Isotopically pure He4

.4-A

The rotation of the rotor and its longitudinal movement inside the transformer case are achieved with the help of a compound connecting rod passing inside the space of the central conductor of the coaxial line. The section of the rod at the place where it emerges from the central lead of the line is made of capron tube, while the section which passes through the vacuum packing is a polished metal rod. Translational motion of the rod is produced by a micrometer screw placed in the upper part of the device, and rotation by a lever fixed to the upper part of the rod. A rotatory joint makes the two motions mutually independent. The specimen to be studied, 5, with the plate tranducer 6 stuck to it, is placed in the holder between the matching devices at the point where the spring contacts 7 of the central lead of the coaxial line emerge. An In wire seal provides for rapid change of specimens and for reliable electrical contact where they are placed. Lithium niobate plates with a fundamental resonance frequency of 100 to 200 MHz were used as transducers for logitudinal and transverse ultrasonic waves. Evaporated film transducers of ZnO or CdS can be used. The device operates in the range 0.8 to 2.0 GHz at harmonics of the fundamental resonance frequency of the piezoelectric transducer. In a 5% frequency band the voltage standing wave ratio is ~ 3. The device described was used for studying the non-linear attenuation of sound in metals 4 and also in studies of the acoustic-magnetoelectric effect in semiconductors.S

Fig.3 Design of the matching device 1 - capacity transformer, 2 - inductive stub, 3 -- transformer case, 4 - movable capacity attachment. 5 - specimen being studied, 6 -- piezoelectric transducer, 7 - spring contact

former is formed by the walls of case 3 (the stator) and the metal attachment on the central lead of the coaxial line 4 (the rotor). Because the stator and rotor have the profile shown in Fig,3b, their relative motion leads to a change in capacitance.

References 1 Korolyuk, A. P., Matsakov, L. Ya., Fal'ko, V. L. JETP 54

(1968) 3 2 Leisure, R. G., Bolef, D. 1. Rev Sci Instr 39 (1968) 199 3 Korolyuk, A. P., Roi, V. F. Patent No 397837;Buli OIPOTZ 37 (1973) 169 4 Korolyuk, A. P., Khotkevich, V. I., Obolenskii, M. A., Belitskii, V. I. J E T P L e t t 18 (1973) 32 5 Korolyuk, A. P., Roi, V. F. Phys and Techn o f Semiconductors 3 (1972) 386

Isotopically pure H e 4 P. P. Fatouros, D. O. Edwards, F. M. Gasparini, and S. Y. Shen Very pure He 4 is often desired in studies of its properties in liquid and solid form. Commercially available He 4 contains a few parts per 10 million of He 3. This amount of He 3, though seemingly small, does have a significant effect for example, on the surface properties of liquid He 4 at very low temperatures 1 and on phonon scattering in solid He4. 2 Mezhov-Deglin 3 has described a method in which commercial He 4 is purified by 'filtering' it through a superleak. We use the same principle but our experimental arrangement is

The authors are w i t h the Department of Physics, the Ohio State University, Columbus, Ohio 4 3 2 1 0 , USA. F M G is at present at the State University of New Y o r k , Buffalo, N Y and SYS is at present at Northwestern University, Evanston, Illinois. This research was supported in part by a grant f r o m the US National Service Foundation GH-31650 A 2. Received 26 September 1974.

CRYOGENICS. MARCH 1975

simpler and gives a purer sample. The experimental set-up is shown in Fig.1. The 400 cm 3 copper vessel in which pure He 4 is collected, is connected to the main bath via a Vycor 4 superleak. A stainless steel needle valve when opened at about 1.1 K allows the superfluid component to flow into the vessel through the superleak. A 200 f2 heater made of manganin wire is wrapped around the bottom of the vessel and may be connected to an external battery. The amount of helium flowing into the vessel can be determined from the level drop in the bath; typically 200 cm 3 h -l of liquid are collected. After the vessel is filled the needle valve is closed and the heater is used, by dissipating 0.5W, first to evaporate the bath liquid and then the liquid in the vessel. Vycor 3 mm in diameter and 1 cm long in a brass-Stycast housing was used. s The measured volume flow rate of

147

room temperature He 4 gas was 1.5 x 10 -7 std cm 3 sq torr -1 The superfluid He 4 flow rate as determined from the level drop was 5.5 x 10 -2 cm 3 o f liquid per second equivalent to 45 std cm 3 s-1 at 1.1 K. Vacuum

¢~

Helium obtained in this way was analysed by mass spectrometry 6 and contained 0.4 -+ 0.2 parts per billion o f He 3 (0.4 + 0.2 x 10-9). In reference 3 the purity was estimated to be better than 50 x 10 "9.

t...I ...........

References

Fig.1

The experimental arrangement

1 Eekardt, J. R., Edwards, D. O., Fatouros, P. P., Gasparini, F. M., Shen, S. Y. Phy Rev Lett 32 (1974) 706 2 Mezhov-Deglin, L. P., Zh Eksp TeorFiz 49 (1965) 66 [Soy Phys, JETP 22 (1966) 47] 3 Mezhov-Deglin, L. P. Prib i Tekh Eksper No 3 (1971) 217 [Cryogenics 12 (1972) 311] 4 Code 7930 porous glass, Corning Glass Works, Coming, New York 5 Wilson, M. F., Edwards, D. O., Tough, J. T. Rev Scilnstr 39 (1968) 134 6 We are grateful to the Bureau of Mines, United States Dept. of the Interior, Amarillo, Texan for analysing our He4 sample

Thin film superconducting quantum interferometers W. Richter

and G. Albrecht

Thin trim superconducting quantum interferometers have a number o f important advantages compared with point contact interferometers. They are insensitive to vibrations and knocks, are stable with time, and do not require adjustment. Various authors have previously made many attempts to prepare stable thin film SQUIDS. Since Josephson tunnel contacts were unreliable, Dayem bridges 1,2 were used in preference. Goodkind and Stolfa 3 used a cylindrical film interferometer with a single contact which was, like a bulk interferometer with one contact, inductively in a radiofrequency oscillatory circuit. 4 A fairly complicated apparatus was required to work with these interferometers. In addition, it was essential in many cases to have stable temperatures since the interferometer has to work at a temperature ~ 0.1 K below the critical temperature, s,6 In the present work a thin film quantum interferometer is described, which works at 4.2 K without temperature stabilization. It has two Josephson contacts and therefore does not require a radiofrequency apparatus for its operation. It can work b o t h as a magnetometer and as a superconducting current measuring device (similar to Clarke's drops 7), since it has a control wire. It can easily be connected into any circuit. It is not critically dependent on establishing an operating point. The

interferometer

U

/

Zcon~. Fig.1 Design of thin film interfer0meter 1 - glass substrate, 2 - InSn contact, 3 - Josephson junction (Dayem bridge), 4 - interferometer area, 5 - control wire, 6 - Ag shunt film; lop -- operating current,/cant -- control current, V - interferometer output voltage

design

A.plane glass plate 10 x 4 x 1 m m 3 acts as substrate for the

interferometer. The design o f the interferometer is shown schematically in Fig. 1. It is a symmetrical structure and The authors are with the Physics Departmentt Friedrich Schiller University, Jena, East Germany. Prib i Tekh Eksper No 4 (1974) 205. Received 25 November 1974.

148

has four InSb alloy (eutectic) contacts to which leads are soldered. Dayem bridges act as Josephson junctions. The bridges are 1 # m long and 1/am wide. They consist of three films o f Ag, Pb, and In deposited on top of one another with a total thickness of 1000/~. The control wires are 10/am

C R Y O G E N I C S . MARCH 1 9 7 5