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The direct determination of dose‐to‐water using a water calorimeter
Author(s) -
Schulz R. J.,
Wuu C. S.,
Weinhous M. S.
Publication year - 1987
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.596004
Subject(s) - thermistor , calorimeter (particle physics) , materials science , calorimeter constant , capillary action , core (optical fiber) , ionization chamber , calibration , analytical chemistry (journal) , composite material , calorimetry , electrical engineering , optics , thermodynamics , chemistry , physics , ionization , chromatography , ion , organic chemistry , quantum mechanics , detector , engineering
A flexible, temperature‐regulated, water calorimeter has been constructed which consists of three nested cylinders. The innermost “core” is a 10×10 cm right cylinder made of glass, the contents of which are isolated from the environment. It has two Teflon‐washered glass valves for filling, and two thermistors are supported at the center by glass capillary tubes. Surrounding the core is a “jacket” that provides approximately 2 cm of air insulation between the core and the “shield.” The shield surrounds the jacket with a 2.5‐cm layer of temperature‐regulated water flowing at 5 l/min. The core is filled with highly purified water the gas content of which is established prior to filling. Convection currents, which may be induced by dose gradients or thermistor power dissipation, are eliminated by operating the calorimeter at 4 °C. Depending upon the power level of the thermistors, 15–200 μW, and the insulation provided by the glass capillary tubing, the temperature of the thermistors is higher than that of the surrounding water. To minimize potential errors caused by differences between calibration curves obtained at finite power levels, the zero‐power‐level calibration curve obtained by extrapolation is employed. Also the calorimeter response is corrected for the change in power level, and therefore thermistor temperature, that follows the resistance change caused by irradiation. The response of the calorimeter to 4‐MV x rays has been compared to that of an ionization chamber irradiated in an identical geometry. For nitrogen‐saturated water, the grand mean of the calorimeter‐to‐ion chamber (Cal/Ion) dose ratio for five experiments conducted over a period of six months is 1.006±0.001. Three experiments with oxygen‐saturated water in the same core yeild a Cal/Ion ratio of 0.991±0.001. These results are consistent with radiochemical models and refined experiments that used water saturated with various gases, and suggest that nitrogen‐saturated water calorimeters of the type described may be used to determine the dose‐to‐water without reference to radiation‐dependent parameters or recourse to corrections for thermal defects.