Abstract: A calorimeter for use in assaying radioactive sources. The calorimeter includes a constant temperature shield and a series of temperature matched shields in which temperature control is provided by electrical resistance, thereby eliminating the need for a water jacket, while providing improved temperature and heat flow control. A labyrinth plug which eliminates air exchange between the sample well and the outside is used to seal the calorimeter. A computer control system monitors the calorimeter to reduce errors introduced by system fluctuations.