Results from the Martian Radiation Environment Experiment Marie

Introduction: One of the three science instruments aboard the 2001 Mars Odyssey spacecraft is the Martian Radiation Environment Experiment, MARIE. MARIE consists of a stack of silicon detectors, augmented by a Cerenkov detector. MARIE is designed to measure a portion of the particle spectrum of the Ga-lactic Cosmic Rays (GCR), as well as the high fluxes of low-energy protons (energies less than about 100 MeV) that are intermittently produced by active regions on the sun in Solar Particle Events (SPE). MARIE is providing the first detailed information about the radiation environment near Mars. The GCR are atomic nuclei, stripped of their electrons , that are accelerated through the interstellar medium by supernova shockwaves. Nuclei of every naturally occurring atomic species are present in the GCR, though ions heavier than iron are relatively rare. The flux is dominated by protons (about 87%) and helium (about 11%), with the remaining two percent divided among all " heavy " ion species. Although the flux of heavy ions in the GCR is small, their contribution to the radiation dose received by astronauts is considerable. The reason for this is that the dose delivered by a particle of a given velocity is proportional to the square of its charge (Z). Thus a neon ion (Z = 10) with an energy of 1 GeV/amu gives a dose 100 times greater than that delivered by a proton at 1 GeV/amu. The contributions of the heavy ions to dose equivalent (the legally-defined measure of exposure) are even larger owing to weighting factors that reach a peak of 20 for densely-ionizing particles. GCR energy spectra for a given species typically have broad peaks, centered at a few hundred MeV/amu, with long tails out to higher energies. Such energetic particles are capable of passing entirely through any plausible amount of shielding that a spacecraft might have. A dose to astronauts from this exotic form of radiation is therefore unavoidable, as has long been realized [1]. Solar Particle Events generally produce large fluxes of protons, occasionally with significant fluxes of heavier ions also present. Typically, the energy spectra of these particles is much " softer " than those of their counterparts in the GCR, and thus they are less penetrating. It is estimated [2] that shielding consisting of approximately 10 g cm-2 of aluminum would reduce the dose from even a large SPE to an acceptable level.