Radiation Shielding Properties of Lunar Soil and Soil Simulant

We have measured selected radiation transport and dose reduction properties of lunar soil, using samples returned by the Apollo missions and several types of synthetic soil glasses and lunar soil simulants, with the objectives of evaluating soil as potential radiation shielding in a lunar habitat and of man-made soil as a surrogate for use in ground-based studies.

Exposure to space radiation may be a limiting factor in future manned lunar missions. In contrast to the brief stays by the Apollo astronauts, in the coming decades humans will remain on the lunar surface for weeks and eventually months at a time. Chronic exposure to highly ionizing ions in the galactic cosmic radiation (GCR) and sporadic acute exposures to protons emitted in solar proton events (SPE) are health hazards that can be mitigated in part by radiation shielding. The spacecraft, spacesuits and rovers will provide only modest shielding, and the expense of transporting material to the moon will allow for little if any supplemental shielding material. An alternative is the essentially unlimited supply of lunar soil, if ways can be found to effectively use it. Reliable synthetics and simulants are needed due to the extreme scarcity of Apollo soil samples.

Beams of protons and heavier charged particles at energies comparable to the most biologically damaging components of the GCR are available at a few particle accelerator facilities, including the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the Japanese National Institute of Radiological Sciences, which was used for the data reported here.

The measurements and associated model calculations indicate that a fairly modest amount of soil—34 cm or less—affords substantial protection against primary GCR nuclei and SPE protons, with only modest residual dose from surviving charged fragments of the heavy beams.