Contains reports on five research projects.Office of Naval Research (Contract N5ori-07823

Cavileer, R. P.

Collins, S. C.

Herlin, M. A.

Mayper, V., Jr.

Prince, M. B.

Siegel, L. A.

Toomey, J. W., Jr.

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IV. LOW-TEMPERATURE PHYSICSProf. J. C. Slater R. P. CavileerProf. S. C. Collins R. D. MaurerProf. M. A. Herlin V. Mayper, Jr.Dr. L. A. Siegel M. B. PrinceN. I. Adams III G. E. ReisB. Bernstein A. R. SearsJ. W. Toomey, Jr.A. HELIUM LIQUEFIERSOn several occasions the liquefiers have been operated with the loverengine alone - letting the refrigeration from the liquid nitrogen's pre-cooling on the radiation shield be substituted for the upper engine. Whenthis was done it was observed that the cooling-down time increased aboutthree-quarters of an hour. Also the rate of liquefaction was lower. Inreassembling the No. 1 liquefier the large or top engine will be omitted,and the inlet gas will be led through an eighteen-foot length of coppertubing which is coiled and which will be immersed in a bath of liquidnitrogen as shown in Fig. IV-1.Such a scheme should truly substitute for the large engine and givethe advantage of being able to maintain a pressure of 200 lbs/sq. in. onthe lover engine and the Joule-Thomson valve. This scheme is applicableonly where there is an ample supply of liquid nitrogen available at alltimes.Figure IV-2 illustrates a vessel which has been constructed to holdliquid nitrogen and accommodate the intermediate supply dewar for liquidhelium. R. P. Cavileer, J. W. ToomeyThe construction of the large helium liquefier and cyclic demagnetiza-tion apparatus is nearing completion. The liquefier has been operated forseveral hours and the various difficulties which have arisen are graduallybeing eliminated.This construction has been supported separately by the Office of NavalResearch, Contract No. N5ori-07823 Amend. 3, T.O. XXIII.S. C. CollinsB. SECOND SOUND VELOCITYThe study of pressure dependence of second sound velocity in liquidhelium II is temporarily delayed. This project will be taken over as asenior thesis in January. Several new projects are also planned as senior-26-bAN I ULtLINSULATIONFig. IV-2EH YDR ATOR:STAINLESS STEELFig. IV-I(IV. LOW-TEMPERATURE PHYSICS)theses during the Spring Term, and will be reported in a later issue.M. A. HerlinC. X-RAY STUDY OF SUPERCONDUCTORSRecent check experiments indicate that, during exposure, the leadsample was not always below the superconducting transition temperature. Theapparatus is being redesigned to insure that the sample is at the temperatureof the helium bath. L. A. SiegelD. INVESTIGATION OF DIPOLE INTERACTION THEORY IN CRYSTALSUsing a torsion balance system, preliminary measurements in the neigh-borhood of 1.1K have been made of the magnetization curve of cesium titaniumalum. These measurements indicate that the design of the experiment issatisfactory. Two runs have been made with the use of adiabatic demagneti-zation to reach the desired extremely low temperature of around 0.01K.These were unsuccessful for several reasons, and measures are now beingtaken to eliminate the troubles. First, we believe that there is a radia-tive heat leak to the sample which will be overcome by silvering the samplecapsule. Second, the transfer gas pressure will be increased to make thetransfer of the heat of magnetization more efficient. M. B. PrinceE. COOLING LIQUID HELIUM BY ADIABATIC DEMAGNETIZATIONThe problem of the experimental behavior of liquid helium in the tem-perature range 0.1 - 0.9*K is of considerable importance in the attempts tounderstand theoretically the mechanism behind its peculiar properties. Ex-periments have been begun with the objective of developing apparatus suitablefor cooling the helium to these temperatures in sufficient quantities andfor a sufficiently long time to allow the convenient performance of experi-ments on it.The usual method of cooling liquid helium is by evaporation, which can,with practicable pumping speeds, get down to about 0.8 or 0.90K. Below this,one must resort to the adiabatic demagnetization of a paramagnetic salt.Since the specific heats of most substances drop off as the cube of thetemperature or faster in this region, the effect of heat leaks becomes veryserious, and one must look for something which will serve as a "heat reser-voir" to reduce the warming rate. Fortunately, there is a peak in the speci-fic heat of a paramagnetic salt at a temperature corresponding to the Curietemperature (where the atomic magnetic moments freeze into a macroscopicalignment), and thus the same salt used for cooling can be used as the-28-(IV. LOW-TEMPERATURE PHYSICS)temperature reservoir. The immediate problem is to find a salt whose speci-fic heat peak falls near the temperature desired, and is high enough so thatit will serve as a reasonably good temperature reservoir. A partially de-hydrated chromic sulfate has shown some promise in preliminary tests, butthe choice of salt is as yet quite uncertain and must be the subject of con-siderable experimentation. V. Mayper, Jr.***-29-

Low-Temperature Physics

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Low-Temperature Physics

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