Preliminary results from tiny lithium fluoride crystals flown on Orion suggest astronauts near the Moon face more radiation than in low Earth orbit, though the gap is smaller than many would assume.
During NASA's uncrewed Artemis I mission in late November and early December 2022, the Orion spacecraft carried more than 10,000 tiny crystals embedded in two human-shaped mannequins to record radiation exposure around the Moon. Among them were several dozen experimental fluorescent lithium fluoride detectors developed at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow, flown as part of the Matroshka AstroRad Radiation Experiment (MARE). The institute also supplied nearly 600 thermoluminescent crystal detectors, which use a different measuring technique. The researchers have now described a new method for analysing the data stored in the crystals, in a paper published in the journal Radiation Measurements.
According to the Polish team, the lithium fluoride detectors work passively, like photographic film: charged particles create defects in the crystal, and when it is lit with blue light these regions glow red, letting scientists trace individual particle tracks under a fluorescence microscope. After applying corrections and software to remove background light, the results differed by no more than 15 percent from those of conventional track detectors across the overlapping range of energy deposition, and the crystals also recorded tracks below the detection threshold of earlier devices, says Prof. Pawel Bilski of IFJ PAN. Comparing the data with the earlier MATROSHKA experiment on the International Space Station, the team reports that near the Moon the Van Allen belts no longer contribute low-energy trapped protons, which can account for most of the dose in low Earth orbit. Instead, high-energy galactic particles dominate, and the dose was spread almost uniformly through the mannequins. Total doses near the Moon are higher than in Earth orbit, Bilski says, but the difference is not as large as one might intuitively expect.
The figures are described as preliminary and come from a single uncrewed flight, so they cannot by themselves define safety limits for future crews. The researchers note that the crystals, only a few millimetres across, are small enough to be worn by people, give sharper track images than plastic detectors that need chemical etching, and can also register neutrons when enriched with lithium-6. These points matter for monitoring radiation on future lunar missions. The MARE research was supported by a grant from Poland's National Science Centre under its OPUS programme. The account here relies on the research institute's description of its own study, so the conclusions should be read as the authors' findings rather than independently replicated results.
