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A cryostat for magneto-optical studies
test sample of 1.5 m length on which a weak part had been made artificially by annealing. Also the small variations of about 15~ can be reproduced entirely; obviously they are caused by a varying diameter of the inner superconducting part of the wire, drawn to the final diameter together with the copper cladding. As can be seen from equation (8) such variations have high effects. Later on, further measuring results as well as flux jumps occurring under certain conditions will be reported. Figure 3. Voltage U at the compensation tape recorder as function of positions on a test wire. The arrow indicates the artificially produced weak part
flux for which the following equation is obtained after Bean's model :4 7~
~0 = -~ I~ojcr 3 = ½lzorJc
REFERENCES
. . . (8)
(where j c is the critical current density, Jc the critical current strength, r the radius of the wire, and /to the induction constant). In Figure 3 the measuring results are shown for a
A cryostat for magneto-optical studies V. N. P A V L O V and V. V. EREMENKO Physico-technical Institute of Low Temperatures, Academy of Sciences, Ukrainian SSR, Khar'kov, USSR. Pribory i Tekhnika ~ksperimenla No. 3, 208 (1967) Received 16 November 1967
] N L O W temperature spectroscopy of crystals, specimens of small size and of substances with high light absorption are commonly used. The exposures which are then necessary to obtain sufficiently clear spectrograms sometimes last for several hours. In the case of magnetooptical investigations, when spectrographs of high resolving power but small light intensity are used to observe fine structure, the duration of an experiment gets even longer. When one considers that it is often necessary to obtain several spectrograms corresponding, for example, to different orientations of the external magnetic field relative to the crystal axes of the specimen, then it is clear that the time for which the liquid helium (or hydrogen) lasts in the cryostat must be increased. In developing the cryostat described we tried first of all to satisfy this main requirement, but also to attain the maximum simplicity in design so that rapid reassembly could be achieved if necessary. It should however be noted that during a year of intensive operation the cryostat has not once had to be dismantled. The essentials of the shape and external dimensions of the cryostat were 170
This paper was partly presented at the 6th International Conference on Physics and Techniques of Low Temperatures of the States Council for Mutual Economic Aid in Wroclav (Poland) on 31 August 1967. The authors are very grateful to Professor Dr Bewilogua for his assistance in this paper.
1. DETTMANN,F., and LANCE, F. Exp. Tech. Phys. 13, 26 (1965) 2. ALEXEJEWSKI,N. E., DUBROWlN, A. W., MICHAILOW,N. N., SOKOLOW,B. J., and FEDOTOW,A. N. Dokl. Akad. Nauk. SSSR 171, 566 (1966) 3. KREMLEV,M. G., SA~OVLOW,B. N., and SKULATCh'ENKO,S. S. Cryogenics 5, 73 (1965) 4. BEAN,C. P., and DOYLE, M. V. J. appl. Phys. 33, 3334 (1962)
laid down by the commercial SP-47G electromagnet which was on hand and in the field of which the studies were carried out. The cryostat (Figure 1) consists of four main parts: the central can a of 850 cm 3 capacity with windows of special design and upper flange system, the 600 cm 3 nitrogen can b, the vacuum jacket c with outer windows, and the upper demountable flange d with moving shaft and specimen rotator. All components of the cryostat are made of non-magnetic materials. The central can is made of bellows sections of wall thickness 0.15 ram, soldered with PSR-45 solder. To give it strength, three strengthening rings, which can withstand a sudden increase of 1 atm in external pressure, are soldered inside with POS-60 solder. Although initial cooling is achieved by simple mechanical contact, the small mass of the can ( ~ 150 g) makes it possible to start the helium transfer 1 h after filling the nitrogen shield with liquid nitrogen. No appreciable increase in the amount of helium to cool the can to 4.2°K is then observed; ~ 200 cm a of liquid helium are used for the cooling. Two transparent windows of fused optical quartz are sealed co-axially in the lower narrowed part of the can--the tail. An adhesive based on the domestic epoxy resin ED-6 was used for vacuum sealing of the optical windows, instead of the expoxy resins used by others.I, 2 Resin ED-6 and cold setting hardener has some advantages over resin 'Epoxy PR', since the latter needs special heat treatment up to 240°C, which can increase the force on the glued joins on cooling to low temperatures. In spite of the very great difference in thermal expansion coefficients of metal and quartz, it was possible to obtain windows free from depolarizing CRYOGENICS • JUNE 1968
effects. The composition of the adhesive by weight is: ED-6 epoxy resin--100, hexamethyldiamine--10, aluminium powder 40. The hardening process was: hold at room temperature for 1 h, then 4 h at 60°C. The surfaces to be joined were carefully de-greased and dried. The strength of the join was checked by many rapid coolings of the widows to liquid hydrogen temperature. The design of the windows is shown in Figure 2. The long central can support tube a is cooled by flow of heat through six M4 screws to the nitrogen shield. The screws are tightly screwed but not soldered. This gives access to the central tube and only two nitrogen
filling tubes need be unsoldered. Tubular shields which greatly reduce the solid angle over which infared thermal radiation can freely propagate, reduced that part of the heat influx to the liquid helium which comes through the windows. The inner surfaces of the tubes are blackened by cutting screw threads and by intense oxidation. The lower parts of the vacuum jacket and the nitrogen shield are made as parallelepipeds. All inner surfaces are polished and thoroughly washed. The vacuum space in the cryostat is common. The specimen under investigation is fixed on a spindle which moves in the packing of the upper demountable
0"1 m m ~ -
! 1 - - Case
2 - - Ring
3 - - Fused quartz window
Figure 2. Inner optical w i n d o w
L
1 a - - Central can b - - Nitrogen can
c - - V a c u u m jacket d - - D e m o u n t a b l e flange
Figure 1. The cryostat CRYOGENICS
• J U N E 1968
1 - - Spindle 2 - Case 3 - Bobbin
4 - - PTFE bearing
5 - - Fixing washer 6-
Silk thread
Figure 3. Specimen holder 171
flange. The spindle has a spiral copper surface at its upper end to cut out direct infrared radiation to the liquid helium bath. This surface cuts out the radiation effectively without affecting the pumping of helium vapour. The design of the specimen holder is shown in Figure 3. To prevent sticking, the bobbin rests on PTFE bearings which fit tightly on the drum at room temperature with a sliding fit in the holes. The conical end surfaces of the outer windows are smeared with GOST 9645-61 grease (Ramsay grease) and the demountable parts of the cryostat are sealed with vacuum rubber, greased with castor oil. The cryostat has been used to study the absorption
Limiting vacuum obtained by adsorbing air on activated charcoal cooled with liquid nitrogen B. G. L A Z A R E V , A. L. DONDE, and V. I. S H A R O N O V Physico-technical Institute, Academy of Sciences, Ukranian SSR, Khar'kov, USSR. Pribory i Tekhnika ~ksperimenla No. ?, 220 (1967) Received 16 November 1967
A V A C U U M better than 10-7 torr in an appreciable volume was achieved in the first high vacuum adsorption pumps using as adsorbent activated wood charcoal (BAU). 1 Examination of the adsorption isotherms of the components of air show that the limiting vacuum over the adsorbent is still better (P < 10-s torr). 2 In the work cited, the vacuum systems did not have an arrangement for degassing the walls by heating, and the limiting pressure in them was determined by equilibrium between the rate of the absorption of gas by the adsorbent and desorption of gas from the walls. In the present work results are given of the determination of the residual air pressure over a charcoal adsorbent when desorption of gases from the walls of the vacuum system is prevented. The arrangement is shown in the figure. The glass cylindrical vessel 1 of 500 cm 3 capacity had two manometer bulbs in its upper part: LM-2 and an Alpert type. Tube 5 is joined to the lower part of the vessel for preliminary pumping. The double-walled container 2 for the adsorbent BAU charcoal is placed in the vessel. The container is made of copper net. The charcoal (30 g) is sprinkled in the space between the cylindrical walls of the container 3. The preliminary pumping was carried out with a rotary pump through a rubber vacuum tube fitted on to the top of tube 5. After pumping to p --~ 10-2 torr the rubber tube was fastened with a clip and the whole system immersed tT2
spectra (in the spectral range from 2 200 to 7 O00A) of single crystal specimens of ferrite garnets and antiferromagnetic dielectrics in steady magnetic fields up to 30 kOe. It can easily be used for low temperature studies of photoelectric and photomagnetic phenomena in semiconductors. REFERENCES
1. RAUCH, C. J., and KERNAN, W. C. Rev. sci. Instrum. 33, 496 (1962) 2. PRIKHOT'KO,A. F., PTUKHA, T. P., and SHANSKII,L. I. J. appl. Spectroscopy 2, 223 (1965) 3. BABENKO, V. P., BROUDE, V. L., MEDVEDEV, V. S., and PRIKHOT'KO, A. F. Prib. i Tekh. l~skper. No. 1, 115 (1959)
in liquid nitrogen up to the manometer bases. A pressure of 5 x 10-9 torr was reached in the system 90 min later. If the preliminary pumping was taken to ~ 1 0 -4 torr and the system then heated at t = 300°C for 2 h and tube 5 sealed off, the pressure reached ,~8 x 10-lo torr after cooling. It is evident that the limiting value of the pressure is raised because of the means of measuring it (the presence of valves with a hot cathode). We suggest that this simple system may have some special application (small high vacuum systems, or as an apparatus for measuring adsorption isotherms). REFERENCES
1. LAZAREV,B. G., and FEDOP.OVA,M. F. JTF29, 862 (1959); JTF 3O, 865 (1960) 2. FEDOROVA,M. F. JTF33, 590 (1963) V
1
,.....-
.,..-
¢
...._
CRYOGENICS . JUNE 1968
flux for which the following equation is obtained after Bean's model :4 7~
~0 = -~ I~ojcr 3 = ½lzorJc
REFERENCES
. . . (8)
(where j c is the critical current density, Jc the critical current strength, r the radius of the wire, and /to the induction constant). In Figure 3 the measuring results are shown for a
A cryostat for magneto-optical studies V. N. P A V L O V and V. V. EREMENKO Physico-technical Institute of Low Temperatures, Academy of Sciences, Ukrainian SSR, Khar'kov, USSR. Pribory i Tekhnika ~ksperimenla No. 3, 208 (1967) Received 16 November 1967
] N L O W temperature spectroscopy of crystals, specimens of small size and of substances with high light absorption are commonly used. The exposures which are then necessary to obtain sufficiently clear spectrograms sometimes last for several hours. In the case of magnetooptical investigations, when spectrographs of high resolving power but small light intensity are used to observe fine structure, the duration of an experiment gets even longer. When one considers that it is often necessary to obtain several spectrograms corresponding, for example, to different orientations of the external magnetic field relative to the crystal axes of the specimen, then it is clear that the time for which the liquid helium (or hydrogen) lasts in the cryostat must be increased. In developing the cryostat described we tried first of all to satisfy this main requirement, but also to attain the maximum simplicity in design so that rapid reassembly could be achieved if necessary. It should however be noted that during a year of intensive operation the cryostat has not once had to be dismantled. The essentials of the shape and external dimensions of the cryostat were 170
This paper was partly presented at the 6th International Conference on Physics and Techniques of Low Temperatures of the States Council for Mutual Economic Aid in Wroclav (Poland) on 31 August 1967. The authors are very grateful to Professor Dr Bewilogua for his assistance in this paper.
1. DETTMANN,F., and LANCE, F. Exp. Tech. Phys. 13, 26 (1965) 2. ALEXEJEWSKI,N. E., DUBROWlN, A. W., MICHAILOW,N. N., SOKOLOW,B. J., and FEDOTOW,A. N. Dokl. Akad. Nauk. SSSR 171, 566 (1966) 3. KREMLEV,M. G., SA~OVLOW,B. N., and SKULATCh'ENKO,S. S. Cryogenics 5, 73 (1965) 4. BEAN,C. P., and DOYLE, M. V. J. appl. Phys. 33, 3334 (1962)
laid down by the commercial SP-47G electromagnet which was on hand and in the field of which the studies were carried out. The cryostat (Figure 1) consists of four main parts: the central can a of 850 cm 3 capacity with windows of special design and upper flange system, the 600 cm 3 nitrogen can b, the vacuum jacket c with outer windows, and the upper demountable flange d with moving shaft and specimen rotator. All components of the cryostat are made of non-magnetic materials. The central can is made of bellows sections of wall thickness 0.15 ram, soldered with PSR-45 solder. To give it strength, three strengthening rings, which can withstand a sudden increase of 1 atm in external pressure, are soldered inside with POS-60 solder. Although initial cooling is achieved by simple mechanical contact, the small mass of the can ( ~ 150 g) makes it possible to start the helium transfer 1 h after filling the nitrogen shield with liquid nitrogen. No appreciable increase in the amount of helium to cool the can to 4.2°K is then observed; ~ 200 cm a of liquid helium are used for the cooling. Two transparent windows of fused optical quartz are sealed co-axially in the lower narrowed part of the can--the tail. An adhesive based on the domestic epoxy resin ED-6 was used for vacuum sealing of the optical windows, instead of the expoxy resins used by others.I, 2 Resin ED-6 and cold setting hardener has some advantages over resin 'Epoxy PR', since the latter needs special heat treatment up to 240°C, which can increase the force on the glued joins on cooling to low temperatures. In spite of the very great difference in thermal expansion coefficients of metal and quartz, it was possible to obtain windows free from depolarizing CRYOGENICS • JUNE 1968
effects. The composition of the adhesive by weight is: ED-6 epoxy resin--100, hexamethyldiamine--10, aluminium powder 40. The hardening process was: hold at room temperature for 1 h, then 4 h at 60°C. The surfaces to be joined were carefully de-greased and dried. The strength of the join was checked by many rapid coolings of the widows to liquid hydrogen temperature. The design of the windows is shown in Figure 2. The long central can support tube a is cooled by flow of heat through six M4 screws to the nitrogen shield. The screws are tightly screwed but not soldered. This gives access to the central tube and only two nitrogen
filling tubes need be unsoldered. Tubular shields which greatly reduce the solid angle over which infared thermal radiation can freely propagate, reduced that part of the heat influx to the liquid helium which comes through the windows. The inner surfaces of the tubes are blackened by cutting screw threads and by intense oxidation. The lower parts of the vacuum jacket and the nitrogen shield are made as parallelepipeds. All inner surfaces are polished and thoroughly washed. The vacuum space in the cryostat is common. The specimen under investigation is fixed on a spindle which moves in the packing of the upper demountable
0"1 m m ~ -
! 1 - - Case
2 - - Ring
3 - - Fused quartz window
Figure 2. Inner optical w i n d o w
L
1 a - - Central can b - - Nitrogen can
c - - V a c u u m jacket d - - D e m o u n t a b l e flange
Figure 1. The cryostat CRYOGENICS
• J U N E 1968
1 - - Spindle 2 - Case 3 - Bobbin
4 - - PTFE bearing
5 - - Fixing washer 6-
Silk thread
Figure 3. Specimen holder 171
flange. The spindle has a spiral copper surface at its upper end to cut out direct infrared radiation to the liquid helium bath. This surface cuts out the radiation effectively without affecting the pumping of helium vapour. The design of the specimen holder is shown in Figure 3. To prevent sticking, the bobbin rests on PTFE bearings which fit tightly on the drum at room temperature with a sliding fit in the holes. The conical end surfaces of the outer windows are smeared with GOST 9645-61 grease (Ramsay grease) and the demountable parts of the cryostat are sealed with vacuum rubber, greased with castor oil. The cryostat has been used to study the absorption
Limiting vacuum obtained by adsorbing air on activated charcoal cooled with liquid nitrogen B. G. L A Z A R E V , A. L. DONDE, and V. I. S H A R O N O V Physico-technical Institute, Academy of Sciences, Ukranian SSR, Khar'kov, USSR. Pribory i Tekhnika ~ksperimenla No. ?, 220 (1967) Received 16 November 1967
A V A C U U M better than 10-7 torr in an appreciable volume was achieved in the first high vacuum adsorption pumps using as adsorbent activated wood charcoal (BAU). 1 Examination of the adsorption isotherms of the components of air show that the limiting vacuum over the adsorbent is still better (P < 10-s torr). 2 In the work cited, the vacuum systems did not have an arrangement for degassing the walls by heating, and the limiting pressure in them was determined by equilibrium between the rate of the absorption of gas by the adsorbent and desorption of gas from the walls. In the present work results are given of the determination of the residual air pressure over a charcoal adsorbent when desorption of gases from the walls of the vacuum system is prevented. The arrangement is shown in the figure. The glass cylindrical vessel 1 of 500 cm 3 capacity had two manometer bulbs in its upper part: LM-2 and an Alpert type. Tube 5 is joined to the lower part of the vessel for preliminary pumping. The double-walled container 2 for the adsorbent BAU charcoal is placed in the vessel. The container is made of copper net. The charcoal (30 g) is sprinkled in the space between the cylindrical walls of the container 3. The preliminary pumping was carried out with a rotary pump through a rubber vacuum tube fitted on to the top of tube 5. After pumping to p --~ 10-2 torr the rubber tube was fastened with a clip and the whole system immersed tT2
spectra (in the spectral range from 2 200 to 7 O00A) of single crystal specimens of ferrite garnets and antiferromagnetic dielectrics in steady magnetic fields up to 30 kOe. It can easily be used for low temperature studies of photoelectric and photomagnetic phenomena in semiconductors. REFERENCES
1. RAUCH, C. J., and KERNAN, W. C. Rev. sci. Instrum. 33, 496 (1962) 2. PRIKHOT'KO,A. F., PTUKHA, T. P., and SHANSKII,L. I. J. appl. Spectroscopy 2, 223 (1965) 3. BABENKO, V. P., BROUDE, V. L., MEDVEDEV, V. S., and PRIKHOT'KO, A. F. Prib. i Tekh. l~skper. No. 1, 115 (1959)
in liquid nitrogen up to the manometer bases. A pressure of 5 x 10-9 torr was reached in the system 90 min later. If the preliminary pumping was taken to ~ 1 0 -4 torr and the system then heated at t = 300°C for 2 h and tube 5 sealed off, the pressure reached ,~8 x 10-lo torr after cooling. It is evident that the limiting value of the pressure is raised because of the means of measuring it (the presence of valves with a hot cathode). We suggest that this simple system may have some special application (small high vacuum systems, or as an apparatus for measuring adsorption isotherms). REFERENCES
1. LAZAREV,B. G., and FEDOP.OVA,M. F. JTF29, 862 (1959); JTF 3O, 865 (1960) 2. FEDOROVA,M. F. JTF33, 590 (1963) V
1
,.....-
.,..-
¢
...._
CRYOGENICS . JUNE 1968