NASA’s Artemis I mission has offered new insight into how radiation exposure near the Moon compares with exposure in low-Earth orbit. Preliminary measurements from experimental lithium fluoride crystal detectors aboard the Orion spacecraft found that lunar-environment radiation produces a more evenly distributed dose across the body, while the total dose is higher than in Earth orbit—but not by as much as researchers might have expected.
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 during the journey. The spacecraft also carried dozens of experimental fluorescent lithium fluoride detectors developed by scientists at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow.
Researchers have now described a new method for analyzing the information stored in those crystals. The approach provides data on cosmic radiation levels as well as the performance of the detectors. Their findings were published in the journal Radiation Measurements.
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.
Lithium fluoride detectors record microscopic changes caused by radiation. When charged particles pass through a crystal, they disrupt its atomic structure and create defects called color centers. Later, scientists can illuminate the crystal with blue light, causing the altered regions to glow red. By examining this fluorescence under a microscope, researchers can 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 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 method to measure radiation exposure.
One advantage of the lithium fluoride system is that it simplifies the examination of radiation tracks. Conventional passive detectors made of plastic must undergo chemical etching before scientists can study the marks left by incoming particles. Lithium fluoride crystals do not require that step, allowing researchers to examine the tracks directly with a standard fluorescence microscope.
That made it relatively easy to observe the trajectories of radiation particles recorded near the Moon. The more difficult task was determining what those tracks meant in terms of radiation exposure. To produce reliable numerical measurements, the researchers had to develop advanced calibration and analysis techniques.
Why deep-space radiation is difficult to measure
Cosmic radiation is far less predictable than the radiation used in a controlled laboratory experiment. In a typical physics experiment, researchers can expose a detector to a beam containing a known particle with a specific energy and direction. In space, radiation arrives from all directions and includes many types of particles and atomic nuclei spanning a wide range of energies.
This complexity 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, while also capturing a blurred glow from portions of the track outside that narrow region. That additional light affects the measured fluorescence intensity, which scientists use to estimate how densely the particles ionized the material.
Different particles also leave distinct patterns. Strongly ionizing particles create continuous, sharp lines, while weakly ionizing particles may appear as scattered, disconnected dots. These variations make it more difficult 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 improved analysis produced results that closely matched those of established detector technologies, while also revealing 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,” Professor Bilsky says.
Artemis I finds differences in lunar radiation exposure
The Artemis I data also enabled 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 recorded during that mission came from low-energy protons trapped in Earth’s Van Allen radiation belts. When those particles reached the mannequin, they deposited energy at different depths, producing an uneven distribution of radiation dose.
The radiation environment near the Moon is different because the Van Allen belts do not extend into the lunar environment. Instead, a large portion of the radiation there comes 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 recorded a dose that was distributed almost evenly throughout the mannequin, unlike the uneven patterns seen in earlier space station experiments.
The researchers also made a finding that may seem counterintuitive. People near the Moon are exposed to more radiation than those in low-Earth orbit, but the difference is not always as large as might be assumed.
“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,” Professor Bilsky says.
A compact detector for future missions
The fluorescent lithium fluoride detector flown on Artemis I was 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 are only a few millimeters in size rather than several centimeters, making them small enough to be worn comfortably by people. This could make them practical for monitoring radiation exposure during space missions.
The crystals also produce clearer particle tracks than traditional plastic detectors, which require chemical etching. Their higher resolution allows scientists to distinguish individual trajectories even when radiation doses are relatively high. In addition, the crystals can record different types of particles and energies, potentially providing a more complete picture of the radiation environment.
The detectors can also identify neutrons, particularly when the crystals are 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 characteristics make lithium fluoride detectors a promising tool for studying complex radiation environments beyond Earth. The Artemis I measurements have improved scientists’ understanding of cosmic radiation near the Moon and demonstrated how compact detectors could help assess radiation exposure during future missions.
The MARE research was supported by a grant from the Polish National Science Center (NCN) under the OPUS program.
Source: www.sciencedaily.com
