Dosimetry of proton radiation fields in space with nuclear emulsions.

Abstract

The bulk of the astronauts' radiation exposure in space is due to trapped protons in the South Atlantic Anomaly or, on a deep space mission, in the radiation belt itself. The energy spectrum of the proton flux in both cases is a broad continuum from zero to several hundred MeV, with low energy protons and protons ending in tissue carrying a substantial fraction of the total dose. As demonstrated with the emulsion data of the Earth-orbital mission Apollo VII, a method using grain counting of tracks traversing the emulsion combined with a count of those ending in it (zero energy) provides sustained accuracy over the entire energy range. Within certain limits, the counts of ending protons at various locations in the space vehicle are proportional to the corresponding total doses. Highly structured directional patterns that are generally characteristic for the low energy particles have been analyzed in detail from the unmanned Apollo VI mission during which a total proton dose of 1.56 rad was observed within the vehicle. The fact that low energy protons with their comparatively high LET contribute substantially to the total dose is reflected in large microdosimetric fluctuations of the absorbed energy in tissue, with Bragg peaks of ending tracks occurring only in a few per cent of the total cell population even at dose levels of 50–100 rad.