A Cryostat for Dispersion Measurements Low T e m p e r a t u r e s
IT. P. Babe nko, M. S. .Brodin, and M. S. Soskin htstitute of Physics, Ukrainia~ Academy of Scie~ces, "U.S.S.R. Received 14 February 1962t
AT the present time studies of the dispersion of light in crystals at low temperatures are essential in order to find effects, predicted by theory, of dispersion. The most convenient method for measuring refractive index in the region of an absorption is an interferometric one, in particular by using a Jamin interferometer. The main difficulty in low temperature studies of this type lies in cooling the specimen to the required temperatures while at the same time preserving the clarity and stability of the interference picture. The most successful practical solution of this problem remains the housing of the interferometer in the cryostat, together with the specimen, first accomplished by Obreivmov and Prikhot'ko I for a temperature of 77 ° K. The same principle was maintained in the construction of a metal cryostat 2 designed for working at T = 20 ° K. In these arrangements the specimen, together with the interferometer, were in the vapour of the liquid coolant. This simple principle cannot be applied in the case of liquid helium, unfortunately, because of the small heat transfer by gaseous helium, as a result of which a specimen contained in a vapour atmosphere and under illumination is at a temperature considerably above 4 ° K. In constructing a helium cryostat for dispersion measurements it was found most successful to house only the interferometer in the vapour, while the specimen was directly in the liquid helium. In the cryostat developed, several constructional drawbacks which existed in the cryostat previously described 2 for measurements at T = 20°K were also removed; these did not make it possible to orient and interchange the specimen while working. The general layout and external form of the well-proved metal cryostat designed for absorption studies 3 was preserved. For this reason, only that part of the cryostat which is substantially new and specific for the dispersion measurements is shown in Figure 1. The removable inset 1 has a shaped tank 2 of volume 1.5 1. with the recess 3 filled with liquid helium. There is a pocket 4 in the recess, 6 m m wide with quartz windows 5, in which the specimen 6 is placed on a boss. The interferometer mirrors 7 are supported on the quartz horseshoe5" R e c e i v e d by PTI~ E d i t o r 6 M a r c h 1961: Pribory i Tekhnika
l~ksperimenta N o . 6, p. 140 (1961). CRYOGENICS
shaped support 8 which is in an atmosphere of helium vapour. The path of the rays in the cryostat is clear from the drawing. The correct fitting of the windows in the cryostat requires great attention as this must prevent flow of the liquid and at the same time must not introduce
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mechanical strains in the windows, which are specially marked on cooling and lead to depolarization of the light passing through. This was achieved by using a screw clamp and a suede ring for sealing, impregnated with vacuum grease and sprinkled with graphite powder. An inset of slightly different construction was made for working in liquid hydrogen vapour: the slot determining the direction of the boss with the specimen, was solid, while the ring into which the holder with the crystal fitted had free openings for the vapour to reach the specimen. 365
The consumption of helium is 0.7 l./hr and of hydrogen 0.2 l./hr. The cryostat proved itself well in operation, the interference picture being sufficiently stable.
2. BRODIN,M. S., MEOVEDEV,V. S., and PRmnOT'KO, A. F. Prib. i Tekh. Eksper. No. 3, p. 96 (1956) 3. BABENKO,V. P., BROUDE,V. S., MEDVEDEV,V. S., and PRIKHOT'KO, A. F. Prib. i Tekh. Eksper. No. 1, p. 115 (1959)
REFERENCES I. V., and PRIKHOT'KO, A. F. Volume hi Celebration of Academician A. F. loffe's 70th Birthday (Academyof Sciences, U.S.S.R., 1950)
This paper has been specially translated for CRYOGENICS and is included by permission o f the Editors of Pribory i Tekhnika t~ksperbnenta. We are also indebted to the Instrument Society o f America and the Plenum Press, who publish their own cover-to-cover translation o f PT# by arrangement with the Russian publisher.
CORRIGENDA The following corrections should be made to the paper by Professor McGee and Mr. Martin in the September 1962 issue. Page 259, left-hand column. The sentence beginning 'Although there is n o . . . ' should be replaced by: Although there is no evidence from high speed mass spectrographic analyses for the presence of H 2 0 4 in the effluent from the gas-phase reaction of H with 02, HO2 is readily observed in such experimental arrangements. 17.18However, H O 2 has not been observed in infra-red absorption in the ices that may be formed upon quenching this effluent (GIGUERE,P. A., and HARVEY,K. B. J. chem. Phys. 25, 373 0956)). Page 267, right-hand column. Reference 111 should read: BERGER, R. Proc. nat. Acad. Sci., Wash. 47, 1434 (i961)