A Polystyrene-Water Calorimeter

The paper describes a new type of calorimeter that can be quickly put into operation for determining absorbed dose at a point in polystyrene. It also describes a unique method of decreasing drifts in electrical signals caused by temperature gradients. Two calibrated thermistors were placed close together between sandwiched polystyrene discs that were immersed in water. The assembly was irradiated with gamma rays from a cobalt-60 source. The dose rate was about 14 mGy/s and exposure times were about 100 s. The standard deviation for a daily set of measurements was about 0.7%. A zero heat defect for polystyrene was assumed. A calculation converted the measurements to absorbed dose in water. The dose in water determined in this way, and with a graphite calorimeter, is 3-4% lower than that measured in an all-water calorimeter previously reported. Drifts in electrical signals are eliminated by a resistance-capacitance circuit placed across a Wheatstone bridge. The rate of potential change across the bridge (caused by the circuit) is adjusted to have an opposite effect to the drifts in electrical signals produced by temperature gradients within the calorimeter. The method can be applied to other calorimeters.