A Sealed Water Calorimeter for Measuring Absorbed Dose

The NIST sealed water calorimeter is intended for direct measurement of absorbed dose to water. This calorimeter was used for a series of approximately 3700 measurements to investigate the so-called heat defect, that is, anomalous endothermic or exothermic effects caused by dissolved gases. The three systems investigated were “high-purity” water saturated with N(2), H(2), and mixtures of H(2)/O(2). The repeatability of the measurements of absorbed dose rates for the (60)Co teletherapy beam was studied with different water fillings and accumulated absorbed dose. Measurements with the H(2)/O(2) system varied with accumulated absorbed dose. Based on the measurements and theoretical considerations, it appears that the H(2)-saturated system is the best choice for eliminating the heat defect. Measurements with both the N(2)- and H(2)-saturated systems are in good agreement with those determined with a graphite and graphite-water calorimeter (for which there is no heat defect).