NASA’s Artemis I mission has offered new insight into cosmic radiation near the Moon and how it compares with exposure in low-Earth orbit. Preliminary measurements from experimental lithium fluoride crystal detectors aboard the Orion spacecraft found that radiation near the Moon is higher overall, but not by as much as might be expected. The results also show that the detectors can reveal particle tracks missed by older radiation-monitoring equipment.
Artemis I was an uncrewed mission that traveled around the Moon from late November to early December 2022. Orion carried more than 10,000 tiny crystals embedded in two humanoid mannequins to record radiation exposure. The mission also included dozens of fluorescent lithium fluoride detectors developed by scientists at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow.
The researchers have now developed a method for analyzing the information stored in those crystals. Their findings, published in Radiation Measurements, provide data on both cosmic radiation levels and the performance of the experimental detectors.
How lithium fluoride crystals detect radiation
“The lithium fluoride-based fluorescence radiation dose detector developed in our laboratory is passive in nature. In other words, it works like a photographic film. When a crystal is exposed to radiation, the tracks left behind by individual particles have to be optically analyzed under a microscope,” explains Professor Pawel Bilsky of IFJ PAN, one of the paper’s co-authors.
When charged particles pass through a lithium fluoride crystal, they disrupt its internal atomic structure and create defects known as color centers. Illuminating the crystal with blue light causes the altered regions to glow red. Researchers can then examine the fluorescence under a microscope to determine the paths taken by individual radiation particles.
The detectors were included in the MARE (Matroushka Astrorad Radiation Experiment) study conducted during Artemis I. Both mannequins contained dozens of the experimental lithium fluoride detectors, including one with a high concentration of the lithium-7 isotope. IFJ PAN scientists also supplied about 600 thermoluminescent crystal detectors, which use a different technology to measure radiation exposure.
A key advantage of the lithium fluoride detectors is that they do not require the chemical etching used to examine tracks in conventional plastic detectors. Scientists can inspect the tracks directly with a standard fluorescence microscope.
That made it relatively easy to observe the trajectories of radiation particles recorded near the Moon. Determining what those tracks meant in terms of radiation exposure was more difficult, however. The team had to develop calibration and analysis methods capable of producing reliable numerical measurements.
Why cosmic radiation is difficult to measure
Cosmic radiation is much less predictable than radiation in a controlled laboratory experiment. In the laboratory, a detector can be exposed to a beam containing a known particle type with a defined energy and direction. In space, radiation arrives from all directions and includes many different particles and atomic nuclei spanning a broad range of energies.
This makes the detailed tracks difficult to interpret. Particles enter the crystal at different angles, sending their trajectories in multiple directions. The microscope records a section of each track within its focal plane, which is about 1 micrometer in size, but it also captures a blurred glow from parts of the track outside that narrow region. That additional light affects the measured fluorescence intensity, which researchers use to estimate how densely the particles ionized the material.
Particles with high ionization power leave continuous, sharp lines. Those that ionize the material more weakly may appear as scattered, disconnected dots. These different patterns make it more challenging to convert fluorescence measurements into useful radiological data.
To address the problem, the researchers introduced several corrections and developed software to remove unwanted background signals. The resulting measurements closely matched those from established detector technology and also revealed radiation events that older equipment had not detected.
“We applied a series of corrections and developed software that appropriately removed background interfering with the measurements, and the results differed by less than 15% from those obtained using conventional track detectors over the entire overlapping range of energy deposit values. We also recorded a large number of particle tracks corresponding to energy deposits below the detection threshold of previously used detectors,” Bilsky says.
Artemis I shows differences between lunar and Earth orbit radiation
The Artemis I data allowed the researchers to compare radiation exposure near the Moon with measurements from earlier experiments on the International Space Station.
More than a decade before Artemis I, the MATROSHKA experiment used humanoid mannequins aboard the space station to study radiation exposure. Some of the radiation measured during that mission came from low-energy protons trapped in Earth’s Van Allen radiation belts. As those particles entered the mannequin, they deposited energy at different depths, producing an uneven distribution of radiation dose.
The environment near the Moon is different because the Van Allen belts do not extend into the lunar environment. Radiation there includes a substantial contribution from high-energy particles originating elsewhere in the galaxy.
These galactic particles have enough energy to pass through the entire human body, depositing only part of their energy along the way. Consequently, the Artemis I detectors measured a radiation dose that was distributed almost evenly throughout the mannequin, unlike the uneven patterns recorded in the earlier space station experiments.
The researchers also found that the difference in total radiation exposure between lunar distance and low-Earth orbit was smaller than might intuitively be expected. Astronauts near the Moon receive more radiation overall, but low-energy protons trapped in the Van Allen belts can account for a substantial share of the dose received in low-Earth orbit.
“Low-energy protons trapped in the Earth’s magnetic field within the Van Allen belts can account for a large portion of the radiation dose received by astronauts in low-Earth orbit. Near the Moon, this substantial contribution disappears. Our measurements from the MARE experiment show that, due to significant levels of galactic radiation, the total radiation dose near the Moon is higher than that in Earth orbit, but the difference is not as large as intuitively expected,” Bilsky says.
A compact detector for future missions
The fluorescent lithium fluoride detectors flown on Artemis I were manufactured using the Czochralski method, a technique for growing crystals. IFJ PAN scientists first developed the technology about 10 years ago and have continued to refine it.
The detectors measure on the order of millimeters rather than centimeters, making them small enough to be worn comfortably by people. This could make them practical for monitoring radiation exposure during future space missions.
They also produce clearer particle tracks than traditional plastic detectors, which must undergo chemical etching before analysis. The improved resolution can help scientists distinguish individual trajectories even at relatively high radiation doses. The crystals can also record different particle types and energies, potentially providing a more complete picture of the radiation environment.
Lithium fluoride detectors can additionally detect neutrons, particularly when the crystal is rich in lithium-6 isotopes. When neutrons interact with lithium nuclei, they trigger a nuclear reaction that creates secondary charged particles. Like other ionizing radiation, those particles leave detectable changes in the crystal.
Together, these features make lithium fluoride detectors a promising tool for studying complex radiation environments beyond Earth. The Artemis I measurements provide a clearer view of cosmic radiation near the Moon and demonstrate how compact detectors could help assess radiation risks for future astronauts.
The MARE research was supported by a grant from the OPUS program of the Polish National Science Center (NCN).
Source: www.sciencedaily.com