A water calorimeter for high energy x-rays and electrons

The current primary standards at NPL for the measurement of absorbed dose to water in high energy photon and electron beams are graphite calorimeters. However, the quantity of interest in radiation dosimetry is absorbed dose to water. Therefore, a new absorbed dose to water standard based on water calorimetry has been developed for use in high energy photon and electron beams. The calorimeter operates at 4 deg C, with temperature control being provided by liquid cooling. The sealed glass inner vessel of the calorimeter was designed to minimise the effect of non-water materials on the measurement of absorbed dose. The temperature sensing thermistor probes were designed and constructed so that glass is the only material in contact with high purity water inside the vessel. Initial measurements of absorbed dose to water made in 6, 10, and 19 MV photons, and 16 MeV electrons agreed, within the measurement uncertainties of approximately 1.5% (95% c.l.), with those determined by graphite calorimetry. These measurements confirmed the feasibility of the calorimeter design. Significant improvements have been made to the temperature measurement system, the water phantom and its temperature controlled enclosure since these measurements were performed, reducing the estimated uncertainty on the measurement of absorbed dose to water to 1% (95% c.I.), although further work is required before the calorimeter can be used as a primary standard