During the Artemis-1 mission in late November to early December 2022, the uncrewed Orion spacecraft flew around the Moon. On board, thousands of small crystals collected data on space radiation. Some of them - several dozen fluorescent detectors based on lithium fluoride - were developed at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow. The method for reading information from these detectors is described in the journal Radiation Measurements.
Such detectors are passive: they work like photographic film. When a particle of ionizing radiation passes through a crystal, it leaves defects in its lattice. During laboratory readout, these defects are excited by blue light and emit a red glow - fluorescence - that can be seen under a microscope. According to one of the authors of the work, Pawel Bilski, after irradiation the particle tracks are analyzed optically.
In the MARE experiment (Matroshka AstroRad Radiation Experiment), the detectors were installed in two mannequins simulating a human. In total, IFJ PAN supplied almost 600 crystal detectors of another type - thermoluminescent - for the experiment.
Reading particle tracks near the Moon turned out to be fast: lithium fluoride does not need chemical processing, a fluorescence microscope is sufficient. Difficulties began when trying to obtain quantitative estimates - complex calibration methods were required. Cosmic rays come from all directions and contain all types of particles and nuclei found in nature over a wide range of energies. Therefore, under the microscope the tracks run in different directions, and the camera captures blurred glow from areas outside the focal plane. In addition, strongly ionizing particles leave solid lines, while weakly ionizing ones leave only chains of individual dots.
After corrections and the development of a program that removes background, the results differed by no more than 15% from the data of conventional track detectors across the entire overlapping range. The researchers also recorded many tracks with energies below the threshold of detectors used until now.
On the ISS in the MATROSHKA experiment, part of the dose in the mannequins was contributed by low-energy protons from the Van Allen radiation belts - regions around Earth that trap charged particles. They lost energy at different depths, so the dose was distributed unevenly. Near the Moon there are no Van Allen belts, but galactic radiation is stronger. Its energies are so high that particles pass through the entire body, leaving only part of their energy. Therefore, the dose distribution inside the mannequins in Artemis-1 was practically uniform.
"Low-energy protons trapped by Earth's magnetic field can provide the main part of the dose for astronauts in low Earth orbit. Near the Moon this contribution disappears. Our measurements show: although the total doses near the Moon are higher because of galactic radiation, the difference is not as great as one might intuitively expect," Bilski notes.
The detectors were grown by the Czochralski method - a method of obtaining crystals from a melt. Their main advantages are a size on the order of millimeters rather than centimeters, better track resolution and a wide range of particles and energies. Detectors enriched with lithium-6 can also record neutron doses.
These are preliminary estimates obtained during the uncrewed flyby. They do not prove that the radiation environment near the Moon is safe or dangerous for future crews, but only refine the expected doses.