ABSTRACT
Thermometry is a central concern of low temperature physics. Physical quantities are often studied versus temperature, and a common place to question experimental results is in the thermometry. The most commonly used type of nuclear orientation thermometry is based on the anisotropic γ-ray decay from radioactive nuclei. Nyquist's Theorem states that due to the Brownian motion of conduction electrons, one will detect a randomly fluctuating voltage (Johnson noise) across a resistor. There was a time in the not so distant past when it was believed, or perhaps hoped, that the electronic paramagnetic susceptibility of certain salts would provide a nearly ideal thermometer at very low temperatures, a thermometer which above all could be understood from theoretical first principles. Although this has proved not to be the case, the cerous magnesium nitrate (CMN) and lanthanum diluted CMN (LCMN) thermometers remain very suitable devices for many purposes down to the low millikelvin regime.
