viton gasket will be pre-baked in high vacuum before being incorporated into the system. 3.2. EXIT CANAL The ion source base plate is a commercially available unit x°) modified to accommodate a smaller diameter exit canal than is ordinarily used. The exit canal was designed to provide a large enough impedance to gas flow so that the dwell time was reasonably large while at the same time allowing a useable ion beam to be extracted. The diameter of the gas canal in its narrowest portion is 0.343 mm for a length of 2.03 mm. The exit canal is shown in fig. lb. 3.3. ION OPTICS

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A quartz sleeve fits over the exit canal assembly and shapes the field due to the extraction voltage to focus the ions through the exit canal*). An end-corrected solenoid surrounds the ion source bottle to provide a uniform, axial magnetic field over the gas volume. The axial magnetic field, which must be uniform because of the demands of the optical pumping process, also enhances the efficiency of ionizationS). Beyond the exit canal, the ions are focussed by a gap lens, whose dimensions are shown in fig. 1. The separation between the gap lens electrode and the base plate surface at the exit canal can be varied continuously from 0.0 to 1.6 cm. An Einzel lens ~°) is used to focus the ion beam further beyond the gap lens. The base plate, an insulating ring, and the gap lens mount are attached to the Einzel lens by means of threaded nylon rods. The Einzel lens is, in turn, mounted on a vacuum manifold constructed from a standard 2" copper tee. On the arm of the tee opposite the Einzel lens, a high vacuum gate valve is mounted to isolate the ion source from the accelerator to which it might be attached.

3.4. Fig. lb. Vertical section showing the extraction geometry and the ion bottle mounting arrangement.

VACUUM SYSTEM

An identical gate valve is mounted on the third arm of the tee to isolate the ion source from an oil diffusion

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FINDLEY et al.

pump. Also attached to the tee are vacuum lines to the gas handling system and to the source bottle fastpump-out line, a line for roughing out the ion source with a mechanical pump, and an ionization vacuum gauge tube for monitoring the vacuum. The vacuum pumping system consists of a 2" oil diffusion pump (CVC type PMCS) and baffle ring mounted below the high vacuum valve and a mechanical forepump. Vacuum safety includes forepressure and thermal protection for the diffusion pump. For the purpose of accelerating the ions from the source, the entire ion source can be raised to a potential of 125 kV above ground. The diffusion pump is cooled with high voltage transformer insulating oil, which is circulated through a heat exchanger at ground potential. Transformer oil is not an efficient coolant, but no difficulties have been encountered because of the ability of the special diffusion pump fluid (Convalex 10) to function properly at high temperatures.

pressure in the ion source bottle. This pressure is monitored by means of a thermocouple gauge tube mounted in the gas supply line near the ion source base plate. The 3He gas is obtained in a very pure form11); however, experience has shown that the gas needs further purification before optical pumping can be accomplished. Therefore, the gas is flowed through a cold trap of simple, all-glass design, which is immersed in liquid helium. The cold trap can be bypassed whenever a high pumping speed is needed in this section of the gas-handling system during clean-up. A high vacuum stopcock separates the gas-handling system from the high vacuum system; this stopcock is opened during the cleaning of the gas-handling system. Another high vacuum stopcock isolates the gas-handling system from the ion source bottle gas input when the former is being cleaned or repaired. An ionization gauge tube is mounted in the gas-handling system to monitor the pressure during the clean-up procedure.

3.5. GAS-HANDLINGSYSTEM The gas-handling system, shown diagrammatically in fig. 2, provides the means of distributing and purifying gases from their storage bottles to the ion bottle. Two gases are used: 3He and 4He. 4He is used in the cleaning up of the ion source to economize on the use of 3He. A variable leak (Granville-Phillips) provides for the metering of the gas flow to maintain a constant

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3.6.

BRIGHTLAMP

The lamp system which furnishes the resonance)ight for optical pumping is similar to systems described in ref.L'2'12). Briefly, the system consists of a high power, 100 MHz rf oscillator which excites an intense discharge in a button-shaped cavity filled to a pressure of 10 to 2 0 T o r r with 4He gas13), a concave mirror behind the discharge to direct the light toward the optical pumping cell, and a circular polarizer, consisting of a linear polarizer and a quarter-wave plate. The system is mounted on an optical bench inside the solenoid coil form.

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WEAKDISCHARGERF

The rf energy for the weak discharge in the ion bottle is furnished by a 50 MHz rf transmitter capable of up to 25 W input to the final amplifier stage. The actual power input used depends on the pressure of the gas in the ion source bottle and on the desired brightness of the discharge. A typical value is 1.5 W for the weakest self-sustaining discharge at 0.4 Torr pressure (the power dissipated in the discharge has not been measured). Rf is matched to the discharge in the ion bottle by an L C coupling network mounted on a lucite platform on an optical bench runner. One side of the output of the L C network is grounded to the base plate of the ion source while the other side is connected to a loop of wire. The platform is moved on the optical bench until the loop of wire is in position against the rear of the ion bottle.

A POLARIZED 3He+ ION SOURCE 3.8. POLARIZATIONMONITOR

Located on the same lucite platform with the r f coupling network is a small coil placed directly below the center of the ion source bottle. This coil is excited by rf at the Larmor frequency of the ground state 3He atoms (3.25 kHz/G) to depolarize the optically pumped atoms in the ion source bottle; this step is required in the optical measurement of the atomic polarization. A lead sulfide infrared detector 14) is used to monitor the change in absorption of the 1.08/~m resonance light when the 3He atoms in the ion bottle are depolarized. This optical signal can be related to the polarization of the 3He atomsa'a/). Light passing through the cell is reflected 90 ° by a slant-mounted miniature mirror located behind the spherical part of the ion source bottle to the detector. The optical signal is observed on a chart recorder. 3.9. ELECTRICALSYSTEM Some of the component power supplies (the power supplies for the extraction voltage, the weak discharge excitation, the bright lamp, the solenoid, and the atomic polarization measurement) must float at the base plate (accelerating) voltage. Therefore, l l0 V ac for these supplies is furnished through an isolation transformer with insulation to withstand 35 kV. In addition, all power to the ion source table is delivered through an isolation transformer with 150 kV insulation. The total power requirement for the polarized ion source in operation is 3 kVA. 4. Operation of the ion source

Preliminary design studies and feasibility tests on the polarized 3He ion source were made on a bench model using 4He gas15). The present design was a result of those tests. Unless otherwise stated, all tests reported in this section were made using 3He gas in a system of the design described in section 3. 4.1. CLEAN-UP PROCEDURE A cleaning procedure has been developed from experience with preparing gaseous 3He targets which is used with only minor modifications in preparing the ion source for operation. Similar cleaning procedures are described in ref.l'2'12). After the components had been cleaned with the appropriate solvents (hexane, followed by acetone, isopropyl alcohol and distilled water) and carefully assembled, the system was pumped on for several days with the diffusion pump while the glass was heated to

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speed degassing. The ion source bottle was covered by electrical heating mantles and warmed to temperatures of 180 °C or so (while the temperature in the baseplate region of the cell was monitored to insure that the Viton gasket was not heated above 150 °C). The small diameter glass tubing on both sides of the cold trap was heated on several occasions with a flame. Other portions of the gas-handling system were painted black and warmed with a keat lamp. The ionization vacuum gauge tube in the gas-handling system was used to monitor the progress of the cleaning procedure. Once the pressure in the gashandling system remained below 10 - 4 Tort when the only pumping on the system was through the ion bottle exit canal, 3He was flowed through the cold trap (cooled to liquid helium temperature) and admitted to the ion bottle. The weak discharge rf was turned to a high level, and the discharge in the ion bottle was observed with a low resolution spectroscope. If the discharge was "clean", as judged by the absence of spectral lines other than those of helium, optical pumping of the aHe gas would most likely be possible, and the clean-up procedure would be terminated at this point. If, however, spectral lines or bands due to impurities were observed in the discharge, further cleaning was necessary. 4He gas (to conserve 3 H e ) w a s flowed through the cold trap into the ion bottle, where a very intense rf discharge was maintained with a diathermy machine. Occasionally the fast pump-out line stopcock was opened slightly to hasten the gas flow through the bottle. When the discharge appeared clean, the diathermy machine was removed, and the gas-handling system was opened to high vacuum to pump out the 4He gas before admitting 3He to the system. After the system had been pumped on for an hour or so and before 3He gas was admitted, a very dim discharge could be lighted in the bottle with the weak discharge rfturned up to a high level. This discharge was probably from helium gas being driven out of the walls of the bottle. The discharge could be quickly extinguished by turning the extraction probe voltage up to + 4 kV; it could not be relighted. At this point, aHe was again admitted to the ion bottle. If atomic polarization was observed in the ion bottle, our experience was that the system would stay clean as long as liquid helium covered the cold trap. If the liquid helium was removed, a repeat of the initial cleaning procedure before the next use was required. 4.2. OPERATING PARAMETERS Tests were made to determine the optimum para-

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D.O. FINDLEY et al.

meters for operating the polarized 3He ion source. For those tests in which an ion current was measured, the 3He+ ions were accelerated to an energy of 300 keV, and the current was measured by stopping the beam on a copper target behind a 0.32 cm diameter aperture located 1.8 m from the exit port of the Einzel lens. The maximum attainable atomic polarization as a function of cell pressure was investigated. We found that the polarization was very nearly constant - 4 % to 6 % - over the range 0.4 to 1.0 Tort. We interpret these results to mean that the dwell time of the average 3He atom did not allow full optical pumping at the higher pressures. (By shutting off the gas supply and observing the time rate of decay of the pressure, the dwell time was measured to be approximately 16 sec.) It is of interest to determine the relationship between discharge intensity and beam current or atomic polarization. There are several ways to define the intensity of the discharge. One might measure discharge intensity by the intensity of the light emitted by the discharge or, perhaps, by the ion density (as measured by the extracted current). Most often, however, the 23S1 metastable density is used as a measure of the intensity of a helium discharge; the metastable density can be calculated from the percentage absorption of the resonance light passing through the ce118'16). For convenience we used the B + voltage of the 50 MHz weak discharge rf source as an index and determined the extracted beam current (at constant pressure and extraction voltage), the metastable density, and the light from the discharge (the latter two at constant pressure with zero extraction voltage). All were measured relative to the values at 70 V B + , where the weakest self-sustaining discharge was obtained. The results of these tests are shown in table 1. Also shown in table 1 are the optically measured'polarizations of the neutral atoms at 0.5 Torr. As to the information contained in table 1 about the dependence of atomic polarization on discharge intensity, the result that the polarization increased slightly from the 70 V to the 80 V'discharge level and remained constant from 80 to 100 V is interpreted as meaning that the speed-up in optical pumping time at the higher discharge levels overcame the depolarizing effects of the heated-up discharge; as in the case of the near constancy of polarization as a function of pressure, this effect was probably due to the short dwell time of the cell. The extraction voltage did not affect the attained polarization. Table 1 also shows a roughly linear increase of extracted current as the

discharge level is increased over the range observed. The relationship between ion source bottle gas pressure and beam current with discharge intensity and extraction voltage held constant (at 70 V and 3.75 kV, respectively) was investigated for pressures from 300 to 1000/~m of mercury. The empirical relation i = 7.2 e x p ( - p / 2 9 0 ) , was established, where i is the current (/tA) measured on the target and p is the ion bottle gas pressure (/tm Hg). As a result of the above tests, we selected a pressure of 0 . 4 T o r r and 100V weak discharge oscillator B + for the operation of the ion source. The extraction voltage was set rather arbitrarily at 3.75 kV; at this voltage there was no problem with sparking in the ion bottle. The gap lens-base plate separation was 2.2ram; no attempt was made to investigate the effect of this separation on the focusing properties of the ion source. The focus and Einzel lens voltages could be adjusted remotely to maximize the target current during an experiment. Tests with the preliminary design of the ion source showed that increasing the axial magnetic field to about 200 G resulted in an almost three-fold gain in beam currents over those currents obtained with a field of about 20 G t s). However, because of the lack of cooling for the solenoid on the ion source table, the high fields could not be obtained for tests with the present design, where the axial magnetic field along the exit canal was approximately 15 G. 4.3. MEASURINGTHE POLARIZATION Most operating experience with the present design of the polarized 3He ion source has been in conjunction TABLE 1 Extracted ZHe + b e a m current, 2zS1 metastable density, ion bottle weak discharge light (all normalized to 70 V data), and nuclear polarization o f aHe neutral species in ion bottle at four weak discharge rf oscillator B + voltages. (All readings at 0.5 Torr pressure with zero extraction voltage unless otherwise noted.)

B+ (V)

Extracted beam current * (arb. units)

23S1 metastable density (arb. units)

Discharge light intensity (arb. units)

s He polarization

70 80 90 100

1.0 1.3 1.5 1.7

1.0 1.6 2.0 2.5

1.0 1.5 2.6 5.0

0.05~0.01 0.06-!0.01 0.06~0.01 0.06~0.01

* + 3.75 kV extraction, 0.4 Torr pressure.

A POLARIZED 3He + ION SOURCE with an experiment to measure the polarization of the 3He ions17). The experiment is shown in fig. 3. In the double-scattering experiment, 3He+ ions from the ion source were accelerated to 300 keV energy and produced protons through the reaction 2H(3He, p)4He (Q = + 18.4 MeV). When initiated by s-waves, this reaction proceeds for the most part is) through a 3/2 + resonance in 5Li, and with this single channel assumption it is easy to establish the relationship between the 3He+ ion beam polarization and the polarization of the 14.7 MeV protons from the reaction; for protons emitted at an angle of 90 ° with respect to the incoming 3He beam, which is longitudinally polarized, the proton polarization is 0.67 times the 3He nuclear polarization and opposite in sign 19) as shown in fig. 3. The polarization of the protons is measured through the left-right asymmetry of their elastic scattering in a high-pressure (35 atm) heliumfilled polarimeter 2°) with an analyzing power of 0.6. The acceleration to 300 keV, which was necessary to obtain a practical proton yield from the reaction, was achieved in the following manner: the ion source and its auxiliary equipment, including the optical pumping apparatus and the vacuum system, were mounted on a table insulated from ground; the deuterium target (DzO-ice) and the polarimeter were mounted on a second insulated table; on a stand between the two tables, two acceleration columns were mounted with a manifold pumped by a 4" oil diffusion pump at ground potential between them; the ion source table was raised to + 125 kV, and the target table to - 165 kV. Data were collected in two separate runs separated by three weeks' time. During the experiment the sign of the beam polarization was reversed every five minutes (by reversing the sense of the optical pumping

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light) to cancel errors due to detector solid angle and efficiency asymmetries in the polarimeter. An averaged scattering asymmetry of 0.0227+0.0033 (standard deviation) was observed, from which the ion beam polarization was calculated to be 0.05-t-0.01. At the same time we found that the atomic polarization in the ion source bottle was 0.05+0.01. Thus the nuclear polarization of the neutral and ionic 3He species were equal within experimental error. With beam currents of 3 to 4 HA measured on target, the counting rate in the polarimeter was 2 events per second. About 18 to 24 h of active data-taking time was required to obtain adequate statistics. Because of the inefficiency of this method of measuring the beam polarization, an extensive search for the optimum operating parameters of the ion source could not be made. Such a search will have to await a more efficient polarization analyzer. 5. Conclusion

5.1. SPECIALQUALITIES We would like to summarize the special features of the Rice 3He polarized ion source, which mostly stem from the optical pumping method used to polarize the ions. The source operates at room temperature and requires very modest magnetic fields and vacuum system. It is simple in construction, and most of the parts are commercially available. The sign of the nuclear spin polarization can be changed easily, with no effect on ion trajectories, simply by changing the sense of circular polarization of the pumping light. No external ionization is required. Operation of the ion source is very economical, the 3He gas consumption being about 9 STP • cma/h. The one difficulty in the operation of the ion source is the necessity of having very pure 3He gas. 5.2. FUTURE DEVELOPMENT

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Fig. 3. Schematic diagram of an experiment to measure the nuclear polarization of the ZHe+ ions from the polarized ion source.

Although the intensity (4HA), the polarization (0.05), and the optical quality (the emittance is estimated to be 1 cm • rad • eV ~) of the ion beam make it suitable for injection into appropriate accelerators for use in nuclear scattering studies and nuclear structure research, the initial activity with the polarized 3He ion source, in conjunction with an accelerator and an efficient analyzing reaction experiment, such as 3He-gHe elastic scattering2~), might well be directed toward improving the source. Certainly a higher polarization than 0.05 would be valuable, and the polarization could probably be increased by increasing the

132

D.O. FINDLEY et al.

dwell time (by increasing the dimensions of the optical pumping cell). Another line of endeavor might be directed toward eliminating the need at the ion source for liquid helium in the gas purification process. 5.3. OTHER METHODS It s h o u l d be m e n t i o n e d t h a t t h e r e a r e o t h e r p o s s i b l e m e t h o d s o f p r o d u c i n g p o l a r i z e d 3 H e i o n s besides t h e o p t i c a l p u m p i n g t e c h n i q u e . W o r k e r s at t h e U n i v e r s i t y o f British C o l u m b i a h a v e b e e n p r e p a r i n g a p o s i t i v e i o n s o u r c e u s i n g i n h o m o g e n e o u s m a g n e t i c fields separation of the hyperfine substates of the ground state (11S0) 3He a t o m at p u m p e d liquid helium temperatures22). T h i s s o u r c e h o l d s t h e p o s s i b i l i t y o f h i g h b e a m p o l a r i z a t i o n s . A n o t h e r p o s s i b i l i t y is the p r o d u c t i o n o f a b e a m o f p o l a r i z e d n e g a t i v e 3He i o n s by a t o m i c physics schemes23'2*).

The authors wish to express their appreciation to L . L . Hatfield, G. C. Phillips and G . K . Walters for their participation and support in this work and also to the staff of the Physics Department Shop for their excellent workmanship and for their hospitality after relinquishing a portion of their work space for the polarized ion source experiment. References 1) F. D. Colegrove, L. D. Schearer and G. K. Walters, Phys. Rev. 132 (1963) 2561. 2) R.L. Gamblin and T . R . Carver, Phys. Rev. 138 (1965) A946. 8) This method for producing polarized 3He ions was originally suggested by G. K. Waiters, M. de Wit and G. C. Phillips; see Phys. Rev. Letters 10 (1963) 108, ref. 5. 4) C. D. Moak, H. Rees¢, Jr. and W. M. Good, Nucleonics 9 (1951) 18. 5) j. F. Williams, Rev. Sci. Instr. 3'7 (1966) 1205 and references cited therein. 6) S.D. Baker, E.B. Carter, D.O. Findley, L.L. Hatfield,

G . C . Phillips, N . D . Stockwell and G. K. Waiters, Phys. Rev. Letters 20 (1968) 738 and 1020 (E). 7) A comprehensive review of He+-He electron exchange may be found in E. W. McDaniel, Collision phenomena in ionized gases (Wiley, New York, 1964) Ch. 4 and 9. 8) L.D. Schearer, Ph.D. Thesis (Rice University, 1966) available from University Microfilms, Ann Arbor, Michigan. 9) L . D . Schearer and G . K . WaRers, Phys. Rev. 139 (1965) A1398. 10) From Texas Nuclear Corporation, Austin, Texas. al) From Mound Laboratory, Miamisburg, Ohio. An analysis furnished with the gas bottle specifies 99.46 mol % of~He, with a gross composition of 99.90 mol % helium, 0.10 mol % hydrogen and negligible tritium. 12) S. D. Baker, D. H. McSherry and D. O. Findley, Phys. Rev., to be published. 13) The 23S-23P resonance light from 4He is more effective for optical pumping than that from 3 He at the pressure at which the ion source operates1). 14) From Infrared Industries, Waltham, Massachusetts. 1.5) N . D . Stockwell, M.A. Thesis (Rice University, 1965) unpublished. 16) R. Byerly, Ph.D. Thesis (Rice University, 1967) appendix I; available from University Microfilms, Ann Arbor, Michigan. 17) D.O. Findley, M. A. Thesis (Rice University, 1967) unpublished. 18) The possibility of a contribution from the ½+ channel is discussed by L.C. McIntyre and W. Haeberli, Nuclear Physics A91 (1967) 369; also Ch. Leemann, H. Bfirgisser, P. Huber, H. Schieck and F. Seller, Helv. Phys. Acta 41 (1968) 438. 19) G.C. Ohlsen, Phys. Rev. 164 (1967) 1268. The author discusses the mirror reaction 3H(d, n)4He. 2o) The polarimeter is similar to a design described by G. J. Lush, T.C. Griffith and D. C. lmrie, Nucl. Instr. and Meth. 27 (1964) 229. 21) R. J. Spiger and T. A. Tombrello, Phys. Rev. 163 (1967) 964. 2e) D. Axen, M. K. Craddock, R. L. Erdman, W. Klinger and J. B. Warren, Proc. 2 "~ Intern. Symp. Polarization Phenomena of Nucleons (ed. P. Huber and H. Schopper; Birkhauser Verlag, Basel, 1966) p. 94; D. Axen, Ph. D. Thesis (University of British Columbia, 1965) unpublished; R . N . Vyse, M.A.S. Thesis (Univ. of Britsh Columbia, 1967) unpublished. 28) p. Feldman and R. Novick, Proc. 2"0 Congr. Intern. de Physique nucldaire, Paris, 1964 (ed. P. Gugenberger; Centre National de la Recherche Scientifique, Paris, 1964) p. 785. 24) B. L. Donnally and G. Thoeming, Phys. Rev. 159 (1967) 87