Improving gamma spectrometry for radionuclide analysis of extraterrestrial samples

With the recent return of extraterrestrial material from the Chang'E 5 and Chang'E 6 missions, and the upcoming Mars Sample Return mission, it is essential to develop optimised methodologies for their analysis. These samples are rare and valuable, typically consisting of low-mass, fine-powdered regolith with very low natural radioactivity. This work, carried out at the Laboratoire National Henri Becquerel (LNE-LNHB), presents an adapted gamma-ray spectrometry methodology to determine the activity concentrations of natural radionuclides in extraterrestrial samples, with a particular focus on understanding the mobility of in planetary regoliths and atmospheres/exospheres. A bespoke sample holder was designed to minimise gamma-ray self attenuation, particularly in the low energy range, while providing the gas-tightness and inert handling conditions necessary to preserve pristine extraterrestrial material. In addition, a high-purity germanium gamma spectrometer with an active anti-coincidence veto was optimised to increase detection efficiency, with a particular focus on the 46.54keV emission of . To validate this methodology, a Martian regolith analog (JSC Mars-1) was analysed immediately after enclosure, with a total measurement time of 36.25 days. This approach allowed for precise quantification of radionuclides in extraterrestrial samples, overcoming the challenges posed by their low mass, precious and difficult-to-handle nature. The measured specific activities (activity per unit mass of material) with expanded uncertainties (k=2) for , , , , and were 28.6 ± 5.7Bqkg^-1, 19.8 ± 3.6Bqkg^-1, 15.4 ± 2.5Bqkg^-1, 0.13 ± 0.03Bqkg^-1, 146 ± 12Bqkg^-1, respectively, compatible with reported data on much larger sample masses. These results demonstrate the capability of this optimised methodology to aid in the radiological characterization of extraterrestrial materials while ensuring minimal sample usage.