Contains reports on four research projects

Cavileer, R. P.

Maurer, R. D.

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. MaurerDr. M. A. Herlin P. L. NicholasDr. L. A. Siegel M. B. PrinceN. I. Adams III J. W. Toomey, Jr.A. HELIUM LIQUEFIERSThe liquefiers have been moved to the new location of the Low-Tempera-ture Laboratory where an adequate surrounding area allows for much easieroperation than did the cramped conditions which existed in the old labora-tory. The liquefiers and their compressors are on a concrete floor whicheliminates much of the vibration experienced in the former location.There are now two independent liquefying setups. The plumbing is soarranged (as shown in Figure IV-1) that any of the integral parts may beshifted from one setup to the other by means of the valving. If troubleshould develop in a manifold, clean-up pot, gas holder or compressor duringa run, the other manifold, clean-up pot, gas holder or compressor can besubstituted within a few minutes so that the run may continue. Obviously,if both liquefiers were operating simultaneously, or if trouble shoulddevelop in a liquefier itself, such a substitution could not be accomplished.Moving the liquefiers on to a concrete floor presented the problem ofplacing tall dewars under the existing liquid draw-off tubes. In theformer location a hole was cut in the wood floor and the dewars were loweredand raised through this hole. To avoid cutting a hole in the concretefloor, the end of the draw-off tube was raised. Now, at this raised height,it is much easier to hold the dewars into which the liquid helium is trans-ferred.The number two liquefier has continued to be adequate for supplyingthe demands for liquid helium during this transition period.It is interesting to note that this liquefier has now been operatingsince August of last year with uninterrupted service. In this year oftrouble-free running it has produced liquid helium ninety-four times. Itmust be pointed out that both the careful operation of the liquefier, andthe engineering and manufacture by Arthur D. Little, Inc. have combined tomake this record possible. R. P. Cavileer, J. W. Toomey, Jr.B. SECOND SOUND VELOCITYApparatus has been designed and constructed for measuring the variationof the velocity of second sound with the change in both temperature and-19-ER PRESSURE 2300 POUNDS/INCH2RESSURE 200 POUNDS/INCH2RESSURE ATMOSPHERICATOR VALVEFig. IV-1 Piping diagram for cryostats.II(IV. LOW-TEMPERATURE PBYSICS)pressure of liquid helium. A pulse method will be utilized as in previouswork of this nature in the laboratory (1). Commercial helium tanks pro-vide the pressure to a second-sound tube system immersed in a liquid heliumbath. The previous timing arrangement has been considerably refined togive greater accuracy in case it is possible to obtain more exact squarepulse signals. This system can measure velocities greater than 10 metersper second in order to cover the lower temperature range of the phenomenon.It is hoped that data can be obtained over part of the range measured byPeshkov (2) and can be extended through the rise in velocity.R. D. MaurerC. X-RAY STUDY OF SUPERCONDUCTORSTwo diffraction patterns have been taken of a lead wire sample main-tained below the superconducting transition temperature. An exposure timeof one hour gave sufficiently intense lines in the back reflection region.Since the sample temperature is dependent upon the level of liquid heliumin the apparatus, it was necessary to add additional helium during theexposure by means of an auxiliary transfer dewar.Due to the very limited amount of sample motion and to the propertiesof the lead itself, the diffraction lines in the first exposure were veryspeckled. By using a very broad sample, the lines were smoothed withoutany appreciable broadening in the back reflection zone.Study of the exposures showed no change in the appearance of the 620doublet below the transition temperature. The resolving power of the ap-paratus would allow a change in cell edge of about one part in one thousandto be detected, and it seems, then, that if any lattice change does takeplace, its magnitude is considerably smaller than the cell deformations inbarium titanate.It is planned, however, to continue the experiments using a slightlydifferent x-ray wave length. This would allow study of a line other than620 which might possibly be insensitive to a structural change of the typein question. L. A. SiegelD. INVESTIGATION OF DIPOLE INTERACTION THEORY IN CRYSTALSEquipment has been built for an experiment to measure the magneticmoment of CsTi(SO04)0.12H20 versus magnetic field near the temperature ofabsolute zero (0.01K). This experiment will differentiate among three theo-ries for dipole interaction in crystals. The theory of Sauer and Temperley(3) is based on nearest neighbor interaction and predicts a curve that is(IV. LOW-TEMPERATURE PHYSICS)equal to zero until a critical field Ho is reached where all the dipolesbecome aligned and the magnetization becomes saturated. A theory ofVan Vleck (4) expands the partition sum of the crystal in inverse powersof the temperature, and the calculation of the interaction fails at lowtemperature where the interaction becomes appreciable. This theory predictsa linear relation between M and H until Ho where the curve gradually levelsoff to a saturated magnetization. The theory of Luttinger and Tisza (5)reduces the problem of the dipole interaction in a crystal to the diagonali-zation of the quadratic form in the components of the dipole moments, ofthe dipole interaction energy and the reduction of any arbitrary dipolearray into these diagonalized basic arrays. This theory predicts a linearrelation from the origin until Ho where the curve breaks abruptly and staysflat to infinite field. From the theories, Ho should be somewhere between50 and 150 oersteds.The experiment should be completed within the next two months.M. B. PrinceReferences(1) R. D. Maurer and M. A. Herlin, R.L.E. Technical Report No. 124.(2) V. Peshkov and Zinoveva, J.E.T.P. USSR 18, 438 (1948).(3) J. A. Sauer and A. N. V. Temperley, Proc. Roy. Soc. 176, 203 (1940).(4) J. H. Van Vleck, J. Chem. Phys. 2, 320 (1937).(5) J. M. Luttinger and L. Tisza, Phys. Rev. 70, 954 (1946).-22-el_ _____^_^~I_^ ~~_ll I ._.i.l~LI111-----

Low-Temperature Physics

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Low-Temperature Physics

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