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 N2, H-2, and mixtures of H-2/O2. The repeatability of the measurements of and dose rates for the Co-60 teletherapy beam was studied with different water fillings and accumulated absorbed dose. Measurements with the H-2/O2 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 N2- 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).