The design of future spacecraft such as the Crew Exploration Vehicle must take into account the radiation shielding properties of both the structural components as well as dedicated shielding materials. Since modest depths of shielding stop the vast majority of Solar Energetic Particles (SEP), the greater challenge is posed by the need to shield crew from the Galactic Cosmic Rays (GCR), which include highly-charged and highly-energetic particles. Here, we report on results from tests performed with beams of 1 GeV/nuc 56Fe at the Brookhaven National Laboratory. A wide variety of targets, both elemental and composite, were placed in the particle beams, and the spectra of particles emerging from the targets were measured using a stack of silicon detectors. Results are presented primarily in terms of dose reduction per g cm-2 of target material, and support the conclusions of an earlier calculation by Wilson et al. showing that performance improves as the shield's mass number decreases, with hydrogen being by far the most effective. The data also show that, as depth increases, the incremental benefit of adding shielding decreases, particularly for aluminum and other elements with higher atomic mass numbers.

This report describes the highlights and progress made in a program of measurements studying radiation transport through materials of interest to NASA. All measurements were preformed at accelerator facilities, primarily using GCR-like heavy-ion beams incident upon various elemental and composite targets. Both primary and secondary particles exiting the target were measured. The secondary particles include both charged particles and neutrons. These measurements serve as useful benchmarks and input to transport model calculations.

Radiation risk management for human space missions depends on accurate modeling of high-energy heavy ion transport in matter. The process of nuclear fragmentation can play a key role in reducing both the physical dose and the biological effectiveness of the radiation encountered in deep space. Hydrogenous materials and light elements are expected to be more effective shields against the deleterious effects of Galactic Cosmic Rays (GCR) than aluminum, which is used in current spacecraft hulls. NASA has chosen polyethylene, CH2, as the reference material for accelerator-based radiation testing of multi-function composites that are currently being developed. A detailed discussion of the shielding properties of polyethylene under a variety of relevant experimental conditions is presented, along with Monte Carlo simulations of the experiments and other Monte Carlo calculations in which the entire GCR flux is simulated. The Monte Carlo results are compared to the accelerator data and we assess the usefulness of 1 GeV/amu 56Fe as a proxy for GCR heavy ions. We conclude that additional accelerator-based measurements with higher beam energies would be useful.

The radiation environment aboard the space shuttle and the International Space Station includes high-Z and high-energy (HZE) particles that are part of the galactic cosmic radiation (GCR) spectrum. Iron-56 is considered to be one of the most biologically important parts of the GCR spectrum. Tissue-equivalent proportional counters (TEPC) are used as active dosimeters on manned space flights. These TEPC's are further used to determine average quality factor for each space mission. A TEPC simulating a 1 micron diameter sphere of tissue was exposed as part of a particle spectrometer to iron-56 at energies from 200-1000 MeV/nucleon. The response of TEPC in terms of frequency-averaged lineal energy, dose-averaged lineal energy, as well as energy deposited at different impact parameters through detector was determined for six different incident energies of iron-56 in this energy range. Calculations determined that charged particle equilibrium was achieved for each of the six experiments. Energy depositions at different impact parameters were calculated using a radial dose distribution model and the results compared to experimental data.