An experimental study of the microdosimetric response of a graphite walled proportional counter in low energy neutron fields

This study describes the measurement of lineal energy spectra carried out with a graphite walled proportional counter (GPC) simulating 1 μm tissue equivalent (TE) site diameter in low energy neutron fields generated by employing the McMaster University 1.25 MV double stage Tandetron accelerator. Experiments were carried out for neutron beams of mean energies extending from 34 to 354 keV in direct neutron beams from the 7Li target using proton beam energies ranging from 1.89 to 2.56 MeV. e observed a continuously increasing trend in microdosimetric averages, y ¯ F and y ¯ D , and the ICRP 60 based quality factor, Q, with an increase in neutron energy. It is interesting to note that standard tissue equivalent proportional counters (TEPCs) give rise to a similar trend for these microdosimetric quantities of interest in the same energy range. Quantitatively, these quantities are also comparable to those obtained with standard TEPCs. This implies that at the microdosimetric level in the neutron energy range we employed in this study, the pattern of average energy deposited by ‘starter’ and ‘insider’ proton recoil events in the gas is similar to those generated cumulatively by ‘crosser’ and ‘stopper’ events originating from the counter wall plus ‘starter’ and ‘insider’ recoil events originating in the sensitive volume of a TEPC. This suggests that the average energy deposition per event in the wall of a TEPC counter is the same as in the counting gas itself, which confirms the similarity of the composition and mass stopping power of two media, i.e., TE wall and TE gas, used in neutron dosimetric measurements.