Low temperature Moessbauer thermometry

The basis of a Moessbauer low temperature thermometer has been discussed by Kalvius et al. The temperature is determined from the relative intensities of the various hyperfine lines as these are affected by the temperature dependent Boltzman populations of the hyperfine levels in the resonant nuclei. One can perform either source experiments or those that make use of the nuclear polarization in the absorber. The second method is more advisable because the radioactive heating is much smaller. Among the various Moessbauer transitions suggested for the absorber thermometer the 21.6 keV transition in 151Eu has acceptable characteristics for the temperature range from 25 to 250 mK. This work is concerned with a fundamental difficulty in absorber thermometry which arises from the fact that the line intensity in a Moessbauer spectrum is also dependent on the absorber thickness, the dependence on that is not a linear one. It is demonstrated that if this thickness effect is not analyzed properly, the experimental results will yield erroneous temperature values, destroying the absolute nature of the Moessbauer thermometer. (Auth.)