HELIUM REFRIGERATOR FOR TEMPERATURE VARYING MÖSSBAUER STUDIES

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HELIUM REFRIGERATOR FOR TEMPERATURE VARYING MÖSSBAUER STUDIES

Y. Chow, M. Fuchs, A. Mukerji

To cite this version:

Y. Chow, M. Fuchs, A. Mukerji. HELIUM REFRIGERATOR FOR TEMPERATURE VARY- ING MÖSSBAUER STUDIES. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-41-C2-43.

�10.1051/jphyscol:1979213�. �jpa-00218501�

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JOURNAL DE PHYSIQUE Collogue C2, supplement au n° 3, Tome 40, mars 1979, page C2-41

HELIUM REFRIGERATOR FOR TEMPERATURE VARYING MOSSBAUER STUDIES

Y.W. Chow, M. Fuchs and A. Mukerji

Division of Natural Soienoe and Mathematics, Medgar Evers College, Cuny and Department of Physios and Astronomy, Lehman College, Cuny, Bronx, N.Y., U.S.A.

Department of Physios and Astronomy, Lehman College, Cuny, Bronx, N.Y., U.S.A.

Résumé.- Un réfrigérateur à cycle fermé d'hélium a été utilisé pour des études par effet Mossbauer où la source est maintenue à température constante (20 K) et où l'absorbeur peut être placé à une température variant entre 20 K et 300 K. Cet appareil a été utilisé pour déterminer le déplacement isomérique pur S (indépendant du mouvement moléculaire) qui est une fonction linéaire de la tempé- rature cinétique T1 = T0 coth T /T. La contribution chimique à <5 peut être trouvée en extrapolant 6(T*) jusqu'à T* = 0. Pour le composé Nal on trouve ainsi 6(0) = - 0,480 ± 0,005 mm/s et TQ = 60±2K.

Abstract.- A closed cycle helium refrigerator has been adopted for studies of M'ossbauer effect at constant source temperature of 20 K and various absorber temperatures from 20 to 300 K. This apparatus has been used to measure isomer shift (6) in the absence of all molecular motion because S is linearly dependent on kinetic temperature T* which is defined as T0 coth T0/T. The zero-point temperature T is determined when 6 becomes linear in Tm. The chemical contribution in 6 can be found by extrapolation of the isomer shift data to T* = 0. In this way TQ and 6 for Nal have been found to be 60 i 2 K and -.480 + .005 mm/s respectively.

The isomer shift obtained from M'ossbauer ve- locity spectrum measurements includes the actual isomer shift and the second-order Doppler effect which is proportional to the mean kinetic energy of

the Mossbauer atoms in the lattice. The second-order Doppler effect can be evaluated by varying the temperature difference between the emitter and the absorber. Following the pioneering suggestion of Mazo and Kirkwood /I/, Collins and Cosgrove /2/ define the kinetic temperature T as T0 coth T0/T which measures the kinetic energy of the solid, including the zero-point energy. TQ is the zero-point temperature defined as . 5JiwQ/k. As the isomer shift is linear in T*, the intercept on the isomer shift axis in the plot of S vs. T* gives the expected isomer shift in the absence of all molecular motion. For a given TQ, S can be plotted as a function of T*. T^Q is determined when S becomes linear in T*.

In order to carry out temperature varying Mossbauer studies where the temperature of the sour-

ce* is kept at 20 K while the absorber temperature is varied from 20 to 300 K, a closed cycle helium refrigerator with a temperature varying resistor

(TVR) manufactured by Cryogenic Technology, Inc.

has been used with the modifications shown in figure 1. From the compressor, compressed helium gas is supplied to the refrigerator to expand in two stages to create two low temperature stations, one at 80 K and the other at 14 K with no load. The temperature of the cold stations can be varied over a range of

14 to 300 K by controlling the TVR. For fine control of the temperature of the sample from 20 to 300 K, an electric heating coil with a temperature control circuit has been installed. Most of the vibrations are channeled away from the sample chamber which is connected to the refrigeration unit by a highly flexible bellows. The absorber holder and the Mossbauer driver are rigidly attached to the chamber. Thermal contact between the cold station and the sample hol- ders is accomplished by flexible oxygen-free, high conductivity copper strips. The staisfactory operation of the apparatus has been checked by comparing the Mossbauer spectra 57Fe in stainless steel (SS) with the refrigerator on and off, and such spectra are shown in figure 2. Evidently, there is no line broadening caused by the operation of the refrigerator.

This apparatus has been used to study Mossbauer effect in 1 2 9i compounds. The Mossbauer effect of

1 2 9I in Na1 2 9I yields a single line of 12 %.Mossbauer

spectra of 1 2 9l in Na1 2 9I have been measured by keeping the source temperature constant at 20 K and varying the absorber temperatures to 20, 40, 55, 65 and 73 K.

One typical spectrum is shown in figure 3. The measured isomer shifts at various absorber temperatures are given in table I. A plot of the isomer shift, 6 vs. T becomes linear at TQ = 60 ± 2 K as shown in figure 4, and the intercept on the isomer shift axis by extrapolation of the isomer shift data to T* = 0 gives -.480 ± .005 mm/s for the isomer shift due to

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979213

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C2-42

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F i g . 1 : Schematic diagram o f t h e a p p a r a t u s .

Fig. 2 : MEssbauer s p e c t r a of 5 7 ~ e i n SS w i t h r e f r i g e r a t o r o n (a) and o f f ( b )

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Table I

F i g . 3 : Mzssbauer s p e c t r u m of 1 2 9 ~ i n ~ a ' " 1 w i t h s o u r c e a t 2 0 K and a b s o r b e r a t 73 K.

Measured isomer s h i f t s i n N ~ a t v a r i o u s a b s o r b e r ~ ~ ~ I t e m p e r a t u r e s

F i g . 4 : R e l a t i o n s h i p between isomer s h i f t and kine- t i c t e m p e r a t u r e .

R e f e r e n c e s

/ 11 Mazo, R.Y. and Kirkwood, J. G.

,

P r o c . Nat. Acad.

S c i . ( U . S . )

2

(1955) 204.

121 C o l l i n s , R.L. and Cosgrove, J . G . , J. I n o r g . Nucl. Chem.

38

(1976) 507.