Frequency of Brain Cell Hits by High-LET Particles in a Low-Earth-Orbit Space Mission

Abstract

The fluence of high-LET particles in space radiation was measured with plastic nuclear track detectors (PNTDs) in the brain of a life-size human phantom in the 9th Shuttle-Mir Mission (STS-91). The relationship between PNTD track-formation sensitivity (S) and LET∞·H₂O was examined using heavy-ion beams at the NIRS-HIMAC and the incident-angle dependence of S was corrected using an empirical function. The particle fluence of space radiation with LET∞·H₂O greater than 10keVμm^-1 in the brain was evaluated as 1.3×10⁴ncm^-2 in a 9.8-day low-Earth-orbit mission at 400km×51.6°. Based on these data, the ratio (the number) of hits to hippocampus cells and cell-nuclei were calculated for long missions. As a result, it was estimated that 45% (2.2×10⁷) cells and 7.0% (3.4×10⁶) cell-nuclei will be hit by high-LET particles (LET∞·H₂O>10keVμm^-1) in a 90-day mission, and 91% (4.4×10⁷) and 25% (1.2×10⁷) of cell-nuclei will be hit in a 1-year mission.