Geant4-based microdosimetry for astronauts' radiation protection
Abstract
Space exploration by means of human missions implies astronauts' exposure to high energy cosmic radiation, which may have harmful health effects. Latest researches predict the cosmic rays exposure during long space missions outside the Earth's geomagnetic sphere, as to the Mars, can double the cancer risk, The main radiation sources are Galactic Cosmic Rays (GCR), made up of approximately 87% protons, 12% alpha particles and 1% heavy ions (e.g. C, O, Si, Fe) with a wide energy range, up to hundreds of GeV/n; Solar Particle Events (SPEs), mainly consisting of protons emitted by the Sun, with energies up to hundreds of MeV/n; trapped protons and electrons of the Van Allen Belts. It is paramount to characterize cosmic radiation and its effects on astronauts' health starting from where astronauts are currently living: at the International Space Station (ISS). A powerful approach to determine the effect of space radiation in astronauts is microdosimetry: Silicon-On-Insulator (SOI) microdosimeters developed at the Centre for Medical Radiation Physics, represent a perfect alternative to the disadvantageous TEPCs, for radiation protection purposes. SOI microdosimeters consist in a matrix of silicon Sensitive Volumes (SV), which mimic a real biological cell distribution. This project includes the characterization, in terms of particles spectra, of the radiation environment typically encountered at the ISS's altitude, outside and inside the Columbus module. Moreover, we performed for the first time the response of the novel 3D SOI "Mushroom" microdosimeter in such environment. The research was done by means of Monte Carlo simulations, confirming the suitability of GEANT4 operating in space environments for human health radiation protection applications. Firstly, we simulated the radiation environment outside the ISS through SPENVIS, a tool provided by ESA along a real orbit at 408km altitude. The obtained spectra were used to model the isotropic radiation field by means of the Geant4 General Particle Source (GPS), as emerging from a sphere of 1km radius, towards the ISS, set in the center. In order to reduce the execution times of the simulation, particles were modeled emerging from a cone with a half-aperture angle $\theta$max, which is the maximum angle that subtends the entire Columbus and directed with a cosine distribution. By adopting this bias, we reduced the effective relevant cross-section surface to a smaller sphere. Based on specifications found in the literature, we modeled the Columbus module as a multilayer cylinder. By the geant4.10.04v, both electromagnetic and hadronic physics interactions were modeled in the simulation. The Geant4 Physics lists that we adopted are the G4EmStandardPhysics\_option3 and the G4HadronPhysicsQGSP\_BIC\_HP. The kinetic energy spectra of all primary and secondary particles produced through the interaction with the multilayer, behind the innermost layer of Columbus' wall, were retrieved as output of the simulation. Secondly, we simulated the response of the 3D SOI "Mushroom" microdosimeter placed in the center of the Columbus considering the radiation environment studied above. The result of the simulation is the frequency of the energy deposition of each single event occurring in the silicon SVs. We calculate the microdosimetric tissue-equivalent spectra for GCR, SPE and trapped particles sources with the bare microdosimeter configuration and with the microdosimeter embedded in a PMMA sphere of 10mm radius. This analysis of those spectra led us to the evaluation of the astronauts' dose equivalent in a 1-day mission as well as the distinction of the dose due to different type of particles. The calculated dose equivalent agrees with the literature. The outcomes of this study confirm the suitability of GEANT4 operating as a tool for cosmic radiation field characterization and modeling the response of microdosimeters for human health radiation protection purposes.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1883P