A Sealed Water Calorimeter for Measuring Absorbed Dose

National Institute of Standards and Technology, Gaithersburg, MD 20899-0001 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, H2, and mixtures of H2/O2. The repeatability of the measurements of absorbed dose rates for the '*"Co teletherapy beam was studied with different water fillings and accumulated absorbed dose. Measurements with the H2/O2 system varied with accumulated absorbed dose. Based on the measurements and theoretical considerations, it appears that the H2saturated system is the best choice for eliminating the heat defect. Measurements with both the N2and H2-saturated systems are in good agreement with those determined with a graphite and graphite-water calorimeter (for which there is no heat defect).