X Ray Spectrometry of Low Energy Photons for Determining Conversion Coefficients from Air Kerma, Ka, to Personal Dose Equivalent, Hp(10), for Radiation Qualities of the ISO Narrow Spectrum Series

The pulse height spectra of the radiation qualities N-10 to N-120 of the ISO narrow spectrum series (N) were measured using a commercial Ge spectrometer and a specially designed lead collimator. For the unfolding of these spectra, three response matrices of the Ge spectrometer for different maximum energies and energy resolutions were calculated by the EGS4 (Electron Gamma Shower version 4) code incorporating the PRESTA and LSCAT options. Then, the pulse height spectra were unfolded with two different algorithms to check the correctness. To verify the measuring and unfolding method, the first half value layers (HVLs) were determined using small ionisation chambers and the fluence spectra. The mean photon energies were calculated, also on the basis of the fluence spectra, and compared with published values. For each radiation quality N-10 to N-120 the conversion coefficients hpK(10;N,a) from air kerma Ka to personal dose equivalent at 10mm depth, Hp (10,a), were calculated for angles of incidence a of 15, 30, 45, 60(and 75(between the unidirectional photon field and the normal to the slab phantom surface. Because the fluence spectra were influenced by the air density, in particular the low energy spectra, the hpK(10;N,a) values were normalised to reference conditions. Differences of up to about 88% between the hpK(10;N,a) values presented in this work and those given in ISO/FDIS 4037-3 were found.