A study of neutron radiation quality with a tissue equivalent proportional counter for a low-energy accelerator-based in vivo neutron activation facility

The accelerator-based in vivo neutron activation facility at McMaster Univ. has been used successfully for the measurement of several minor and trace elements in human hand bones due to their importance to health. Most of these in vivo measurements have been conducted at a proton beam energy (Ep) of 2.00 MeV to optimise the activation of the selected element of interest with an effective dose of the same order as that received in chest X rays. However, measurement of other elements at the same facility requires beam energies other than 2.00 MeV. The range of energy of neutrons produced at these proton beam energies comes under the region where tissue-equivalent proportional counters (TEPCs) are known to experience difficulty in assessing the quality factor and dose equivalent. In this study, the response of TEPCs was investigated to determine the quality factor of neutron fields generated via the 7Li(p, n)7Be reaction as a function of Ep in the range 1.884-2.56 MeV at the position of hand irradiation in the facility. An interesting trend has been observed in the quality factor based on ICRP 60, QICRP60, such that the maximum value was observed at Ep=1.884 MeV (E n=33±16 keV) and then continued to decline with increasing Ep until achieving a minimum value at Ep52.0 MeV despite a continuous increase in the mean neutron energy with Ep. This observation is contrary to what has been observed with direct fast neutrons where the quality factor was found to increase continuously with an increase in Ep (i.e. increasing En). The series of measurements conducted with thermal and fast neutron fields demonstrate that the 14N(n, p)14C produced 580 keV protons in the detector play an important role in the response of the counter under 2.0 MeV proton energy (En ≤ 250 keV). In contrast to the lower response of TEPCs to low-energy neutrons, the quality factor is overestimated in the range 1-2 depending on beam energy < 2.0 MeV. This study provides an insight to understanding the response of TEPCs in low-energy neutron fields where the neutrons are moderated using a polyethylene moderator. (authors)