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

Abstract

The poor agreement in the photon calibrations of tissue-equivalent ionisation chambers between different participants in two 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 chamber are discussed. Calibrations of a TE ionisation chamber were made in five photon beams (250 kV, 300 kV, 137Cs, 60Co and 4 MV) and the different calibration factors were compared both for in-air and in-phantom measurements. Good agreement was found in these factors provided that 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 that revised values of C lambda were used for the in-phantom measurements at the two higher energies.

Determination of absorbed dose and kerma in a neutron field from measurements with a tissue-equivalent ionisation chamber

“…The results of absorbed-dose measurements in the user's radiation beam should be independent of calibration quality, which implies that the result of dividing a, by the product ( W,)c(s,,g)c[(~.en/~)t/(~en/~)m]c should not be dependent on photon energy. Williams and Greening (1980) measured a, in several photon spectra between 250 kV and 4 MV x-rays in both air and a phantom against a secondary standard exposure chamber. Using the assumption that (Wg)c and (smJc do not vary over this energy range they concluded that this quotient did remain constant provided the correct factors were applied for displacement and wall attenuation.…”