Shielding effectiveness: A weighted figure of merit for space radiation shielding

The risk to space crew health and safety posed by exposure to space radiation is regarded as a significant obstacle to future human exploration missions to the Moon, Mars, and beyond. Engineers developing future spacecraft or planetary surface habitats can benefit from detailed knowledge of a broad range of possible materials that could provide improved protection to space crews from the deleterious effects of prolonged exposure to the space radiation environment. As one step towards providing this knowledge base, we have developed an empirical weighted figure of merit, referred to as shielding effectiveness, that quantifies the ability of a candidate material to shield space crews from the space radiation environment. The shielding effectiveness, as formulated in this study, accounts for the competing physical aspects of target and projectile fragmentation to provide a comprehensive assessment of radiation protection with regard to passive shielding for space applications. The empirical data used in determining shielding effectiveness was obtained from proton and heavy ion accelerator-based experiments wherein Al₂O₃:C optically stimulated luminescence dosimeter and CR-39 plastic nuclear track detector were irradiated behind candidate space radiation shielding materials of varying composition and depth. As a test case, the experimental setup was exposed to nominal beams of 1 GeV protons, and 1 GeV/n ²⁸Si and ⁵⁶Fe heavy ions, the latter serving as a sample of the high linear energy transfer portion of the galactic cosmic ray spectrum. Established radiation dosimetry techniques were used to obtain linear energy transfer spectra, absorbed dose, and dose equivalent as a function of depth. Based on the measurement results, a shielding effectiveness value was computed, quantifying the efficacy of the candidate material as a function of depth, with cumulative weighting factors accounting for the measured percent composition of baryonic matter in the galactic cosmic ray spectrum, and the measured percent contribution to absorbed dose and dose equivalent. The methodology for shielding effectiveness was tested using the common materials of aluminum, copper, graphite, and water, with polyethylene serving as the standard reference. The preliminary shielding effectiveness values for these materials confirm the low Z principle for effective space radiation shielding, and, furthermore, these values tend to be lower when the effectiveness calculation is based on dose equivalent. Of the common materials studied here, at a bulkhead depth of 5 g/cm², all materials provide a similar level of radiation protection to within standard error. In addition, this method can be used to supplement and/or verify similar findings obtained from transport models.