Helium refrigerator for temperature varying Moessbauer studies

A closed cycle helium refrigerator has been adopted for studies of Moessbauer 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 (delta) in the absence of all molecular motion because delta is linearly dependent on kinetic temperature T* which is defined as T0 coth T0/T. The zero-point temperature T0 is determined when delta becomes linear in T*. The chemical contribution in delta can be found by extrapolation of the isomer shift data to T*=0. In this way T0 and delta for NaI have been found to be 60+-2 K and -.480+-.005 mm/s respectively