DELTA CAMPAIGN - AN ACCOUNT OF CONTROVERSY SURROUNDING TEMPERATURE SCALE FOR CMN

Theory predicts that the susceptibility of cerous magnesium nitrate will closely approximate Curie's law below 1 K and into the millikelvin region. Specifically, χ = c(T - θ)-1, with θ = - 0·27 mK. Single crystal specimens substantiate this prediction down to about 6 mK, below which strong deviations begin to appear and the susceptibility reaches a limiting, maximum value for temperatures below 2 mK. On the other hand, thermometers fashioned (for convenience) from powdered CMN and shaped into cylinders of diameter equal to height, appear-from the properties of 3He and expectations as to their temperature dependence-to obey Curie's law down to 3 mK and below. Such drastic differences are of considerable importance, inasmuch as CMN is the only practical thermometer currently available for this region of temperature. At various times, interested parties have expressed strong views that either the single crystal thermometric results or those for powder cylinders were wrong, or speculated that both might be correct. A condensed survey is here presented of the details of the dilemma, recent attempts to resolve it, and the situation at present. © 1969.