Hydrogen (H-2) 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 Am-241-Be, Am-241-B, Cf-252 and Cf-252 + D2O. Based on the energy deposited in the gas cavities (tissue-equivalent (TE) propane and H-2) of the TEPC, density scaling correction, f rho, to be applied on H-2 gas to achieve tissue-equivalence is calculated for the above sources at 1 mu m 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 H-2-based microdosimetric distributions are comparable when the density of H-2 is scaled appropriately. Mean quality factor (Q) over bar 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 H-2-based (Q) over bar values are comparable to within 3-15%, and for the ISO sources, the comparison is within 5-8%. For the ISO neutron sources, (Q) over bar values are in the range of similar to 10-15.
