Abstract
The construction and operation of an isothermal microcalorimeter are described. The instrument relies on the triple point of water for the maintenance of the constant temperature envelope. This ensures a temperature stability to ±0.0001 degC. Thermistor thermometry is used and the smallest detectable temperature change in the calorimeter is 0.00012 degC corresponding to a power change of 0.5 μW. The microcalorimeter has been employed for the measurement of alpha and beta emitting radioactivities; for example, an activity of about 4 Ci of tritiated water was measured with an estimated accuracy to ±2.5%.