Improving Methods on Radiological Characterization of Extraterrestrial Samples using Low-Level Gamma Spectrometry

IMPROVING METHODS ON RADIOLOGICAL CHARACTERIZATION OF EXTRATERRESTRIAL SAMPLES USING LOW-LEVEL GAMMA SPECTROMETRY. IdL. Chacartegui Rojo1,2 , B. Sabot1, F. Girault2 and P-Y. Meslin3, 1Université Paris-Saclay, CEA, LIST, Laboratoire National Henri Becquerel (LNE-LNHB), F-91120 Palaiseau, France, inigodeloyola.chac[email protected], +33 (0)1 69 08 48 68, 2Universite Paris Cite, Institut de Physique du Globe de Paris, CNRS, Paris, 75005, France, 3Institut de Recherche en Astrophysique et Planetologie, UPS/CNRS/CNES, Toulouse, 31400, France The return of lunar material – totalling about 386 kg – has enabled unprecedented geochemical and geophysical investigations of the Moon [1]. These rare and scientifically valuable samples, consisting mainly of regolith collected from airless surfaces, provide unique opportunities to study the long-term evolution of the lunar surface, the surface-bounded exosphere and the transport of volatiles. Among the naturally occurring radionuclides, radon-222 (a decay product of Ra-226 from the U-238 chain) is of particular interest because of its mobility and its role as a tracer of regolith-atmosphere exchange. On the Moon, radon outgassing contributes to the transient nature of the lunar exosphere and may be related to deeper outgassing sources or near-surface transport mechanisms. To support these investigations, a highly sensitive gamma-ray spectrometry system, optimised for the radiological characterisation of small-mass (~1 g) lunar samples, has been developed at the Laboratoire National Henri Becquerel (LNHB). The setup is based on a high-purity germanium (HPGe) detector equipped with an active anticoincidence shielding system to suppress cosmic background [2]. A custom gas-tight sample holder was designed to reduce gamma-ray self-attenuation, especially in the low energy range, and to preserve the pristine nature of the sample. The system was recalibrated to improve detection efficiency below 100 keV, with particular emphasis on the 46.54 keV emission of Pb-210, a key tracer in the radon decay chain. A total of 70 days of blank measurements were acquired to ensure proper background subtraction and to achieve minimum detectable activities consistent with the expected radioelement content of lunar materials. The methodology was validated using a wellcharacterised regolith analogue, JSC Mars-1, a Martian simulant with radionuclide concentrations similar to those observed in samples returned from Chang’E 5 [3] and previously characterised by natural radionuclide concentration [4]. This gamma spectrometry approach is the first step in a broader experimental framework aimed at understanding radon behaviour in lunar regolith. Accurate quantification of U-238 and Ra-226 provides the source term necessary to estimate radon production. These data will be used in follow-up radon release experiments using liquid scintillation counting to determine the fraction of radon released from mineral grains, followed by adsorption experiments to study radon retention on mineral surfaces. Together, this suite of measurements will help to constrain radon mobility and the thermophysical properties of lunar regolith. This work is directly relevant to the Detection of Outgassing Radon (DORN) instrument, which performed in-situ alpha spectrometry of radon and polonium isotopes on the lunar surface as part of the Chang’E 6 mission [5]. The radiological data obtained by gamma spectrometry provide critical reference values for interpreting the DORN measurements and for modelling radon transport and exospheric dynamics. By linking laboratory analysis, in-situ data and model development, this study contributes to the broader effort to characterise the lunar volatile cycle and the processes shaping the environment. The optimised methodology developed here allows accurate and non-destructive measurement of radionuclides in very limited amounts of extraterrestrial material. Its high sensitivity, combined with careful control of environmental conditions, ensures minimal sample degradation – critical for current and future sample return missions. In addition to providing important radiological data, this technique opens the door to improved models of planetary surface evolution and volatile dynamics, supporting broader efforts in planetary exploration and habitability assessment. References: [1] Barnes J. and Davidson J. (2024) Eos, 105. [2] Ferreux L. and Bouchard J. (2016) Appl. Radiat. Isot., 109, 425–429. [3] Tian H. Et al. (2021). Nature, 600, 59–63 [4] Meslin P.-Y. et al. (2011). Geochimica et Cosmochimica Acta, 75, 9. [5] Meslin P.-Y. et al. (2020), 51st LPSC, 1741.