Dosimetry Research Using Low Pressure Proportional Counters for Neutrons with Energies up to 60 MeV

Low pressure proportional counters (PC) with walls made of A-150 plastic and graphite were used to measure ionisation yield spectra and kerma in radiation fields designed to produce monoenergetic neutrons in the energy range of 20-70 MeV at the Swiss Institute for Nuclear Research (SIN). The investigation at nominal energies of 27.8, 39.7 and 60.3 MeV has shown that the PC is a suitable instrument for dosimetry research in this energy range also. The spectrum information can be used to study absorbed dose build-up in detail. The combined graphite and A-150 measurements were used to evaluate gas-to-wall absorbed dose conversion factors. The kerma ratio of carbon to A-150 follows the shape of curves for theoretical values but is systematically lower. This is explained by a contamination of the beam with low energy neutrons.