The Measurement of Locally Absorbed Dose of Megavoltage X rays by Means of a Carbon Calorimeter

Abstract

A carbon calorimeter is described which has been used to measure the energy locally absorbed from beams of X-radiation generated at 4, 7.5, 14 and 20 MV and from [gamma] rays from Co60. The calorimeter is of the''isothermal twin type and does not need a thermo-static bath. Calorimetrlc measurements have been compared with ionlzation in a carbon chamber of known volume. The calorimeter and chamber were designed to be as similar as possible, so that they measure, to the same degree of approximation, the energy absorption at a point in a block of carbon and the ionization in a small cavity at the same point With cobalt i rays the measurements give W = 34.0 [plus or minus] 0.2 eV per ion pair in air. The ionization in the carbon chamber has been compared with that in a Baldwin-Farmer chamber exposed under particular conditions in a Perspex block, and a calibration for the Baldwin-Farmer chamber in terms of absorbed dose in water has been derived.