Cryogenic thermometry: a review of recent progress

Since the accurately plishment report on

early 1960’s, a great deal of progress has been made in our ability to measure temperature in the region below 100 K. This period of accomis reviewed in an attempt to present a brief but complete up-to-date status cryogenic thermometry. The bibliography contains over 200 references.

Cryogenic

thermometr&:

a review

of recent

progress

L. G. Rubin

In the early 1960’s, the science and technology of cryogenic thermometry was in an active, advanced, and welldocumented state. A wealth of information was contained in both the state of the art and review material to be found in Volume III of Temperature: Its Measurement and Control in Science and Industry (Reinhold, New York, 1962). Useful review articles by Corruccini,’ Timmerhaus,2 and Orlova3 subsequently appeared, giving good coverage up to about 1962. The purpose of this paper is to extend that coverage to the present, at least for the temperature region below about 100 K. The review is divided into six categories: 1. resistance and diode thermometry 2. thermocouples 3. gas, vapour pressure, and acoustic thermometry 4. paramagnetic thermometry

and

nuclear

magnetic

resonance

5. other methods of thermometry 6. instrumentation

and techniques.

While the accompanying bibliography is extensive, it is impossible to avoid overlooking some contributions that should have been included. There is a rich source of added references in the cited publications, however, and this fact was used to limit the bibliography to manageable (for the author) proportions. No significance should be attached to the order in which references are listed. Resistance

and diode

thermometry

Since a great deal of importance was assigned to the platinum resistance thermometer (PRT) by the JPTS-684.5 this represents an appropriate starting point. The choice of the PRT as the standard instrument for the 13.81 K to 630.74 C region could not have been made without the benefit of the enormous amount of carefully acquired data collected by hundreds of dedicated workers in the field. Although this should be obvious, it is explicitly stated because the majority of the work was done prior The author is with the Francis Bitter National Magnet Laboratory, Mass. Inst. of Technology, Cambridge Massachusetts 02139, USA. An earlier briefer version of this paper appeared in the Leeds and Northrup Technical Journal, No. 6 (1969). Received 11 November 1969. I4

to the period covered by this review. However, activity in the area continues, as attested to by the number of references.6-21 One important, though disappointing, discovery was concerned with the Sondheimer-Wilson-Kohler formula for deviations from Matthiessen’s rule.g~iO It now seems unlikely that this formula will be practically useful in predicting the behaviour of, or classifying the acceptability of, the most accurate PRTs.10 However, the continuing improvement of PRT manufacturing techniques have made such a criterion less critical. The increased purity of the platinum used in modern PRTs has made it possible to tighten the minimum requirements on W (100 C)* for instruments used to realize the IPTS-68. From a value of > l-390 in the 1927 scale. it has now risen to 3 1.3925. Workers in the field of cryogenic platinum resistance thermometry have become accustomed (resigned?) to dealing with many interpolation methods. These have included the Callendar-Van Dusen equation, along with its a, /3, and 6 coefficients, and the ‘Z function’, along with its many variations. More recently, a number of different analytical expressions have been derived, making use of the reduced resistance W(t) and deviations sW(t) determined by calibration of the unknown thermometer at various fixed points. In the case of the 13.81 to 90.188 K region as defined by IPTS-68, there are six fixed points; the reference function involves twenty coefficients and four polynomial equations for the deviations. It is comforting to note that the masimum difference between the IPTS-68 and, for example, the NBS-55 scale, is only about O-015 K. No further discussion of the why and how of the interpolation procedures and temperature scales will be given here. It is considered sufficient to try to (1) make the casual reader aware of the situation, (2) provide adequate references for those who need more information, and (3) indicate the direction of other advances in platinum resistance thermometry. Although the use of the PRT for cryogenic thermometry at hydrogen temperatures and above is taken for granted, it has not been generally appreciated that these transducers are quite useful below 10 K if accuracy of the order of ten millidegrees is acceptable. It has been *The reduced resistance W(f) of the thermometer resistance at the ice point.

is defined as R(L)/R(O C), the ratio at temperature t to its resistance

CRYOGENICS.

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1970

demonstratedii-15 that with adequate calibration procedures, and sufficiently pure and strain-free thermometers, it is possible to fit a fifth-degree polynomial to experimental points between 2 and 16 K to an accuracy of about + 0.01 K. Of equal importance is the fact that with a detection limit of ten nanovolts in the measuring system, and a sufficiently high measuring current, a precision of as good as + O-002 K (and no worse than 0.005 K) is attainable in this range.14 All the work reported in references 5 through 9 was based on capsule-type (with either the Meyers or Barber construction) thermometers, the most accurate units available. In many situations, accuracy can be traded off for other benefits such as higher sensitivity, smaller size, lower cost. Thus, suppose one could choose a PRT with a volume of from &to g that of a capsule-type, and with an RO c of from 4 to 50 times greater. In a particular experiment, such advantages might overcome the associated degradation in accuracy. Commercial units of this type are available and have been successfully employed 16-19.94to cover the range above 10 K with accuracies no worse than hundredths of a degree. Another approach is to sacrifice accuracy in return for greatly simplified calibration procedures. It has been shownzO that units calibrated only at 4.2 K, at 0 C, and between 2 and 4.2 K were usable over the 2 to 273 K range with an accuracy of f 0.05 K. Alternatively, calibrations at the oxygen point and at about 50 K only, enabled a function to be generated, accurate to f 0.1 K over the 12-90 K range.*’ In addition to platinum, other metals can be used in resistance thermometers. Iridium, because its lattice vibrations are still significant at lower temperatures, has been investigated .22-24 As one might expect, indium thermometers are more sensitive than platinum at the lower temperatures, viz, by about a factor of 10 in the 4.2 to 15 K region. If made carefully enough, they exhibit a reproducibility as high as f 0.01 K over the entire 4.2 to 290 K range .24 Unfortunately, the reproducibility from one thermometer to another (even if made from the same spool of wire) is much worse. Also, due to a superconducting transition at 3.4 K, indium cannot be used below this point. A very interesting resistance thermometer has been made from a-manganese. 25 Between about 2 and 16 K, the ideal resistivity (total resistivity minus residual resistivity) of a-manganese is proportional to T*. It is useful to about 60 K and possesses an enormous dp/dT (two thousand times greater than platinum at 10 K). Unfortunately, this characteristic cannot be fully exploited because the material cannot be drawn into wire form. Other alloys that have been tried include a standard brass for the O-3-0.7 K region *6 and a leaded brass (- 2% lead) that proved useful between 2 and 5 K.*’ An interesting sensor combines individual nickel and manganin wire grids connected in series so as to provide a linear characteristic ( + 3 K maximum deviation) over the entire cryogenic range .*8 It is probable that the technology of a-manganese, indium, and other similar materials could be improved. But, with the advent of successful germanium thermometers, such an effort is not likely to be made. In reviewing the progress made in germanium resistance thermometry, one is struck by the fact that there have been few major changes in transducer design and CRYOGENICS

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construction since the early part of the review period.*+Jr Arsenic, gallium, and antimony doping continue to be used, although there is a better understanding of how to tailor the concentrations to meet specific requirements. Thus, there are reports of measurements down to 1 Ki9*3*-34 and even lower,35-37 and as high as 25 K.30.32 The upper limit is not well defined; most of the commercial suppliers* offer units that are usable to 100 K with ‘trade-offs’ in low temperature sensitivity. Above about 25 K, metallic resistance thermometers are more sensitive. This may not be as important a consideration as continuity of measurement or sensor size. Most manufacturers now have miniature units which are not only smaller than their own standard packages, but smaller than precision platinum sensors. One example of a semi-miniaturized package, and its effect on thermal time constants, has been reported.38 Probably the biggest advances in germanium thermometry has been in: a. the greater confidence level instilled in users by continuing reports of excellent reproducibility J@-33J9140 b. the publication of various analytical temperatureresistance relations important for data processing and error evaluation 19,33,40-43 C. attempts to exploit diffusion techniques in order to overcome the size and time-constant limitations of bulk resistance thermometers. Several promising reports have been given on the resulting diffused germanium resistors.44,45 The carbon resistance thermometer (CRT) has maintained its popularity in the field because it continues to provide high sensitivity at low temperatures in a low cost, very small package. Preferences vary as to the exact type, size and resistance of CRTs. The AllenBradley radio resistors in sizes from 0.1 to 1 W,t and resistance values from 10 to 500 R, remain the most popular choices above 1 K,46-52 and are used occasionally below 1 K.s3 The Speer Carbon Company radio resistors in various sizes and resistances are usually selected for the region below 1 K37J0p55*56yi49and may prove useful to as low as 0.02 K. [For those who are not familiar with their behaviour, a typical Speer 3 W, 220 0 resistor will measure roughly 1 kQ at 1 K, 20 kR at 0.1 K and 300 kS2 at O-015 K, the last at about IO-*J W dissipation.1491 Other realizations of the CRT have been in the form of glassy carbon s7 and carbon films.@+6s Typical of the wide variety of characteristics that may be obtained with colloidal graphite suspensions are the units described in a recent paper .63They are not as reproducible as standard CRTs, but their resistance remains in the kR region over as wide a range as 1.2-80 K. In terms of progress, the situation for the CRT is almost identical to that of germanium thermometers. While the reproducibility of the CRT is roughly an order of magnitude poorer than its cousin, there is little hesitation by most workers in making use of it, particularly as the body of literature on selection criteria and preparation and installation procedures continues to grow. “Thereareat least six commercial sources of germanium thermometers. Readers who would like information these suppliers, or any others (of transducers and equipment, or both) are invited to contact the author tit is not well known that the type CB, ,+ W, Allen-Bradley tors are obsolete and have been replaced by the type Since they are constructed differently, they have different characteristics. See references 51 and 84.

resistance relating to measuring directly. resisBB, t W. somewhat

15

One investigator 34finds an ‘aging’ rate of 10-J K/week for Allen-Bradley resistors at 4.2 K and about twice that for Speer resistors at 1 K. This can be taken care of by sufficiently frequent recalibration. Since this implies that recalibration is always necessary after thermal recycling, it is only fair to refer to some evidence that CRTs are stable to O-l%, even after two years of recycling.54 In one type of application-differential thermometry-the CRTs are used in pairs 37.64and any aging effect is less critical. There is also no lack of published resistancetemperatu.re relations for both AllenBradley 34J-VW%66-6~,69 and Speer 69,70resistors. Even in the attempt to achieve fast response, the analogy between carbon and germanium thermometers continues to hold. Response curves well into the kHz region were obtained on units fabricated by cutting very thin discs perpendicular to the long axis of a 1 W radio resistor 50 or spraying a carbon black suspension on a gold film nattern. There are other members of the non-metallic, negative temperature coefficient, cryogenic sensor family. &con carbide units, cut from ‘Globar’ heating elements were tested in the 4-10 K range.‘l Potentially far more significant are the results reported for commercially available thermistors at 4.2 K.‘* The fact that their sensitivity was found to be high-a dR/RdT of 1.4 K-lwas not surprising. However, the reproducibility with respect to thermal cycling turned out to be a remarkable 5 x 10-d K. An important statement made by the authors in connexion with this result was that there was ‘no conflict in the experimental data as comparable data under well-defined conditions has not previously been reported.’ The emergence of the p-n junction diode as a cryogenic thermometer is indeed a recent phenomenon. There are nine published papers73-81 and two commercial suppliers concerned with this device. Eight years ago little, if anything, was known about them. Germanium,73 silicon,74-77 and gallium arsenide ‘*-*O junctions have been investigated, but only the gallium arsenide units are available commercially. Their biggest advantage is unquestionably the wide temperature range over which reasonable resolution is maintained, viz, perhaps 10 millidegrees over the 2 to 300 K range.‘*-79 Because one measures the forward voltage drop across the junction at a constant current bias, the equipment needed to read-out is very similar to that used for the ‘potentiometric measurement of resistance thermometers. Thus, their place at the end of the section on resistance thermometers is not completely artificial. At least one analytical interpolation formula for gallium arsenide units is available81 and others will undoubtedly follow as their popularity increases. Before concluding this section, there should be some discussion of the effects of external influences on resistance thermometer characteristics. Hydrostatic pressures as high as 7 x 106 N/m2 ( > 60 atmospheres) have been found to exert little influence on carbon*2*83 and germaniuma3 thermometers. The effects of static magnetic fields can be much stronger. A number of the references cited up to this point present some pertinent, if scattered, In data on this subject. 8,25,30,45,47-49,52,55,56,58,63,72,18,80 addition, a recent paper 84 covers the results of measurements on & W, 4 W, and & W Allen-Bradley carbon 1

16

resistors over the 1.8-18 K range in static magnetic fields up to 150 kG. This is the initial phase of a programme that is proceeding at the author’s laboratory in the hope of providing a complete and unifying treatment of the problem. In the second phase, germanium and platinum resistance thermometers have been investigated,*5 and several other sensors are expected to be included in the near future. Preliminary results on gallium arsenide diodes86 indicate a much larger magnetic field effect than has been reported.80 At 4.2 K, changes of N - 0.5 K at 75 kG and N - 2.0 K at 150 kG were observed; at 77 K, the values were N - 1.5 K at 75 kG and N - 5 K at 150 kG. Only comparatively small bias current and orientation effects were seen. Thermocouples

The many advantages offered by thermocouples in the measurement of temperature are too well known to bear repeating. In the cryogenic region considered in this review, however, most of the familiar materials have serious drawbacks-lack of sensitivity and poor reproducibility, or both. Although the severity of the first of these disadvantages was reduced by the advent of the gold-2.1 atomic % cobalt leg (used against copper, silver-normal, manganin, etc.), the problem of poor reproducibility persisted.e’*** The variations found among samples-even from the same reel of wire-were probably due to the nature of the alloy. Because the cobalt is in supersaturated solution, several extraneous factors, such as thermal cycling and mechanical working of the wire, can actually alter the amount of cobalt in solution over a period of time. This has not prevented the gold-cobalt from being used within its limits. There are recent reports on the increased sensitivity attainable with gold-cobalt versus chromel.*9*9’J The one development that has finally led to a favourable position for thermocouples in cryogenic thermometry has been the emergence of dilute gold-iron thermoelements. Their value was first appreciated in 1962, in a practical sense,91 and the technology has been improving ever since. Gold plus 0.02 % iron-or Au-Fe(2) in the notation of the rest of this section-and Au-Fe(3) thermoelements were measured versus both silver-normal and copper over the temperature range 2 to 40 K.g2 A thorough investigation of Au-Fe(Z) and Au-Fe(7) versuschromel-P over the range O-4 to 85 K has been recently reported.93*94 Included were data on alloy inhomogeneity from length to length, temperature-cycling reproducibility, aging stability, and annealing effects. Other data have been reported for Au-Fe(Z), Au-Fe(3), Au-Fe(3*5), Au-Fe(6), and Au-Fe(7). These have been either in the form of absolute thermopower values, or coupled with silvernormal, copper, or chromel-P thermoelements.93-r03 The importance of the latter, variously referred to as ‘second or ‘passive’ thermoelements, is sometimes overlooked. However, valuable information is available *05~106 and helps one to select a material with low thermal conductivity and electrical resistivity, and a reproducible, positive thermopower, particularly in the vital region below 20 K. Interest in other potentiahy useful diIute alloys has extended to the combination of rare earth metals in gold and silver. 107 Preliminary indications were given CRYOGENICS.

FEBRUARY

1970

of a high value of thermopower in cerium- and europiumin-gold alloys. As in the case of resistance thermometers-and for the same reasons-it is important to know the effects of magnetic fields on thermocouples. Some data are available on Au-Fe elements in fields of up to 20 kG9*.95,96 and some preliminary results for fields up to 77 kG.*O* Of great potential importance is the recent demonstration 104that the use of an isothermal path for cryogenic thermocouple wires as they emerge from an applied magnetic field can ensure a field-independent thermocouple output. This encouraging development, along with the others described in this section augurs well for thermocouple thermometry. This is particularly true in the case of several of the varieties of gold-iron thermoelements used with chromel-P, where one can achieve a sensitivity of IO-20 uV/K over the range 2-80 K, along with a reproducibility usually better than 1%. Over the upper half of this range, the merits of the chrome1 versus constantan thermocouple (ISA type ‘E’) should not be overlooked; its sensitivity is the highest available above 40 K.101 Gas, vapour pressure, and acoustic thermometry Gas thermometry is usually associated with the thermodynamic temperature scale, rather than with any of the practical scales such as IPTS-68. Gas-thermometer measurements do have to be made very carefully and precisely, and so are apt to be slow; the gas bulb itself is relatively large and the over-all system tends to be awkward and inconvenient to use (at least in comparison with the thermometers discussed previously). On the other hand, to the experimenter who can live with these drawbacks, there is the tremendous advantage of being able to indepenciently calibrate a temperature sensor or check the accuracy of a temperature measurement. Another feature which is very significant in some experiments, is the lack of sensitivity to magnetic fields. Most recent papers in this field emphasize the results of measurements,32*108-ii6 rather than elaborating on the details of technique and construction which are generally assumed to be well known. Nevertheless, there is a reasonable amount of the latter information in the same references. This includes gas bulb and capillary size and design, details of the thermal and vacuum systems and controllers, type of pressure measuring instruments employed, etc. Equally important are the discussions of the many corrections involved (thermomolecular, dead volume, gas non-ideality, etc.). The convenience and sensitivity of vapour pressure thermometry have insured its popularity, notwithstanding the limited, discontinuous temperature ranges over which it may be used. The vapour pressure-temperature relationship of various gases does have a thermodynamic basis for its application to precision thermometry.ii7 Improved data on this relationship have become available for nitrogen,ii8~ii9 oxygen,119*120 argon,iiQ~i2i neon,i** hydrogen,119,123 and aHe.‘*+126 Empirical formulae for 3He and 4He have been developed for use with computers.**7vi*s There has also been an improved understanding of phenomena which can lead to unsuspected sources of error in vapour pressure measurements. Investigations have been reported on Kapitza resistance,‘*9 thermal relaxation,‘30 thermomolecular pressure corrections,i3i and thermal transpiration.i3* It CRYOGENICS B

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1970

has been suggested 133that by the proper dimensioning of the volume of a vapour pressure thermometer, it can be used as a combination condensation thermometer and gas thermometer. The acoustical thermometer has recently gained prominence due to the efforts of NBS to supplement gas thermometry and pressure-volume isotherm determinations in the 2 to 20 K region. A constant frequency, variable-path acoustical interferometer was developed to measure the speed of sound in helium gas.134 With this instrument, the isotherms of the speed of sound, as a function of pressure, were determined and extrapolated to zero pressure. Values of temperature were calculated from the resulting intercepts approximately every degree from 2 to 20 K. A new temperature scale-the NBS Provisional Scale 2 to 20 (1965)-was subsequently determined. From this and other work,‘35 it is apparent that the new thermometer and scale will prove useful, particularly in the difficult 4 to 14 K region. Two recent investigations have already made use of the new scale: one uses it to obtain new values of a virial coefficient of helium,‘36 and the other determines comparison values for other sca1es.i” Paramagnetic and nuclear magnetic resonance thermometry The principles of paramagnetic thermometry have been known and used for some time. By measuring the susceptibility x of substances which obey the CurieWeiss law, x = C/(T - 8) where C and 0 are constants, it is possible to determine T. Because the sensitivity of this method decreases with increasing temperature, it has usually been employed only at temperatures below 1 Koften as low as several millidegrees. Until recently, comparatively few laboratories were able to attain such low temperatures. However, with the advent of the JHe4He dilution refrigerator and superconducting magnetsto provide adiabatic demagnetization refrigeration-this picture has been strikingly altered. One of the results has been an increased interest in thermometry for this difficult region. Without going into great detail, it may be stated that the paramagnetic salt, cerium magnesium nitrate (CMN), is the pre-eminent choice for both a magnetic cooling and thermometric medium. 137In single crystal spherical form, its suceptibility follows the Curie-Weiss law down to about 0.006 K, which means that down to this point, the absolute thermodynamic temperature T is identical with the ma,qnetic temperature (denoted T*) defined by the Curie-Weiss law. Much of the literature in this field has been concerned with the problem of quantitatively determining the deviation (A) between Tand T*: a. in the region below 0.006 K 137--139 b. above 0.006 K, where the CMN sample is either not spherical, or not single crystal, or is lacking both features 140-144 c. for any other reasons, such as high values of the ratio of magnetizing field to temperature.r45 Fortunately, the thermometry is already sufficiently reliable so that many experiments requiring an accurate knowledge of the temperature below 1 K were made possible.S3.‘46-‘5*1’53 These include work on the thermal and/or magnetic properties of CMN itself. 17

The current status of CMN thermometry may be summed up by saying: a. spheroidal shape, single crystal specimens are usable for values of T between 0.006 and 2 K, with T = T* 139 b. the same specimens are also usable for values of T between 0.002 and O-006 K, with the (T - T*) relation determined by gamma-ray heating137 and nuclear orientation methods 138 spheres of a group of orientated single crystals C. should give better than 1% accuracy at temperatures down to 0.02 Kr44 d. carefully prepared cylindrical samples ofpowdered CMN can give l-2% accuracy at 0.02 K.143,147Although such samples (particularly those with a 1: 1 diameterlength ratio) have been carefully investigated at lower temperatures, there is a lack of agreement on the exact value of the delta deviations.r52prs4 The preceding summary should be considered strongly subject to change, since the field of CMN thermometry presently is in a rather turbulent state. One way to illustrate this is to quote from Hudson’s recent paperr both its title-‘The delta campaign: an account of the controversy surrounding the temperature scale for CMN’-and one sentence-‘The final act of this modest drama remains to be written.’ There has been other work on paramagnetic thermometry. Besides CMN, salts such as manganese ammonium sulphate and neodymium ethylsulphate (NES) have been used for cooling and thermometry. The temperature scale associated with NES was redetermined by comparing the results of gamma-ray heating, nuclear orientation, and magnetic susceptibility measurements.155 Some compounds in the rare earth trihalide family have been of interest because of their possible value as thermometric fixed points. Below 1 K, both CeC13 and NdC13 have exhibited a sharply peaked zero field differential susceptibility, the position of which could be determined to within a millidegree.*56 Temperatures have been derived from frequency measurements on a tuned circuit, the inductance of which encloses some paramagnetic salt such as CMN or manganese ammonium sulphate.ls7 The temperaturedependent magnetic field difference between a pair of ep r lines (such as is available in the K2CuC14.2Hz0 system) can provide a convenient temperature calibration technique.*58vls9 There are a variety of zero-field resonances in solids where the resonant frequency is temperature dependent. One such resonance has been exploited by several workers in the field for precision cryogenic thermometry -the nuclear quadruple resonance (nqr) of 35Cl in KC1 03. The most recent work 160-164covers temperatures from as low as 12 K to well beyond the cryogenic regions defined by this review. In fact, with an upper limit perhaps as high as about 500 K, the size of the range by itself is a tremendous advantage. It is not the only advantage, however, when one considers a reported sensitivity of 0.2 millidegree and an accuracy of better than 1 millidegree at 77 K. These values are actually representative of the entire 50-297 K range, but deteriorate by an order of magnitude by 20 K.163.164 The limitations of the nqr thermometer at the lowest 10

temperatures are not generally found in systems based on nuclear magnetic resonance (nmr). While the application of steady-state n m r techniques to thermometry is not new, there are advantages to be gained from a pulsed n m r system. The latter has been successfully used to measure spin-lattice relaxation times in the O-022 K region. In addition, with refrigeration supplied by the famous Clarendon Laboratory method of nuclear cooling, the system also was able to detect nuclear spin temperatures as low as 10m5K.‘65 Very recent results 166 have demonstrated the great promise shown by zero-field nmr in ferromagnetic and antiferromagnetic materials. In particular, a system based on the sharp 19 F resonance in antiferromagnetic MnF2 along with a frequency locking autodyne circuit has demonstrated reproducibility to 0.1 millidegree at 20 K. Even more important, there is every reason to believe that this performance would be available over the entire IO-40 K range. Thus the MnFz system nicely complements the KC1 O3 system and leads one to predict a rosy future for the new field of quantum electronic thermometry. Other

methods

of thermometry

This section deals with those methods of cryogenic thermometry that do not comfortably fit into one of the previously presented categories, or still tend to be in a preliminary stage of development. One potentially significant example of this genre is the thermal-noise thermometer. This device originally appeared promising at temperatures as low as 2 K, with accuracies quoted as I%,‘67 or 10% 168at the low end of the range. The higher of the two quoted accuracies was undoubtedly a result of a correlation technique involving three resistors and two amplifiers.*67 More recently, it has been suggested 169 that with the aid of a simpler and more effective correlation scheme,r70’171 the temperature range could be extended to below 1 K and the accuracy improved. Another approach utilizes noise voltage pulse-counting techniques which can provide more information than a measurement of rm s noise power.17*s173Thus, a sensitivity of better than 0.01 K at 4.2 K is achievable with a five minute counting period, assuming optimum design of the system.r73 Perhaps the superconducting Josephson junction will eventually provide the basis for the most accurate noise thermometer. Since it has been shown that the line width of Josephson radiation was temperature dependent,174 a subsequent suggestion was made to use the phenomenon in a thermometer system with a theoretically possible millidegree resolution.17s Because of the many possible advantages offered by noise thermometers in general-absolute temperature measurements over a wide range, absence of sensing power, and freedom from dc magnetic field effects-it should not be overlooked. An entirely different aspect of superconductivity-the fact that different materials have different transition temperatures-has been exploited for thermometry. The sharp transitions made by pure metals (Pb, In, Ga, Zn, Cd, etc.) were studied for suitability as fixed points.r76 Alloys of copper and lead 177and of cadmium and zinc,178 with much broader transitions, enabled measurements to be made in a series of narrow temperature intervals in the O-6-7 K range. CRYOGENICS

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The quartz-crystal thermometer has proved to be a useful and very precise instrument at temperatures in the ambient range. Unfortunately, the sensitivity drops drastically at much lower temperatures, even for optimum vibration modes and crystal cuts. What is not generally appreciated is that what remains can still be quite useful. At nitrogen temperatures,179 tenths of a millidegree could be resolved, although the reproducibility after cycling was only about 0.01 K. At 4 K, with a properly cut 10 MHz crystal, approximately 50 Hz/K to 100 Hz/K sensitivity is realizable.r*O*r** Apart from the obvious advantage of digital output, the device is attractive because of the negligibly small influence of magnetic fields.18r Another thermometer with a natural digital output is based on inductors with a large temperature coefficient of inductance.‘82 Special alloys with new and unusual magnetic properties were used in a system similar to that employed for quartz-crystal thermometers. Resolution as high as a millidegree at 2 K was obtained, and the sensitivity was actually increasing at 1.2 K. An extension to low temperatures of studies on the pyroelectric thermometer Is3 has added another valuable tool to aid the experimentalist, particularly those engaged in calorimetry. The applicability of various ferroelectric ceramics to the measurement of temperature change or rate of change has been demonstrated. From the standpoint of its low value of noise equivalent temperature change (about a microdegree at 10 K), the low temperature pyroelectric thermometer is at present superior to any other device at temperatures above about 10 K. As a bonus, it should prove to be unaffected by magnetic fields, just as another member of the same generic family. The latter, a capacitive sensor, utilizing the temperature dependence of the dielectric constant, has been briefly studied .184 The most successful type was reproducible to about 0.01 K at 4.2 K, with respect to thermal cycling. Accuracy no worse than + 0.05 K over the entire 2-300 K range seems feasible. The lack of a convenient primary standard between about 30 and 50 K may be overcome if a recently proposed principle can be fully implemented. It utilizes the known equilibrium composition-temperature relationship for para-ortho mixtures of molecular hydrogen.185 A precision of about 0.01 K at 40 K has been demonstrated, along with a relatively rapid response. Before leaving the subject of temperature sensors, a note of explanation is probably called for. Because of the author’s professional interest in high magnetic fields, perhaps the concern for magnetic-field-independent temperature measuring devices is excusable. Many laboratories now own or will soon purchase superconducting magnets, so this problem can be expected to affect an increasing number of experimentalists. Instrumentation and techniques The title of this section implies a body of material that could easily exceed the length of all that has gone before. While there is no intention of presenting such a complete treatment, it might be useful to indicate some of the important recent advances in the equipment and apparatus that is associated with the read-out for cryogenic sensors. The significant variable involved in gas and vapour CRYOGENICS.

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1970

pressure thermometry is pressure in the range 10-3-103 ‘c. Mercury, oil, or water manometers continue to be very important in the upper four decades. They are still susceptible to improvement, as demonstrated in reports concerning a micrometer U-tube manometer.r86-187 For example, uncertainties of only 4 x lO-4 r plus one part in 104 of the reading were estimated for an oil-filled unit. A two-j&d manometer has been suggested for improved performance.188 Cold-trapped McLeod gauges have been a long accepted standard for low pressures.189 However, they do have some serious shortcomings, including systematic errors recently attributed to mercury pumping132.190-r92 and thermal transpiration.192 A very promising substitute has been found in the capacitance manometer.r93,r94 Not only are accuracies of several per cent achievable at pressures as low as 1O-4 r, but the measuring range extends over many orders of magnitude. Similar performance can be expected from the quartz Bourdon tube pressure gauge.s3 There are other electricaloutput pressure measuring systems which use transducers of different types. They may all prove useful over various parts of the required pressure range. The technology associated with the measurement of small ac and d c voltages has burgeoned in the last ten years. Commercially available d c instruments can detect as little as a nanovolt at reasonable impedance levels and in the presence of large common mode signals. Battery operation and advanced guarding techniques greatly reduce problems due to ground loops and some kinds of pick-up. Amplifier precision in some of these instances is phenomenal: one unit (d c) provides 5 ppm input-to-output linearity, along with better than 0.01% gain stability and 10 nV peak-to-peak noise. Despite the favourable amplifier-detector situation, problems associated with measurements at low temperatures still remain, viz, thermal emfs in the leads and Johnson noise levels usually well below amplifier noise levels. Fortunately, cryogenic technology-both superconducting and the normal variety-has been advancing so rapidly that a number of palliatives are available. Reports on low temperature amplifiers and modulators, using superconducting techniques,195-200 or not,201,202,203 are very helpful. This is particularly true in the case of very low source resistances, that is, thermocouples and some metallic resistance thermometers. One of the devices described-a superconducting parametric amplifier r98-has been the basis of a picovoltmeter which is now commercially available.*99 A C techniques are often preferred or necessary in many of the situations encountered in cryogenic thermometry. AC amplifiers have improved in performance, most notably in their low noise front-ends. This has been due to new techniques, better design, and availability of low noise nuvistors, and bipolar and field-effect transistors.204~209 For those who are interested, an excellent review of low noise audio-frequency amplifiers has recently been published.207 Also associated with ac techniques are the instruments variously known as phasesensitive detectors, lock-in amplifiers, coherent amplifiers, synchronous detectors, etc. There are over a dozen commercial instruments available, most of them with excellent performance characteristics. The manufacturers are able to supply application literature, but one very recent and useful application should be noted.2” In 19

effect, it is the transformer-coupled telemetering of temperature information from a carbon thermometer mounted’ on a microbalance suspension in a cryogenic environment. Another choice (besides ac versus dc) facing the user of resistance thermometers is that of bridge or potentiometric methods. Fortunately, the technology in both areas has continued to advance and many of the contributions have appeared in the literature. In addition to a recent over-all review of both methods,2’2 there are papers on bridges,zlJ-219 a c methods 223-230 and potentiometric techniques,‘9v2’9-222 (with application in both areas). The Dauphinee potential comparator is favoured by many for resistance thermometer applications. For those who are unfamiliar with its design and capabilities, a good description is given in a less-recent over-all review of potentiometric methods.231 Modified versions of this circuit have also proven useful.232-234 There also have been improvements in many auxiliary instruments and devices. There are very high stabilities available in constant current supplies (even in the microampere region), Zener diode references, resistors, KelvinVarley dividers, and ratio transformers. When combined with previously mentioned voltage amplifiers, detectors, and meters, very accurate and versatile measuring systems are made possible. Also not to be overlooked are the high quality analogue and digital integrators and the signal averagers and correlators that

REFERENCES (Note. References to the two Soviet journals Prihory i Tekhnika Eksperimwla and Izmerirel ‘IKI~UJ Tekhnika are given with the page numbers in their English translations, Ins/rrrtnen/s oad Esperirnenrol Techniqaes and Mecsrfrenzenf Techniqrws. respectively.) 1. CORRUCCINI, R. J. Adoances in Ct:yofeRenic Ea,rineering 8, 315 (1962) 9 2. TIMMERHAUS, K. D. Cryogenic Technology, p. 196 (Wiley, NY, 1963) 3. ORLOVA, k. P. Meas. Techn., No. 4, 489 ( 1964) 4. COMITY CONSULTATIF DE THERMOMI!TRIE, Melrolqslia 5, 35 (1969) 5. BENEDICT, R. P. L. and N. Tech. Jour., No. 6, 2 ( 1969) 6. BEDFORD, R. E., PRESTON-THOMAS, H., DURIEUX, M., and MUIJLWIJK, R. Merrologia 5, 45 (1969) I. SHAREVSKAYA. D. I., ORLOVA, M. P., BELYANSKY, L. B., and GALOVSHKINA, L. B. Proc. ICEC2, p. 222 (ISTP, London, 1968) 8. ORLOVA, M. P., ASTROV, D. N., ALSHIN, B. I., and ZORIN, R. V. Proc. ICECZ, p. 231 (ISTP, London, 1968) 9. BERRY, R. J. Can. J. P/lvs 41,946 (1963) 10. CORRUCCINI, R. J. J. kes. Nat. Bar. -Std. 69C. 283 (1965) II. VAN DIJK, H. Phvsica 30. 1498 (1964) 12. BERRY, R. J. Car; J. Whys 45, 1463 (i967) 13. Kos, J. F., and LAMARCHE, J. L. G. Can. J. P+ 45, 339 (1967) 14. BERRY, R. J. Merrologia 3, 53 (1967) 15. ORLOVA, M. P. et al Metrologia 2, 6 (1966) 16. HOLLAND, M.G., RUBIN, L. G..and WELTS, J.Temperature, Its Measurement and Control in Science and Industry, Vol. 3, pt 2, 795 (Reinhold, NY, 1962) 17. ‘GEHRINC, F. D., and GERSTEIN, B. C. Reu. Sci. Instr. 38, 280 (1967) 18. JOHP&ON; W. V., and LINDBERG, G. W. Rev. Sci. Ins/r. 39, 1925 (1968) 19. KLEIN, M. V., and CALOWELL, R. F. Reu. Sci. Insfr. 37, 1291 (1966) 20. YET-CHONG, C., and FORREST, A. M. J. Sci. law. 1, 839 (1968) 21. BJ:ODSKII, A. D. Meas. Techn., No. 4, 455 (1968) 22. JAMFS, B. W., and YAT~, B. J. J. Sci. Imtr. 40, 193 (1963) 29

make very low signal levels. more accessible to those willing to. trade time for improved signal-to-noise ratio. It would appear safe to say that contemporary equipment and instruments are well suited to their complementary role with the wide range of contemporary cryogenic transducers. Although this review has not been intended to cover cryogenic techniques, the subject of low temperature thermal contact agents and adhesives is sufficiently pertinent to thermometer users to justify at least a brief mention. Various greases have been used to help reduce thermal boundary resistances. These have included silicon, Apiezon N, Apiezon L, Eccotherm and Cry-Con greases. Comparison data on the thermal conductivity of most of these materials has been published.235,23GJ37 The Cry-Con grease (a copper loaded, electrically insulating grease) has the highest thermal conductivity, and can be quite useful above I K. However, in the pressed contacts often used at lower temperatures, it is apparently inferior to Apiezon N. The latter’s lower viscosity, and hence its ability to form very thin bonding layers, is at least partially responsible for this. The thermal conductivity of several electrically insulating adhesives has been measured; the results are reported, along with a useful collection of data on other materials over the 4-300 K range.238 This work was supported of Scientific Research.

by the US Air Force Ofice

23. ORLOVA. M. P., AsrRov, D. N.. and MEDVEDEVA, L. A. Cryqpetrics 4. 95 ( 1964) 24. Kos, J. F., DROLET, M., and LAMARCHE, J. L. G. Can. J. fhys 45. 2787 (I 967) 25. MEADEN, G. T. CrljoRenics 6. 275 (I 966) 26. GORDON. J. E., and AW.TUTZ, L. I. Cryppenics 5, 329 (1965) 27. MIKHAILOV, N. N., and GOVOR, A. Y. Cryqpenics 3, 205 (1963) 28. Manufacturer’s specifications 29. BLAKEMORE, J. S. Rev. Sci. Ins/r. 33, 106 (1962) 30. BLAKEMORE, J. S., SCHULTZ, J. W., and MYERS, J. G., Rev. Sri. Ins/r. 33, 545 (1962) 31. ORLOVA. M. P.. ASTROV, D. N., and MEDVEDEVA, L. A. Cryogenics 5, 165 ( 1965) 32. OSRORNE. D. W., FLOTOW, H. E., and SCHREINER, F., Rec. Sci. lnsfr. 38, 159 ( 1967) 33. CLAIBORNE, L. T.. HARDIN, W. R., and EINSPRUCH, N. G. Rea. Sci. Insrr. 37, 1422 ( 1966) 34. COCHRAN, J. F.. SHIFFMAN, C. H., and NEIGHBOR, J. E. Reu. Sri. Instr. 37, 499 (I 966) 35. ANTCLIFFE, G. A., EINSPRUCH, N. G., PINATTI, D. G., and RORSCHACH, H. E. Rev. Sri. Instr. 39, 254 (1968) 36. CULBERT, H. V., and SUNGAILA, Z. Cryqenics 8, 386 (1968) 37. HARRISON, J. P. Rev. Sci. Insfr. 39, 45 (1968) 38. SARVER, C. E., and BLAKEMORE, J. S. Cryogenics 7, 299 (1967) 39. EDLOW, M. H., and PLUMB, H. H. J. Res. Nat. Bar. Std. 7OC, 245 (1966) 40. EDLOW. M. H., and PLUMB, H. H. J. Res. Nat. Bar. S/d. 71C, 29 ( 1967) 41. CATALAND, G., and PLUMB, H. H. J. Res. Nat. Bar. Std. 70A, 243 f 1966) 42. AHLERS, G., and MACRE, J. F. Rev. Sci. Insrr. 37, 962 (1966) 43. SCHRIEMPF, J. T. Cryogenics 6, 362 (1966) 44. HERDER, T. H., OLSON, R. 0.. and BLAKEMORE, J. S. Rev. Sci. Insrr. 37, 1301 (1966) 45. COHEN, B. G.. TRETOLA, A. R., and LILIENTHAL, R. Rev. Sci. Insfr. 37, 1689 (I 966) 46. SCHULTE, E. H. Cryogenics 6, 321 (1966) 47. AMUNDSEN. T. Cryog&ics 7, 368 (I 967) 48. HUDSON, W. R. Rev. Sci. Iasfr. 39, 253 (1968) 49. CHARI, M. S. R. Indian J. Pure and Appl. Phys 5, 482 (1967) CRYOGENICS

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50. MEZHOV-DEGLIN. L. P., and SHAL’NIKOV, A. I. Insl. Exp. Techn., No. 5, 1288 (1965) 51. BORCHERDS. P. H. Crweenics 9. 138 (1969) 52. BELANGER, b. C. Rev: &i. Ins&. 40, iO82 il969) N. H., and GRENIER, C. G. Phvs 53. BLEWER, R. S., ZEBOUNI, Rev. 174, 700 (1968) H. Proc. LTI I, discussion of paper Dl.3, 506 54. WEINSTOCK (1968) 55. BLACK, W. C., ROACH, W. R., and W~IEATLEY, J. C. Rev. Sci. Insw. 35, 587 (I 964) 56. EDELSTEIN, A. S., and MESS, K. W. Physica 31, 1707 (1965) 57. KODAMA, T., WADA, S., SHIGI, T., and OKUDA, T., Proc. ICECI, p. 47 (ISTP, London 1967) 58. HORNUNG, E. W., and LYON, D. N. Rev. Sci. Ittsrr. 32, 684 (1961) 59. DUPR& A., VAN ITTERBEEK, A., MICHIELS, L., and VAN NESTE, L. Cryogenics 4, 354 (1964) 60. CANNON, W. C., and CHESTER, M. Rev. Sci. Insk. 38, 318 (1967) 61. BROWN, C. R., and MATTHEWS, P. W. Rev. Sci. Insrr. 39, 616 ( 1968) 62. TERRY, C. REV. Sci. Ittstr. 39, 925 ( 1968) 63. STAR, W. M., VAN DAM, J. E., and VAN BAARLE, C. J. Sci. Instr. 2, 257 (1969) 64. SOUSA, J. B. Cryogenics 8, 105 (I 968) 65. JELLISON, J. C., and COLLIER, R. S. Advances in Cryqeenic _ ” Eq~itteerittg 14, 322 (I 969) 66. KALINKINA. I. N. CrvoFenics 4. 327 (1964) 61. CUNSOLO, ‘S., SAN;&, M., and VIC~NTINI-MISSONI, M. Cryogenics 5, 168 (I 965) 68. CRAIG, P. P. Crygpenics 6, 112 ( 1966) 69. RAFALOWICZ, J. and SUJAK, B. Acra Phys Polon. 25, 193 (1964); Acra Phys Polon. 25, 599 ( 1964) 70. HETZLER, M. C., and WALTON, D. Rev. Sci. Itutr. 39, 1656 (1968) 71. BROWN, R. E., HUBBARD, W. M., and HABEN, J. F. Rev. Sci. Itrsrr. 33. 1282 ( 1962) 72. SCHLOSSER, W. F., and MUNNINCS, R. H. Rev. Sci. Ins,r. 40, 1359 (1969) 73. BARTON, L. E. Eledronics 35, 38 (4 May 1962) 14. UNSWORTH, J., and ROSE-INNES, A. C. Cryogenics 6. 239 (1966) 75. LOGVINENKO, S. P., and BROVKIN. Y. N. Itrstr. Exp. Techn., No. I, 221 (1968) 76. SHEATHERS, P. R. Cryogenics 8, 393 (1968) 77. MEULEMANS, H. L. F., and VERBEKE, 0. Cryqyenics 8, 398 (I 968) 78. COHEN, B. G., SNOW, W. B., and TRETOLA, A. R. Rev. Sci. Insrr. 34, 1091 ( 1963) 79. DMITRENKO, I. M., LOGVINENKO, S. P., IVANOV, N. I., and KOLOT, Z. M. Cryqenics 6. 239 ( 1966) 80. ARENDS, J., and WRIGHT, R. C. Cryogenics 9, 281 (1969) 81. PRADDAUDE, H. C. Rev. Sci. Ittstr. 40, 599 (1969) 82. LESTER, D. H., and BRONSON, J. C. Cryqrenic En,,ineeriop ., _ I. News 26, (Dec. 1968) 83. DEAN. J. W.. and RICHARDS. R. J. Advances in Crvopenic .. CI Et@tt~erin,~ i3, 505 (1968) 84. NEURINGER, L. J., and SHAPIRA, Y. Rev. Sci. Instr. 40, 1314 (I 969) 85. RUBIN, L. G., NEURINGER, L. J., and PERLMAN, A. (forthcoming) 86. RUBIN, L. G. (forthcoming) 87. POWELL, R. L., BUNCH, M. D., and CORRUCCINI, R. J. Cryogenics 1, 139 (1961) 88. POWELL, R. L., CAYWOOD, L. P., and BUNCH, M. D. Temperature, Its Measurement and Control in Science and Industry, Vol. 3, pt. 2. p. 65, (Reinhold, NY, 1962) 89. KOEPPE, W. Cryogenics 7, 172 (1967) 90. KOEPPE, W. Proc. ICECZ, p. 213 (ISTP, London, 1968) 91. MACDONALD, D. K. C., PEARSON, W. B., and TEMPLETON, I. M. Proc. Roy. Sot. A266, 161 (1962) 92. BERMAN, R., BROCK, J. C. F., and HUNTLEY, D. J. Cryogenics 3, 70 (1963) 93. ROSENRAUM, R. L. Rev. Sci. Instr. 39, 890 (1968) 94. ROSENBAUM, R. L. Rev. Sci. Instr. 40, 578 (1969) 95. BERMAN, R., BROCK, J. C. F., and HUNTLEY, D. J. Advances in Cryogenic Engineer@ 10, 233 (I 964) 96. BERMAN, R., BROCK, J. C. F., and HUNTLEY, D. J. Cryogenics 4, 233 (1964) CRYOGENICS.

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1970

97. FINMMORE, D. K., OSTENSON, J. E., and STROMBERG, T. F. Rev. Sci. Instr. 36, 1369 (1965) 98. ROSENBAUM. R. L., ODER, R. R., and GOLDNER, R. B. Cryogenics 4, 333 (1964) J. T., and SCHINDLER, A. I. Cryogenics 5, 174 99. SCHRIEMPF, (1965) 100. SCHRIEMPF, J. T., and SCHINDLER, A. I. Cryogenics 6, ‘301 (1966) 101. SPARKS, L. L., POWEI.L, R. L., and HALL, W. J. NBS Report 9712 102. BERMAN, R., KOPP, J., SLACK, G. A., and WALKER, C. T. Phys Lerr. 27A, 464 (1968) 103. KUTZNER, K. Cryqrenics 8, 325 (1968) 104. RICHARDS, D. B., EDWARDS, L. R., and LEGVOLD, S. J. Appl. Phys 40, 3836 (1969) 105. CRISP, R. S., and HENRY, W. G. Cryogenics 4, 361 (1965) 106. SPARKS, L. L., and HALL, W. J. NBS Report 9719 107. GAINON, D., DONG& P., and SIERRO, J. Sol. St. Comm. 5, I51 (1967) 108. Temperature, Its Measurement and Control in Science and Induitry, Vol. 3. (Reinhold, NY, 1962). There are 3 papers in Part 1: MOESSEN, G. W., ASTON, J. G., and ASCAH, R. G., p. 90; BARBER, G. R., p. 103; BOROVICK-ROMANOV, A. C. et al, p. I 13. Also, see reference 16. 109. FRANCK, J. P., and MARTIN, D. L. Can. J. Phys 39, 1320 (1961) 110. BARBER, C. R. Bril. J. Appt. PhJu 13, 235 (1962) 111. BARBER, C. R., and HORSFORD, A. Merrolqpia 1, 75 (1965) 112. ROCERS, J. S.. TAINSH, R. J., ANDERSON, M. S., and SWENSON, C. A. Metrologia 4, 47 (1968) 113. MARTIN, D. L. Ph,~s Rev. 141, 576 (1966) 114. HOLTEN, D. C. Advances in Cryogenic Engineering 9, 406 (1963) 115. COFFEY, H. T., and FAYCHAK, G. J. Proc ICECI, p. 49 (ISTP, London, 1967) T., MITSUI, K., TAKAHASHI, M., andSHmAToRI,T. 116. MOCHIZUKI, Proc. ICECZ, p. 65 (ISTP, London, 1968) 117. VAN DIJK, H. Physica 32, 945 (1966) 118. STROBRIDGE, T. R. NBS Tech. Note 129 (1962) 119. RODER, H. M., MCCARTY, R. D., and JOHNSON, V. J. NBS Tech. Note 36/ (1968) 120. MOCHIZUKI, T., SAWADA, S., and TAKAHASHI, M. Japan J. A/@. Phys 8, 488 ( 1969) 121. BOWMAN, D. H., AZIZ, R. A., and LIM, C. C. Can. J. Phys 47, 267 ( 1969) 122. GRILLY, E. R. Cryogenics 2, 226 (1962) 123. BARBER, C. R., and HORSFORD, A. Brit. J. App. Phys 14, 920 ( 1963) 124. ROBERTS, T. R., SHERMAN, R. H., SYDORIAK, S. C., and BRICKWEDDE, F. G. Progress in Low Temperature Physics, Vol. 4, Chapter IO (John-Wiley, NY, 1964) ’ 125. MCCONVILLE. G. T.. WATKINS. R. A.. and TAYLOR. W. L. Ann. Acad. sci., Fennicae: Se; A VI ‘No. 210, 44 (1966) 126. WATKINS, R. A., TAYLOR, W. L.. and HAUBACH, W. J. J. Chem. Phys 46, 1007 (1967) 127. MONTGOMERY, H. Cryogenics 5, 230 (1965) 128. SAMBONGI, T., and MAEDA, I. J. Phys Sot. Japan 21, 2128 (1966) 129. MONTGOMERY, H., and PELLS, G. P. Brir. J. Appl. Phys 14, 525 ( 1963) 130. VAN MAL, H. H. J. Sci. Insfr. 2, 1 I2 (1969) 131. MCCONVILLE, G. T. Cryogenics 9, 76 (1969) 132. EDMONDS. T.. and HOBSON. J. P. J. Vactrum Sci. Technol. 2. 182 (1965) 133. BEWILOGUA, L. Proc. ICECZ, p. 222 (ISTP, London, 1968) 134. PLUMB, H., and CATALAND, G. Merrologia 2, 127 (1966). This is the latest in a series of papers by these authors dating back to 1962, some also involving EDLOW, M. H. References to the earlier works are given in this very complete paper. 13.5 BRODSKII, A. D. Meas. Techn., No. 6, 671 (1967) 136. BOYD. M. E., LARSEN, S. Y., and PLUMB, H. J. Res. Nat. Bur. Std. 72A, 155 ( 1968) 137. HUDSON, R. P., and KAESER, R. S. Physics 3, 95 (1967) 138. FRANKEL, R. B., SHIRLEY, D. A., and STONE, N. J. Phrs Rev. 140, Al020 (1965); 143, 344 (1966) 139. ABEL, W. R., ANDERSON, A. C., BLACK, W. C., and WHEATLEY, J. C. Physics 1, 337 (1965) 140. WHEATLEY, J. C. Ann. Acad. Sci. Fennicae; Ser A VI No. 210, 15 (1966) 21

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FEBRUARY

1970