Ionisation distributions and A-150 plastic kerma for neutrons between 13.9 and 19.0 MeV measured with a low pressure proportional counter

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Abstract
Tissue equivalent proportional counters (TEPC) operated at low gas pressures can be used to measure simultaneously ionisation distributions induced by single events and, with a low overall uncertainty, absorbed dose or kerma. TEPC measurements were performed in a low scattering environment for monoenergetic neutrons with energies of 13.9, 15.0, 17.0 and 19.0 MeV. Neutron fluence and spectral neutron fluence were determined with a proton recoil telescope and an NE 213 scintillation counter. Control measurements permitted detailed analysis of and correction for scattered and parasitic neutrons. The kerma factors for A-150 plastic at 13.9, 15.0 and 17.0 MeV obtained from the TEPC measurements agree very well with calculated values. At 19 MeV the measured kerma factors are significantly lower. The ionisation spectra provide information on kerma fractions due to different types of secondary charged particles. The measured spectra suggest that the difference in kerma factors at 19.0 MeV is mainly due to differences in kerma related to alpha particles, especially by the 12C(n,n'3 alpha ) reaction. The results show that the combination of accurate fluence and TEPC measurements enables the determination of basic data for fast neutron dosimetry and neutron interactions.