Calculation of quality factors for space radiations at Low Earth Orbit: Monte Carlo-based microdosimetric approach.

This study aims to calculate quality factors (Q) for the radiation environment at Low Earth Orbit (LEO) using the Monte Carlo-based microdosimetric techniques. The study considers Galactic Cosmic Ray (GCR) ions (Z = 1 - 28), trapped protons, and albedo neutrons. Q is calculated with (2.8 g/cm²polyethylene and 10 g/cm²thick Aluminum) and without shielding. FLUKA code is used to model a uniform radiation environment incident on a spherical spacecraft having the above shielding material while a spherical Tissue Equivalent Proportional Counter (TEPC) is positioned at the center of this spherical envelope. The initial fluence spectra of the above radiation fields are based on OLTARIS code. Microdosimetric distributions in the cavity of the TEPC are calculated separately for GCR ions, trapped protons and albedo neutrons. Using the microdosimetric distributions, Q values for these radiations including mission Q value are calculated based on Theory of Dual Radiation Action (TDRA) model, and the formalisms based on ICRP60 and ICRU40 reports. Q initially increases with Z of the ion and beyond Z = 22, it becomes insensitive to Z. Depending on the calculation model, mission Q value is in the range of: 1.91 - 2.45 for without shielding, 1.67 - 2.07 for 2.8 g/cm²polyethylene shielding and 1.81 - 2.48 for 10 g/cm²Aluminum shielding. The microdosimetry-based mission Q values corresponding to 2.8 g/cm²polyethylene shielding calculated according to ICRP60 and ICRU40 models compare well with the published measured values of previous space missions. The calculated TDRA-based Q values compare well with the values derived from OLTARIS code. A significant reduction in the dose-equivalent is achieved with 10 g/cm²Al shielding as compared to 2.8 g/cm²polyethylene. Overall, the study enhances the understanding of how shielding influences Q and dose-equivalent.