Radiation tests of the extravehicular mobility unit space suit for the international space station using energetic protons

Measurements to characterize the shielding properties of the EMU space suit and a human phantom were performed using 155 and 250 MeV proton beams at the Loma Linda University Medical Center (LLUMC). The beams simulate radiation encountered in low-Earth orbit (LEO), where trapped protons having kinetic energies on the order of 100 MeV are abundant. Protons at these energies can penetrate many g/cm2g/cm2 of matter and deliver a dose to the skin and internal organs. The dose can be enhanced or reduced by shielding, either from the space suit or the self-shielding of the body, but minimization of the risk depends on knowledge of the detailed energy spectrum and on the dose responses of the critical organs. Nuclear interactions of energetic protons in materials produce highly ionizing secondary radiation that increases dose and dose-equivalent beyond what would be expected simply from increasing ionization energy loss along the Bragg curve. Here, we present results obtained using silicon detectors in the LLUMC proton beams. Bare-beam data were taken to characterize the beams and calibrate the detectors. Data were also taken with the detectors placed inside a human phantom within the EMU suit. Because many secondaries have very high LET and short range, they are best measured in passive track detectors such as CR-39 or in much thinner silicon detectors than those used here. Our data complement the CR-39 data in the LET range below 5keV/μm, where CR-39 is insensitive. Our results suggest that optimizing the radiation shielding properties of space suits is a formidable task—simply adding mass may not reduce the net risk, because adding material to reduce the dose delivered at or near the skin by low-energy particles can increase the dose delivered by more energetic particles to sites deeper in the body. The depth-dose relation therefore depends critically on the energy distribution of the incident protons.