Calorimetric and ionimetric dosimetry intercomparisons I: U.S. neutron radiotherapy centers

A water calorimeter for neutron dosimetry

A water calorimeter has been used for the direct measurement of absorbed dose to water in a d(15) + Be neutron beam. The absorbed dose measured with the calorimeter was compared with that measured with an Exradin ionisation chamber, constructed of A-150 plastic. The doses measured by the ionisation chamber were calculated according to the European (ECNEU) protocol. Absorbed dose to tissue measured with the calorimeter was 4.3% lower than that measured with the ionisation chamber. Relative to ionisation chamber dosimetry, dose measurements with the calorimeter in the neutron beam were 9% lower than similar measurements in 4 and 9 MV photon beams. The significance of the results is discussed in terms of the heat defect in water.

Neutron dosimetry with spherical ionisation chambers III. Calculated results for tissue-equivalent chambers

Values are given of the average energy required to produce an ion pair, Wn, and of the dose conversion factor, r, for tissue-equivalent chambers filled with either tissue-equivalent gas or air, irradiated by neutrons. The model for finite spherical cavities previously presented in part I was used for the calculations, which employed the W values and stopping powers of the charged particles produced in the materials described in part II. Neutron energies ranging from 0.4 to 14 MeV were considered: many of these energies were chosen because of their particularly large or small total cross-sections in order to explore the range of fluctuations of r and Wn. The results are therefore not very suitable for spectral averaging. Cavity sizes ranging from the infinitesimal Bragg-Gray to the infinite cavity were studied. It was found that the changes of r with cavity size and with neutron energy are smaller for the TE-TE chamber than for the TE-air chamber, but for Wn they are about equal; the TE-TE chamber should therefore be considered the ionisation chamber of choice but absolute doses cannot be determined with it to better than +/- 8%.