Absolute Temperatures below 1°K : Chromic Methylammonium Alum as a Thermometric Substance

The method of gamma-ray calorimetry has been employed to determine the entropy-temperature relation for a single-crystal of chromic methylammonium alum in the range 0.015°—1°K. A critical discussion of possible sources of error is given; the accuracy in T is estimated to be approximately 5% throughout the region investigated. There is close agreement between the Hebb-Purcell theoretical S—T relation and the present data down to 0.07°K. The fact that the susceptibility-entropy relation varies considerably for different specimens at temperatures below 0.06°K suggests that S—T measurements are of limited value in that region, especially from a thermometric point of view.