FLUKA capabilities for microdosimetric analysis

Delta-ray transport is important in microdosimetric studies, and how Monte Carlo models handle delta electrons using condensed histories is important for accurate simulation. The purpose of this study was to determine how well FLUKA can simulate energy deposition spectra in a tissue-equivalent proportional counter (TEPC) and produce a reliable estimate of delta-ray events produced when a TEPC is exposed to high-energy heavy ions (HZE) like those in the galactic cosmic-ray (GCR) environment. A 1.27-cm spherical TEPC with a low-pressure gas simulating a 1-μm site, typical of the one flown on the ISS, was constructed in FLUKA, and its response was compared to experimental data for an (56)Fe-ion beam at 360 MeV/nucleon. Several narrow beams at different impact parameters were used to explain the response of the same detector exposed to a uniform field of radiation. Additionally, the effect that wall thickness had on the response of the TEPC and the range of delta rays in the tissue-equivalent (TE) wall material was investigated, and FLUKA produced the expected wall effect for primary particles passing outside the sensitive volume. A final comparison to experimental data was made for the simulated TEPCs exposed to various broad beams in the energy range of 200-1000 MeV/nucleon. FLUKA overestimated energy deposition in the gas volume in all cases. The FLUKA results differed from the experimental data by an average of 25.2% for y(F) and 12.4% for y(D). It is suggested that this difference can be reduced by adjusting the FLUKA default ionization potential and density correction factors. Accurate transport codes are desirable because of the high cost of beam time for experimental evaluation of energy deposition spectra produced by HZE ions and the flexibility that calculations offer in the TEPC engineering and design process.