Development of a water calorimeter as the absorbed dose-to-water primary standard for China

Abstract

K. Wang , S. Jin , X. Yang , J. Zhang , M. McEwen , C. Cojocaru , C. Ross . National Institute of Metrology (NIM), China; National Research Council of Canada (NRC), Canada In collaboration with the National Research Council Canada (NRC), the National Institute of Metrology, China (NIM) is developing a new primary standard for absorbed dose to water at radiotherapy dose levels. The standard is based on a water calorimeter and is designed to operate in Co60 and megavoltage photon beams from a linear accelerator. The main features of the calorimeter are as follows: i) operation at 4 oC to eliminate the problems associatedwith convection in water phantoms at room temperature; ii) a low-noise temperature measurement circuit able to resolve temperature differences at the mK level; iii) capability to investigate different water purities and determine the heat defect due to dissolved gases; iv) potential extension to determination of absorbed dose in high-energy electron beams and HDR brachytherapy; v) determination of absorbed dose to water with a standard uncertainty of better than 0.4 %. Commissioning of the calorimeter involved a comparison with the present NIM standard of absorbed dose and a comparison with the NRC primary standard water calorimeter. Other investigations carried out included: the effect of environmental changes on the temperature control; repeatability of the system over the course of several weeks; stability of thermistor calibrations to determine the radiation-induced temperature rise; capability of the linac to deliver consistent doses (suitable for primary standards measurements) over the course of a day and determination of correction factors for beam uniformity for a range of detector geometries. A full uncertainty analysis indicates that the target uncertainty specification can be achieved. This new standard will significantly reduce the uncertainty of ion chamber calibrations for Chinese radiotherapy centres and open up new areas of research for the NIM.