The photon calibration of a tissue-equivalent ionisation chamber for neutron dosimetry

Abstract

The poor agreement in the photon calibrations of tissue-equivalent ionization chambers between different participants in 2 major neutron dosimetry intercomparisons is noted. The methods of converting measurements made with a calibrated secondary standard exposure meter to the absorbed dose in the wall of a tissue-equivalent [TE] chamber are discussed. Calibrations of a TE ionization chamber were made in 5 photon beams (250 kV, 300 kV, 137Cs, 60Co and 4 MV) and the different calibration factors were compared for in-air and in-phantom measurements. Good agreement was found in these factors provided allowance was made for the attenuation and scattering in the wall and build-up cap of the TE chamber for the in-air measurements, and revised values of C.lambda. were used for the in-phantom measurements at the 2 higher energies.