Artemis I radiation data reveals surprising cosmic ray levels near the Moon

An international team led by the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) has released new radiation data from NASA's uncrewed Artemis I mission, which orbited the Moon in late November and early December 2022. The findings, published in Radiation Measurements, come from experimental detectors made of lithium fluoride crystals placed inside two human-shaped mannequins aboard the Orion spacecraft.

More than 10,000 tiny crystals were embedded in the mannequins, including several dozen experimental fluorescent lithium fluoride detectors developed at IFJ PAN. These detectors work by capturing microscopic changes caused by radiation. When charged particles pass through a crystal, they disrupt its atomic structure, creating defects called color centers. Scientists later illuminate the crystal with blue light, causing these altered regions to glow red. By examining that fluorescence under a microscope, researchers can identify the paths taken by individual radiation particles.

"The fluorescent radiation dose detectors based on lithium fluoride, developed at our institute, are passive in nature," said Prof. Pawel Bilski of IFJ PAN, a co-author of the paper. "In other words, they work like photographic film: once the crystals have been exposed to radiation, the tracks left by individual particles must be analyzed optically under a microscope."

Unlike traditional passive plastic detectors that require chemical etching, the lithium fluoride crystals can be examined directly with a standard fluorescence microscope. The researchers developed sophisticated calibration and analysis techniques to interpret the complex cosmic radiation environment, where particles arrive from all directions across a wide range of energies.

"After applying a series of corrections and developing software to appropriately remove the background interfering with the measurements, our results differed by no more than 15% from those obtained using conventional track detectors across the entire overlapping range of energy deposition values," Bilski said. "Moreover, we also recorded numerous particle tracks corresponding to energy deposits below the detection threshold of the detectors used to date."

Comparing the Artemis I data with measurements from a similar experiment aboard the International Space Station more than a decade earlier, the researchers found a key difference. Low-energy protons trapped in Earth's Van Allen radiation belts produced uneven radiation doses in the ISS mannequins. Near the Moon, where the Van Allen belts do not extend, galactic cosmic rays produced a nearly uniform radiation dose throughout the mannequins.

"Low-energy protons, trapped by our planet's magnetic field within the Van Allen belts, can account for the majority of the radiation dose received by astronauts in low Earth orbit. In the vicinity of the Moon, this substantial contribution disappears," Bilski said. "Our measurements from the MARE experiment indicate that, although total radiation doses near the Moon are higher than those in Earth orbit due to the considerable levels of galactic radiation, the difference is not as large as one might intuitively expect."

The compact detectors, measuring just a few millimeters, were made using the Czochralski crystal-growing method, a technology the institute has refined over the past decade. The findings come from the MARE (Matroshka AstroRad Radiation Experiment) study, part of the broader Artemis I mission.