TISSUE-EQUIVALENCE OF H2 GAS FOR MICRODOSIMETRY IN NEUTRON FIELDS: A GEANT4 MONTE CARLO STUDY.

Hydrogen (H2) as filling gas in Tissue-equivalent proportional counter (TEPC) for measurements of microdosimetric distributions in neutron fields is investigated using the Monte Carlo-based Geant4 toolkit. The neutron fields considered are monoenergetic neutrons (1 keV-14 MeV) and ISO reference neutron sources 241Am-Be, 241Am-B, 252Cf and 252Cf + D2O. Based on the energy deposited in the gas cavities (tissue-equivalent (TE) propane and H2) of the TEPC, density scaling correction, ${f}{\rho }$, to be applied on H2 gas to achieve tissue-equivalence is calculated for the above sources at 1μm site size. The calculated value of ${f}{\rho }$ for the ISO-neutron sources is 0.40, which suggests that energy deposition in the gas cavities is predominantly due to crossers. In the case of monoenergetic neutrons, depending upon the energy, ${f}_{\rho }$ is in the range of 0.11-0.45. The study shows that the TE propane and H2-based microdosimetric distributions are comparable when the density of H2 is scaled appropriately. Mean quality factor $\overline{Q}$ calculated based on microdosimetric distribution increases initially with neutron energy up to 100 keV and thereafter decreases with energy. Depending upon the neutron energy, TE propane and H2-based $\overline{Q}$ values are comparable to within 3-15%, and for the ISO sources, the comparison is within 5-8%. For the ISO neutron sources, $\overline{Q}$ values are in the range of ~10-15.