A study of some non-hydrated paramagnetic substances for cooling below 1°K.

The desirable properties of the paramagnetic substances are discussed and the possibility of using some metallic alloys as well as dielectric crystals doped with proper substances capable of lowering the temperatures is illustrated. For this purpose some low temperature investigations of MgO:Mn and of Mg:Ce alloys is presented. The MgO:Mn doped with 0.0325% Mn gives a final temperature of 0.04°K when demagnetized from a field of 18 Kgauss and approximately 1.3°K. The specific heat measurements resulted in a value of CT 2/R = 6x10-3 for the "high temperature" tail of the specific heat. The entropy determinations performed on the Mg:Ce alloy system on the other hand indicated that the magnetic entropy at about 1°K is approximately of the same value as that of the conduction electrons.Therefore it seems that this particular alloy system is not suitable for appreciable lowering of the temperatures by adiabatic demagnetization. The specific heat of a well behaved pure substance is proposed as a resonably sensitive thermometric standard. Its usefulness is illustrated by calibrating a secondary thermometer using the proposed method. The difficulties encountered in low temperature calorimetry are described and our solution of them is given. A discussion is also included of the difficulties involved in using mechanical heat switches. A double mechanical heat switch is proposed and a design of it is included together with some illustrations of its usefulness