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A cryostat for studying the effect of light on the electron paramagnetic resonance spectra of substances at helium temperatures
improve thermal contact between components on which no pressure is exerted. A very thin layer of grease between the contacts improved Q tenfold until the greases solidified (270 ° K and 230 ° K for the 'N' and 'T' greases respectively), whereupon conduction fell abruptly to a value which was significantly lower than that of the 'dry joint'.
Discussion Differential thermal contraction between the mating surfaces accounts for the increase in ~ for successive coolings when the junction is undisturbed. As the block cools, contraction occurs, and since this takes place under an applied force the rubbing of the mating surfaces will tend to rupture the oxide films present on both contacts and will lead to a greater area of metal to metal contact; the lateral mcwements are of the order of 0.010 in. whereas the point contacts are of microscopic dimensions. It can be seen that for values of Fless than 8 kg.wt Q is proportional to F which is in agreement with the results of both Jacobs and Starr I and of Berman. 2 The results are also in quantitative agreement with those obtained by Berman; 2 he obtained a value of 7.13 roW/° Kkg.wt
for copper to copper contacts at a temperature o f approximately 80 ° K compared with the value o f 7.31 mW/° Kkg.wt obtained in this research for copper to brass contacts at 77 ° K. The saturation of ~ beyond F = 8 kg.wt is possibly explained by the build up o f the oxide layer at point contacts preventing any increase in the area of contact of both the electrically conducting and insulating portions of the mating surfaces; the former is borne out by the constant electrical conductance of the joint above F = 8 kg.wt. Measurements of the electrical and thermal conductance and the use of the Wiedemann-Franz law lead to the conclusion that the majority of the heat energy is carried across the copper to brass joint b y phonons rather than by electrons, a result also obtained by Berman. 2 One of us (M.R.) is grateful to S.R.C. for the award o f a Studentship without which this work would not have been possible.
REFERENCES
1. JACOBS,R. B., and STARR, C. Rev. sci. Instrum. 10, 140 (1939) 2. BERMAN,R. J. app1. Phys. 27, 318 (1956) 3. ASHWORTH,T. Ph.D. Thesis (Manchester University, 1966)
A cryostat for studying the effect of light on the electron paramagnetic resonance spectra of substances at helium temperatures
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in: utat or
O. G. K O S H E L E V t T HE effect of light on electron paramagnetic resonance (e.p.r.) spectra is of interest in the study of the spectra produced by impurity atoms in the crystal at energy levels in the optical range; also in studying the kinetics of electron transitions between these levels. A 3.2 cm wavelength e.p.r, cryostat was developed for the purpose to study substances at temperatures from 4-2 to 1.6 ° K, achieved by pumping liquid helium. The construction of the cryostat is shown in Figure 1. The light from source 1 passes into the waveguide through the optical filter 2 and the waveguide elbow, bent along a narrow wall. The upper part of this elbow is the grating 4, 0.1 mm thick brass strips, 1 mm high soldered in section 5 of the waveguide with 2 mm spacing. These strips prevent the v.h.f, wave from passing upwards while at the same time they do not appreciably weaken the light entering the waveguide. The waveguide elbow 3 is t Physics
Department, Moscow State University, U.S.S.R'
Pribory i Tekhnika ~ksperirnenta No. 6, 216-218 (1966). Received 6 June 1967. 37(}
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CinG-projector lamp 12 Glass filters Optical filter 13 Grating Waveguide elbow 14 Resonator Grating 15 Piston Waveguide section 16 Specimens Brass jacket 17 Infrared spectrometer Flanges 18 Lightpipe (fixed to cryoRubber washer stat in place of lamp 1) Teflon washer 19 Mirror Stainless steel wave- 20 Screws for adjusting the guide section mirror 11 Demountable waveguide section Figure 1. Design of the cryostat C R Y O G E N I C S • DECEMBER 1967
soldered into the brass jacket 6, which, together with the filter 2, flanges 7, and rubber washers 8, seals this section. Unlike the cryostat described by Levitt and Honig, 1 the light propagates along the same waveguide as the v.h.f. wave. The diameter of the cryostat in the magnet pole gap can therefore be reduced appreciably (to 29 mm). A standard 23 × 10 × 1 copper waveguide is used in the cryostat. Section 10 of the waveguide is made of 0.2 m m thick stainless steel to reduce the evaporation rate of the liquid helium. The lower part 11 of the waveguide is demountable; depending on the aim of the experiment, sections of waveguide with or without filters are screwed on here. Filters are used to remove the effect of thermal radiation from the upper parts of the waveguide which are at a temperature of ~ 3 0 0 ° K. Three glass plates 12, 1 mm thick and fixed at their edges to the waveguide with black paper, are used as filters. The v.h.f, waves reflected from them cancel one another out since the distance between the plates is -~7 mm (~ of the wavelength in the waveguide). The rectangular reflecting resonator 14 with a type H0J2 wave is screwed on to waveguide 11. The resonator is made from a section of standard waveguide; its upper wall is grating 13, as in the upper part of the waveguide, but with a distance of 6 mm between the strips of foil in the central part, to achieve the required coupling between the waveguide and the resonator. The Q of the loaded resonator is 2 000-3 000. The resonator is tuned by piston 15 which is moved by a 2 mm thick rod of manganin wire. The wire is vacuum sealed with a rubber washer. The test and control specimens 16 are stuck with BF-2 adhesive to the piston or to the side wall of the resonator. The waveguide and resonator are placed in a glass tube, sealed at the bottom, so that liquid helium should not enter them. This considerably enhances the stability of the spectroscope in operation. The resonator
An automatic regulator
temperature
The author is grateful to V. S. Vavilov for his constant interest in the work and also to A. I. Shal'nikov for help in carrying out experiments at low temperatures. REFERENCES 1. LEvITr, R. S., and HONIG, A. J. Phys. Chem. Solids, 22, 269 (1961) 2. FEHER, G., and GERE, E. Phys. Rev., 114, 1245 (1959) 3. WZLSON,D. K., and FEHER, G. Phys. Rev., 124, 1068 (1961) 4. KOSHELEV,O. G. Fiz. twerd, tela 8, 593 (1966) 5. VAVILOV,V. S., KOVAL', Y. P., and KOSHELEV,O. G. Fiz. twerd. tela 8, 2395 (1966)
I.,
I
G. PONOMAREVt
WHEN making measurements in the range from helium t°r°°m temperatures using a bl°ck meth°d'l it is relatively easy to traverse the whole temperature range several times for one filling cycle of the apparatus with liquid helium. The v o l u m e o f t h e b l o c k c a n b e u s e d a s t h e bulb of a gas thermometer 2 so that the determination of temperature can be made with great accuracy, However, when the block is in a region of large temperature gradient, it is very difficult to establish the required temperature manually. A great improvement in temperature stabilization can be achieved, by using an automatic temperature regulator. An astatic follower t Physics Faculty, Moscow State University, U.S.S.R. Pribory i Tekhnika ~ksperimenta No. 6, 218-220 (1966). Received 6 dune 1967. C R Y O G E N I C S • DECEMBER 1967
;
~ 1
for measurements
in t h e r a n g e 10-150 ° K Y.
and the other components are cooled by heat exchange through helium exchange gas. The temperature is measured with a carbon resistor fixed to the outside of t h e resonator. A 170 W, K-30 cin6-projection lamp is used as light source. The intensity of light falling on the specimen was, because of reflection from the walls of the waveguide, ,~15 times greater than would occur without a waveguide for the same distance to the source. The intensity of the light and its spectral composition w e r e changed by various diaphragms and filters placed between the cryostat and lamp 1 (and are not shown in Figure 1). The diaphragms are screens with orifices of different sizes. An IKS-11 infrared spectrometer was used for studying spectral dependences. A K-30 lamp also served as the source of radiation. The light intensity, judging from the magnitude of its influence on the electron spin-lattice relaxation of a silicon phosphor, was then ~ 3 0 t i m e s greater than in a cryostat with a quartz light pipe.2, a When working with an infrared spectrometer 17 instead of lamp 1, the light pipe 18, also made from a waveguide, is fixed on the cryostat. Results obtained with the cryostat discussed are described by Koshelev 4 and by Vavilov et al. s
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Figure 1. Block diagram of the automatic temperature regulator 371
Discussion Differential thermal contraction between the mating surfaces accounts for the increase in ~ for successive coolings when the junction is undisturbed. As the block cools, contraction occurs, and since this takes place under an applied force the rubbing of the mating surfaces will tend to rupture the oxide films present on both contacts and will lead to a greater area of metal to metal contact; the lateral mcwements are of the order of 0.010 in. whereas the point contacts are of microscopic dimensions. It can be seen that for values of Fless than 8 kg.wt Q is proportional to F which is in agreement with the results of both Jacobs and Starr I and of Berman. 2 The results are also in quantitative agreement with those obtained by Berman; 2 he obtained a value of 7.13 roW/° Kkg.wt
for copper to copper contacts at a temperature o f approximately 80 ° K compared with the value o f 7.31 mW/° Kkg.wt obtained in this research for copper to brass contacts at 77 ° K. The saturation of ~ beyond F = 8 kg.wt is possibly explained by the build up o f the oxide layer at point contacts preventing any increase in the area of contact of both the electrically conducting and insulating portions of the mating surfaces; the former is borne out by the constant electrical conductance of the joint above F = 8 kg.wt. Measurements of the electrical and thermal conductance and the use of the Wiedemann-Franz law lead to the conclusion that the majority of the heat energy is carried across the copper to brass joint b y phonons rather than by electrons, a result also obtained by Berman. 2 One of us (M.R.) is grateful to S.R.C. for the award o f a Studentship without which this work would not have been possible.
REFERENCES
1. JACOBS,R. B., and STARR, C. Rev. sci. Instrum. 10, 140 (1939) 2. BERMAN,R. J. app1. Phys. 27, 318 (1956) 3. ASHWORTH,T. Ph.D. Thesis (Manchester University, 1966)
A cryostat for studying the effect of light on the electron paramagnetic resonance spectra of substances at helium temperatures
4 r-.
in: utat or
O. G. K O S H E L E V t T HE effect of light on electron paramagnetic resonance (e.p.r.) spectra is of interest in the study of the spectra produced by impurity atoms in the crystal at energy levels in the optical range; also in studying the kinetics of electron transitions between these levels. A 3.2 cm wavelength e.p.r, cryostat was developed for the purpose to study substances at temperatures from 4-2 to 1.6 ° K, achieved by pumping liquid helium. The construction of the cryostat is shown in Figure 1. The light from source 1 passes into the waveguide through the optical filter 2 and the waveguide elbow, bent along a narrow wall. The upper part of this elbow is the grating 4, 0.1 mm thick brass strips, 1 mm high soldered in section 5 of the waveguide with 2 mm spacing. These strips prevent the v.h.f, wave from passing upwards while at the same time they do not appreciably weaken the light entering the waveguide. The waveguide elbow 3 is t Physics
Department, Moscow State University, U.S.S.R'
Pribory i Tekhnika ~ksperirnenta No. 6, 216-218 (1966). Received 6 June 1967. 37(}
(b 0 oO
2 ir,i
1
.
[i-I
To cr'yostat
4 E--!,~.15
1 2 3 4 5 6 7 8 g 10
CinG-projector lamp 12 Glass filters Optical filter 13 Grating Waveguide elbow 14 Resonator Grating 15 Piston Waveguide section 16 Specimens Brass jacket 17 Infrared spectrometer Flanges 18 Lightpipe (fixed to cryoRubber washer stat in place of lamp 1) Teflon washer 19 Mirror Stainless steel wave- 20 Screws for adjusting the guide section mirror 11 Demountable waveguide section Figure 1. Design of the cryostat C R Y O G E N I C S • DECEMBER 1967
soldered into the brass jacket 6, which, together with the filter 2, flanges 7, and rubber washers 8, seals this section. Unlike the cryostat described by Levitt and Honig, 1 the light propagates along the same waveguide as the v.h.f. wave. The diameter of the cryostat in the magnet pole gap can therefore be reduced appreciably (to 29 mm). A standard 23 × 10 × 1 copper waveguide is used in the cryostat. Section 10 of the waveguide is made of 0.2 m m thick stainless steel to reduce the evaporation rate of the liquid helium. The lower part 11 of the waveguide is demountable; depending on the aim of the experiment, sections of waveguide with or without filters are screwed on here. Filters are used to remove the effect of thermal radiation from the upper parts of the waveguide which are at a temperature of ~ 3 0 0 ° K. Three glass plates 12, 1 mm thick and fixed at their edges to the waveguide with black paper, are used as filters. The v.h.f, waves reflected from them cancel one another out since the distance between the plates is -~7 mm (~ of the wavelength in the waveguide). The rectangular reflecting resonator 14 with a type H0J2 wave is screwed on to waveguide 11. The resonator is made from a section of standard waveguide; its upper wall is grating 13, as in the upper part of the waveguide, but with a distance of 6 mm between the strips of foil in the central part, to achieve the required coupling between the waveguide and the resonator. The Q of the loaded resonator is 2 000-3 000. The resonator is tuned by piston 15 which is moved by a 2 mm thick rod of manganin wire. The wire is vacuum sealed with a rubber washer. The test and control specimens 16 are stuck with BF-2 adhesive to the piston or to the side wall of the resonator. The waveguide and resonator are placed in a glass tube, sealed at the bottom, so that liquid helium should not enter them. This considerably enhances the stability of the spectroscope in operation. The resonator
An automatic regulator
temperature
The author is grateful to V. S. Vavilov for his constant interest in the work and also to A. I. Shal'nikov for help in carrying out experiments at low temperatures. REFERENCES 1. LEvITr, R. S., and HONIG, A. J. Phys. Chem. Solids, 22, 269 (1961) 2. FEHER, G., and GERE, E. Phys. Rev., 114, 1245 (1959) 3. WZLSON,D. K., and FEHER, G. Phys. Rev., 124, 1068 (1961) 4. KOSHELEV,O. G. Fiz. twerd, tela 8, 593 (1966) 5. VAVILOV,V. S., KOVAL', Y. P., and KOSHELEV,O. G. Fiz. twerd. tela 8, 2395 (1966)
I.,
I
G. PONOMAREVt
WHEN making measurements in the range from helium t°r°°m temperatures using a bl°ck meth°d'l it is relatively easy to traverse the whole temperature range several times for one filling cycle of the apparatus with liquid helium. The v o l u m e o f t h e b l o c k c a n b e u s e d a s t h e bulb of a gas thermometer 2 so that the determination of temperature can be made with great accuracy, However, when the block is in a region of large temperature gradient, it is very difficult to establish the required temperature manually. A great improvement in temperature stabilization can be achieved, by using an automatic temperature regulator. An astatic follower t Physics Faculty, Moscow State University, U.S.S.R. Pribory i Tekhnika ~ksperimenta No. 6, 218-220 (1966). Received 6 dune 1967. C R Y O G E N I C S • DECEMBER 1967
;
~ 1
for measurements
in t h e r a n g e 10-150 ° K Y.
and the other components are cooled by heat exchange through helium exchange gas. The temperature is measured with a carbon resistor fixed to the outside of t h e resonator. A 170 W, K-30 cin6-projection lamp is used as light source. The intensity of light falling on the specimen was, because of reflection from the walls of the waveguide, ,~15 times greater than would occur without a waveguide for the same distance to the source. The intensity of the light and its spectral composition w e r e changed by various diaphragms and filters placed between the cryostat and lamp 1 (and are not shown in Figure 1). The diaphragms are screens with orifices of different sizes. An IKS-11 infrared spectrometer was used for studying spectral dependences. A K-30 lamp also served as the source of radiation. The light intensity, judging from the magnitude of its influence on the electron spin-lattice relaxation of a silicon phosphor, was then ~ 3 0 t i m e s greater than in a cryostat with a quartz light pipe.2, a When working with an infrared spectrometer 17 instead of lamp 1, the light pipe 18, also made from a waveguide, is fixed on the cryostat. Results obtained with the cryostat discussed are described by Koshelev 4 and by Vavilov et al. s
i
I
t
[t~eductionl',
k1. . . . r . .l . ~. . h. . . ;
II III III IIII
'll J i II
iI
I-T':I ~
-1 _Thermocouple 1, ~2~--I I
I
I pac
I " x; I I
I i 24V
I
6v
LPho ocellll I i GQ_~br!dge ~-~AmplJfJer Ii ~.~--j~l..i "-.-~ x , I I i ~ -nu ~ot I i /~1
_J
,.
/_.
.a t
Brock 1 Temperature measuring system 2 Out-of-balance signal 3 Block moving mechanism
Figure 1. Block diagram of the automatic temperature regulator 371