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Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 09: Stability of a water calorimetry system as a primary standard for absorbed dose to water
Author(s) -
Cojocaru Claudiu,
Mui Bryan,
McEwen Malcolm,
Ross Carl,
Klassen Norman,
Marchington David
Publication year - 2016
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 - 10.1118/1.4961863
Subject(s) - absorbed dose , dosimetry , dosimeter , thermistor , calorimetry , materials science , primary standard , radiation , ionizing radiation , calorimeter (particle physics) , irradiation , radiochemistry , nuclear engineering , nuclear medicine , chemistry , physics , calibration , nuclear physics , optics , thermodynamics , medicine , engineering , quantum mechanics , detector
Purpose: To investigate the stability of a water calorimetry system as a primary standard for absorbed dose to water using measurements performed in cobalt‐60 and high‐energy linac photon beams over a span of more than a decade. Methods: Calorimetry measures adsorbed dose directly by recording the amount of heat created when ionizing radiation passes through matter. The radiation‐induced temperature rise was measured using two thermistors calibrated against the NRC temperature primary standard, using an AC bridge with lock‐in amplifier for precise measurement. The calorimeter system was operated under thermal equilibrium at 4 °C (to eliminate convection) with drifts in water temperature less than 0.1 mK/min. Seven water vessels of various designs were used to make repeated measurements over the course of 17 years. Results: The standard uncertainty achieved for a set of ten calorimeter measurements (4 Gy delivered) was generally well below 0.15 % while the variation between multiple sets for a given vessel was consistent with this value. The long‐term stability of the system combined with inter‐vessel variations indicated that there was good control of the radiochemistry (chemical heat defect). Conclusions: The measurements performed over a period of several years showed that the combined water calorimeters showed stability at +/− 0.25 % level. Thus, rather than relying on a particular vessel as an artifact one can realize the Gray through the more generalized method of combining a glass vessel, high‐purity water and thermistor probes. This provides increased robustness in the dissemination of absorbed dose to Canadian users.