Mn-54 in Fe for Ultralow Temperature Thermometry

Experimental investigations into the accuracy of nuclear orientation thermometry are reported. Mn-54 in iron, a primary thermometer useful from 2-40 mK, gives consistent thermometry using both scintillation (3 x 3 in.-NaI) and solid-state (40-cc-Ge) detectors. Co-60-in-iron temperatures are consistent with the Mn-54 temperatures above 10 mK, but are about 10 % higher at 4 mK. Sb-125 in iron registers a temperature that is perhaps 2 or 3 % lower over the 4-15 mK region. Some of the techniques required for the utilization of this type of thermometry are discussed.