# Improving gamma spectrometry for radionuclide analysis of extraterrestrial samples

Íñigo de Loyola Chacartegui Rojo, Benoit Sabot, Frédéric Girault, Pierre-Yves Meslin

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Íñigo de Loyola Chacartegui Rojo, Benoit Sabot, Frédéric Girault, Pierre-Yves Meslin. Improving gamma spectrometry for radionuclide analysis of extraterrestrial samples. Applied Radiation and Isotopes, 2025, 226, pp.112164. 10.1016/j.apradiso.2025.112164. cea-05299835

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Submitted on 6 Oct 2025

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Highlights

Improving gamma spectrometry for radionuclide analysis of ex-
traterrestrial samples.

Íñigo de Loyola Chacartegui Rojo, Benoit Sabot, Frédéric Girault, Pierre-Yves Meslin

* Novel methodology developed for low-mass and low-level gamma spec-trometry analysis

* Special sample holder designed for a 1-gram pristine extraterrestrial sample

* Re-characterization of HPGe detector efficiency to improve sensitivity at 46.54 keV for $^{210}$Pb

* Apparent specific activities of $^{238}$U, $^{226}$Ra, $^{232}$Th, $^{235}$U, and $^{40}$K measured on a 1.07 g JSC Mars-1 simulant
Improving gamma spectrometry for radionuclide
analysis of extraterrestrial samples.

Íñigo de Loyola Chacartegui Rojo<sup>a,b</sup>, Benoit Sabot<sup>a</sup>, Frédéric Girault<sup>b</sup>,
Pierre-Yves Meslin<sup>c</sup>

<sup>a</sup>Universite Paris-Saclay, CEA, LIST, Laboratoire National Henri Becquerel
(LNE-LNHB), Palaiseau, 91120, France

<sup>b</sup>Universite Paris Cite, Institut de Physique du Globe de Paris,
CNRS, Paris, 75005, France

<sup>c</sup>Institut de Recherche en Astrophysique et Planetologie,
UPS/CNRS/CNES, Toulouse, 31400, France

**Abstract**

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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 <sup>222</sup>Rn in planetary regoliths and atmospheres/exospheres.

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A bespoke sample holder was designed to minimise gamma-ray self atten-
uation, 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 an particular focus on the 46.54 keV emission of <sup>210</sup>Pb. To validate this
methodology, a Martian regolith analog (JSC Mars-1) was analysed immedi-
ately after enclosure, with a total measurement time of 36.25 days.

This approach allowed for precise quantification of radionuclides in ex-
traterrestrial samples, overcoming the challenges posed by their low mass,
precious and difficult-to-handle nature. The measured specific activities (ac-
tivity per unit mass of material) with expanded uncertainties (k=2) for <sup>238</sup>U,

Email address: inigo-delayola.chacarteguirojo@cea.fr (Íñigo de Loyola
Chacartegui Rojo)
$^{226}$Ra, $^{232}$Th, $^{235}$U, and $^{40}$K were 28.6 ± 5.7 Bq kg$^{-1}$, 19.8 ± 3.6 Bq kg$^{-1}$, 15.4 ± 2.5 Bq kg$^{-1}$, 0.13 ± 0.03 Bq kg$^{-1}$, 146 ± 12 Bq kg$^{-1}$, 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.

Keywords: gamma spectrometry, regolith, radon, extraterrestrial environment, planetary body

# 1. Introduction

In total, about 386 kg of extraterrestrial material has been returned to Earth. The majority, 99.96%, comes from lunar missions, with much smaller but scientifically invaluable amounts from asteroid, comet, and solar wind missions. These extraterrestrial sample return missions have significantly advanced our understanding of the solar system by bringing back materials from various celestial bodies [1]. The key characteristics of extraterrestrial samples are their rarity and limited availability to the scientific community. Since most of them come from the surface of airless planetary bodies, they originate mainly from impact-induced regoliths and have grain sizes ranging from micrometers to several millimeters [2]. The study of the radionuclides present in these samples provides unique opportunities for radioactive dating and tracking of natural processes with broad applications in geophysics and environmental sciences [3]. Of particular interest is the study of $^{222}$Rn. Within the $^{238}$U decay chain, $^{222}$Rn is known to be one of the major contributors to atmospheric radioactivity due to its ability to escape from rock grains and be released into the atmosphere upon decay of its parent radionuclide, $^{226}$Ra [4]. With a half-life of 3.8 days, it is used as a tracer gas to study lithospheric dynamics and atmosphere-lithosphere exchanges [5]. In extraterrestrial environments such as the Moon, $^{222}$Rn is considered as an important tracer of lunar outgassing [6, 7]. Its use as a tracer gas can also improve models of volatile transport in the subsurface and exosphere/atmosphere of the Moon and other planets such as Mars and Mercury [8, 9, 10, 11, 12]. Therefore, analysis of $^{222}$Rn mobility in extraterrestrial regoliths can reveal key regolith properties needed to refine volatile transport models. However, handling extraterrestrial material presents several challenges not only because of its low mass and rarity, but also because of its pristine state after
collection and return to Earth. These samples have trace material concentrations ranging from 1 to 10000 ppbv and are typically delivered with masses no larger than ~1 g. As a result, non-destructive analyses are required to determine their bulk properties, and both storage conditions and analytical setups must ensure minimal exposure of the sample to terrestrial conditions. For this reason, such materials are stored in curation centers that provide inert conditions and should only be moved outside of such centers for short periods of time.

A first approach to radon analysis is to determine the content of naturally occurring radionuclides, specifically the $^{238}$U decay chain, but also, concomitantly, the $^{232}$Th decay chain and $^{40}$K, which are the most abundant naturally occurring radionuclides in the Earth's crust. For this purpose, gamma-ray spectrometry is the reference method for the measurement of naturally occurring radioactive materials (NORM). Due to the limited availability of extraterrestrial material, the typically low concentration of NORM in extraterrestrial regoliths, the needs for non-destructive techniques, and the short period of time a sample should remain outside of curation facilities, the use of low-level gamma spectrometry stands out as the most appropriate technique. Among all the available devices for gamma-ray spectrometry, high-purity germanium (HPGe) detectors are the most widely used solution due to their excellent energy resolution, which allows the discrimination of close gamma-ray lines and a better signal-to-noise ratio [13]. To achieve the sensitivity required to detect low concentrations of NORMs, the detector must be shielded from external radiation using both passive and active shielding. To increase counts and reduce acquisition time, the detection efficiency, defined as the fraction of gamma rays emitted by the sample that are detected, must be as high as possible over the desired energy range. Several developments have been made to improve detection efficiency [14], including the design of $4\pi \alpha\beta-\gamma$ detectors; and improved shielding systems [15, 14, 16], that implement and optimise modern anti-coincidence veto systems using digital electronics and advanced background characterization techniques.

In this paper, we present the improvements made to the low-level gamma spectrometry setup at the Laboratoire National Henri Becquerel (LNHB-LNE), based on an HPGe detector. These modifications were designed to accommodate a 1-gram Chang'E 5 lunar sample and to analyze its NORM content over an acquisition period of one month. With a particular focus on radon gas, the system was optimised to measure the activity of radionuclides from the $^{238}$U and $^{232}$Th chains, allowing the determination of the $^{222}$Rn pro-
duction term for further analysis of its emanation coefficient.

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## 2. Materials and methods

### 2.1. Analog extraterrestrial sample description

Based on the reported concentration of 1.05 ppm of $^{238}$U in a lunar sample returned from the Chang'E 5 mission [17, 18], and assuming a sample mass of 1 g, we define a target detection threshold of approximately 12 mBq.

Based on this value, we have selected a sample whose reported concentration of $^{238}$U is similar to that found on Chang'E 5 samples. JSC Mars-1 is a Martian regolith simulant derived from weathered volcanic ash from Pu'u Nene, Hawaii, USA and is designed to mimic the composition and spectral characteristics of bright Martian surface regions [19, 20].

Although not a lunar analog, it has already been analysed by gamma spectrometry, and was found to have similar NORM abundances as Chang'E 5 and a large range of Apollo samples [21]. These data are valuable for evaluating the design of the experimental setup and the methodology used. In addition, this sample has a high emanation coefficient of $64.4 \pm 6.4\%$ [21], which means that a large fraction of the $^{222}$Rn gas is able to escape from the sample grains. This high emanation coefficient is also useful for testing the tightness of the system against radon leakage.

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### 2.2. Requirements and constraints

An extraterrestrial sample is a low-mass, low-radioactivity material that must be transported from the curation facility to a laboratory in a specially designed holder to minimise the risk of contamination. This requirement applies not only to NORMs, but to any form of contamination, particularly by $O_2$ and $H_2O$, to preserve the scientific integrity of the sample for long-term study by multiple research communities.

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The reported value of 12 mBq of $^{238}$U serves as a practical sensitivity requirement for our experimental design and evaluation of the minimum detectable activity (MDA) performance. Therefore, our experimental setup must be sensitive enough to provide a result with an acceptable level of uncertainty while keeping the acquisition time reasonable. This is critical to ensure that the bulk lunar sample is not stored outside the curation facility for extended periods of time.

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To define a methodology that meets these requirements, it is necessary to analyze which characteristics of the experimental setup influence the determination of the sample activity and how they do so. The activity of a radionuclide is derived from the spectral analysis of a gamma peak according to:

$$A = \frac{C_{NET} - C_{NUL}}{t\varepsilon_p T_{\varepsilon} I_{\gamma}} F_{COI} F_{CORR}, \quad (1)$$

where $C_{NET}$ are the peak counts after curve fitting and Compton signal suppression, $C_{NUL}$ are the Background counts on the same Region of Interest (ROI) and $t$ is the acquisition time. $\varepsilon_p$ is the full-energy peak (FEP) detection efficiency of a reference sample and geometry at the emission energy and $T_{\varepsilon}$ is the efficiency transfer term between the reference sample and the measured sample. The other terms are $I_{\gamma}$ which is the gamma emission intensity, $F_{COI}$ and $F_{CORR}$ are the coincidence-summing correction and additional correction factors, respectively. The expression of the MDA is given by [22, 23, 24]:

$$MDA = \frac{(k_{1-\alpha} + k_{1-\beta}) \sqrt{5B \cdot FWHM}}{t\varepsilon_p T_{\varepsilon} I_{\gamma}}, \quad (2)$$

where FWHM is full width at half maximum resolution of the detector at the emission energy, B is the sum of the averaged counts over 10 channels within the vicinity of the peak and the background counts within the ROI of the emission, and $(k_{1-\alpha} + k_{1-\beta})$ are the confidence factors for the false positives ($\alpha$) and false negatives ($\beta$) occurrences.

Of all these terms, $\varepsilon_p$ and $T_{\varepsilon}$ are the terms that vary most significantly to the change of the geometric and material properties of our setup. $F_{COI}$ and $F_{CORR}$ also depend but in a lesser way and the remaining parameters are defined either by the spectral peak properties used to determine the activity of the radionuclide or by the intrinsic properties of the germanium crystal. Therefore, the sample holder design and measurement geometry must be optimised to maximise the product $\varepsilon_p T_{\varepsilon}$, thereby reducing the MDA and improving the sensitivity of the instrument.

Within the $^{238}$U decay chain, we have paid particular attention to $^{210}$Pb, the longest-lived radioactive decay product of $^{222}$Rn. Studying its unsupported fraction in the sample could provide insight into $^{222}$Rn transport across extraterrestrial surfaces and into radon night-time accumulation within such a boundary. $^{210}$Pb is characterised by a single gamma emission at a low energy of 46.54 keV, making it a challenging target for detection with sufficient sensitivity.
## 2.3. Detector description

To achieve the sensitivity and resolution required to analyse a sample with low mass and low NORM concentration, we used a 66.0 ± 0.5 mm diameter and 220 cm³ N-type reverse-electrode coaxial high-purity germanium detector (GeHP3). It has a relative efficiency of 51.6% compared to a thallium-activated sodium iodide (NaI(Tl)) in standardised conditions at the 1.3 MeV emission of <sup>60</sup>Co and a peak to Compton ratio (P/C) of 62.4. The detector is installed in a laboratory buried under 1.5 m of reinforced concrete and 1 m of soil. There is 50 hPa of overpressure to prevent radon from exhalating from the walls.

The detector, with model number GR5021, was manufactured and installed by Canberra Industries Inc. (Mirion Technologies, Belgium) in 2001. It is housed in a measurement chamber that combines passive and active shielding Figure 1. The passive shielding consists of several layers of materials arranged from the outside in: first, 10 cm of low-activity lead (A<50 Bq kg<sup>-1</sup>) to attenuate external gamma radiation; second, 3 mm of cadmium alloy (A<50 Bq kg<sup>-1</sup>) to absorb thermal neutrons; third, 5 cm of very low-activity lead (A<10 Bq kg<sup>-1</sup>) to attenuate gamma radiation emitted by the neutron-activated cadmium; finally, 4 mm of copper shielding to attenuate X-rays originating from the lead shielding.

Surrounding this passive shield there is an active shielding consisting of 5 cm thick BC-408 plastic scintillators, coupled to five photomultiplier tubes (PMTs) that collect the scintillation light signals Figure 1. These signals are summed and amplified before being analysed by an anti-coincidence veto system with an extendable gate that allows a variable dead time [25].
<figure><img src="image_2.png" /><figcaption>Figure 1: Schematic cross section of the GeHP3 detector and its shielding (not to scale). The anti-cosmic veto consists of 5 plastic scintillators and 5 PMTs surrounding the passive shielding. The passive shielding is a combination of different layers: low activity lead (10 cm thickness on each side, expected bottom part: 15 cm), cadmium (4 mm), 5 cm of ultra-low activity lead and 6 mm of copper.</figcaption></figure>

The measurement chamber is continuously purged with nitrogen gas from the detector's liquid nitrogen Dewar at a flow rate of 30 L h⁻¹. This purging effectively removes radon gas from the cavity, reducing its concentration from an average volume activity of 20 Bq m⁻³ in the room to undetectable levels, approximately one hour after closure. The room temperature is fixed at 20 ± 1 °C.

Although some studies of environmental radioactivity conducted under mountains may achieve lower background levels [26, 27], the current setup and its shielding are optimal for this application, since the goal is to analyze
natural radionuclides in natural samples, specially radon decay products, which can reach high levels in underground facilities. Although effective radon mitigation strategies are available for such environments, the natural radon concentration in underground laboratories is generally higher than in surface labs. In our case, the combination of an efficient anti-radon system, low ambient background, and on-site accessibility renders the current configuration technically and operationally adequate for the number and frequency of measurements we intend to perform using this methodology. Moreover, at this level of isolation, the main source of contribution in the analysis arises not from the environment, but from the sample itself. For example, if we aim to study the <sup>210</sup>Pb emission located in the low-energy range (<100 keV), the signal will be located within the Compton continuum. It will also be overlapped by X-ray fluorescence from the radionuclides in the sample, the detector materials, and the sample holder. This occurs regardless of how well the measurement system is isolated from external radiation. Although a Compton suppressor system could be added, it is not feasible for us due to budgetary and operational constraints.

Careful determination of the background signal is a critical step in measuring the activity of naturally occurring radionuclides. Therefore, it is essential to perform a thorough analysis of the instrument background signal and its variations over long measurements.

The detector window is made of a carbon-epoxy composite, chosen for its low atomic number NORM content, which minimises gamma and X-ray attenuation while maximising detection efficiency and keeping residual natural radionuclides concentrations low. The end-cap and crystal holder are both made of low background aluminium. The preamplifier is located outside the detector housing.

In addition, the system originally included a 1.12 mm thick copper screen right on top of the detector window to shield the detector from X-rays originating from the samples. However, one of the goals of this study was to improve the sensitivity of the detection system to the 46.54 keV <sup>210</sup>Pb gamma emission, which was strongly attenuated by this screen. Therefore, the copper layer was removed to increase the detection efficiency in this energy range.

This modification introduced a significant variation in the detector's efficiency, making it necessary to reconstruct the efficiency curves of the instrument and re-characterise its overall performance. For this purpose, we used ETNA (Efficiency Transfer for Nuclide Activity measurements)[28], a software developed at LNHB, to calculate efficiency transfer coefficients and
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2.4. Sample holder design

The design of an optimised sample holder for gamma spectrometry must
ensure that the sample geometry minimises gamma-ray self-attenuation and
that the construction material neither absorbs nor adsorbs gases while al-
lowing gamma-ray transmission. The holder must be hermetic, both under
vacuum and at a given pressure, and its internal volume should be minimised
to prevent secondary radionuclide deposition due to radon gas escape from
the sample, that could alter the measurement geometry. Finally, the design
should favor off-the-shelf components to reduce costs and ensure availability.

The optimal sample geometry was determined for a cylindrical volume
corresponding to 1 g of the JSC Mars-1 sample. The primary optimization
factor was the detection efficiency in the low-energy range, which is critical for
the analysis of certain radionuclides such as $^{210}$Pb. Using the ETNA software,
multiple efficiency transfer simulations were performed for a cylinder with
varying diameters while keeping the total volume constant. The efficiency
transfer values were then analysed for different energy values and cylinder
radii, identifying the optimal radius for each energy. A weighted average was
applied to these values, assigning higher weights to geometries that improved
low-energy efficiency.

To ensure air tightness and material compatibility of the sample holder,
a 316L stainless steel CF40 flange was selected while the seal is made with
a copper gasket. The interaction between these materials provides a high-
vacuum seal and prevents any material from entering or leaving the flange.
This aspect is particularly critical for $^{222}$Rn, which could potentially escape
from the holder and create a secular disequilibrium between the parents
($^{238}$U, $^{234}$Th, $^{226}$Ra) and the decay products ($^{214}$Pb, $^{214}$Bi). To evaluate this
possibility, we compared the measured activities of $^{226}$Ra, $^{214}$Pb, and $^{214}$Bi. If
there were disequilibrium, the activities would not match, indicating a $^{222}$Rn
deficit inside the sample holder cavity.

2.5. Background characterisation

One of the particular challenges in the analysis of environmental radia-
tion, and radon in particular, is that the NORMs of interest present in the
sample, which belong to the $^{238}$U and $^{232}$Th decay chains, are also present in
the detector surroundings and correspond to the major source of interference.
For this reason, a thorough characterization of the background is essential to properly distinguish it from the signal emitted by the sample.

Initially, ten background acquisitions were performed using the experimental setup without a sample holder, resulting in a total acquisition time of 51.11 days. Later, to evaluate the potential contribution of the sample container itself, two additional acquisitions were carried out using the same detector: one with the CF40 sample holder used for this study, and one with a pristine, unused holder. Each measurement lasted approximately 14.4 and 6.7 days, respectively. This comparison aimed to assess whether radon daughter deposition during previous sample measurements contributed to the observed signal.

## 2.6. Auto-attenuation

As mentioned in subsection 2.2, $\epsilon_p$ and $T_\epsilon$ depend not only on the measurement geometry but also on the sample materials and thus, to estimate the detection efficiency, it is important to evaluate how much the sample attenuates the gamma rays it emits. For this purpose, we used an experimental setup at LNHB consisting of a HPGe detector, two collimators with a beam diameter of 8 mm, a sample holder and a support for an external radioactive source Figure 2. This geometry ensures that the gamma rays intersect the sample surface at angles sufficiently close to 90°, making the application of Equation 3 a valid approximation.

Self-attenuation analysis is a relative measurement using an external radioactive source, such as $^{152}$Eu and $^{133}$Ba, chosen for its wide range of emission peaks over different energies, extending down to 40.12 keV. Two acquisitions are performed: one with an empty sample holder and one with the holder containing the material. By analyzing how the intensities of multiple $^{152}$Eu and $^{133}$Ba emission peaks are reduced, the linear attenuation coefficient ($\mu$) is determined from the ratio of their peak areas, as defined by:

$$ \mu(E) (\text{cm}^{-1}) = \frac{-\ln\left(\frac{N(E)}{N_0(E)}\right)}{h}, \qquad (3) $$

where $h$ is the thickness of the sample in cm, $N(E)$ is the count rate of a specific emission at energy $E$ (corrected for source decay) with the sample and $N_0(E)$ is the count rate without the sample.

Finally, the linear attenuation coefficient $\mu(E)$ was obtained by fitting the experimental data to a power law model of the form $\mu(E) = a \cdot E^b$ using
weighted non-linear least squares regression. The weights applied in the fit correspond to the absolute uncertainty of each measured $\mu$ value. The resulting fit parameters were then used to evaluate $\mu$ at specific photon energies relevant to the gamma emissions of interest. This set of data was used as input to the ETNA software to calculate the efficiency transfer term $T_e$ from the reference geometry to our measurement geometry.

<figure><img src="image_3.png" /><figcaption>Figure 2: Set-up of the auto-attenuation experiment done at LNHB, the free space for the sample is adjusted depending on the sample thickness.</figcaption></figure>

## 2.7. Efficiency curves

After removing the 1.12 mm thick circular copper screen (see subsection 2.3), the detection efficiency increased and could be modeled using ETNA. However, the resulting efficiency transfer values $T_e$ were high, for example, at the emission energy of 46.54 keV for $^{210}$Pb, the coefficient was 145.44, too large to be used in the activity calculations due to its uncertainty. To address this, new efficiency calibration curves were determined
experimentally using a set of standard radioactive sources with known activities. Two standard geometry configurations were used: configuration 1 was a point source placed 10 cm from the detector, and configuration 2 was a standardised SG50 geometry. This geometry consists of a container made of polyethylene ($C_2H_4$) with an inner diameter of 36.6 mm filled with a 50 mL solution of radionuclides in 0.1 M HCl reaching 47.52 mm in height. This container was placed in direct contact with the detector.

In order to fully characterise the instrument after the removal of the copper screen, total and FEP efficiency curves were generated:

### 2.7.1. Total efficiency curve

The total efficiency $\epsilon$ of a detector for a given energy E is defined as the ratio between the number of interactions (FEP or Compton scattering) caused by photons of energy E and the total number of photons of the same energy emitted by the source [29]. It corresponds to the probability of interaction between a gamma ray of energy E emitted by a source and the detector. This curve thus characterises the total response of the detector and is needed to calculate the coincidence-summation correction factor $F_{COI}$, which is used for further calculations.

The total efficiency of the instrument was determined using configuration 1. The set of radioactive sources was chosen to be mono-energetic and is detailed in the inset of Figure 6.

The total efficiency value was obtained by integrating the FEP emission and its Compton contribution, capturing all gamma-ray interactions within the germanium crystal. When necessary, X-ray emissions from the source were removed to eliminate unwanted contributions not related to gamma-ray emissions. This was done by manually modifying the channel counts within the ROI of the X-ray peaks, replacing their values with estimates interpolated from the surrounding Compton continuum.

After obtaining the experimental values, we used the ACORES fitting algorithm [30], developed at LNHB, to perform a polynomial fit of the efficiency $\epsilon$ as defined by:

$$ \ln(\epsilon) = \sum_{k=1}^{P} a_k \cdot (\ln(E))^{k-1}, \qquad (4) $$

where $a_k$ is the set of fitting coefficients. This procedure was applied consistently to all efficiency curves.
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## 2.7.2. Full-energy peak efficiency curve

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The peak efficiency $\epsilon_p$ of a detector for a given gamma-ray energy E is defined as the ratio of the number of photons that deposit their full energy in the detector, i.e. those contributing to the FEP, to the total number of photons of energy E emitted by the source. Unlike total efficiency, which includes all detectable interactions, peak efficiency accounts only where the entirety of the gamma ray energy is deposited in the active volume of the germanium crystal.

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To obtain this curve, radioactive sources with multiple emissions over different energy ranges and stable decay products were selected to avoid interferences.

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This efficiency value corresponds to $\epsilon_p$ in Equation 1 and 2, and serves as a reference for applying efficiency transfer corrections to the measurement geometry. To obtain the FEP efficiency curve, we used configuration 2.

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## 2.8. Gamma-ray spectrum analysis

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The gamma spectrum analysis was performed using InterWinner 7 (ITECH Instruments, France) [31]. For each full-energy peak (FEP), the region of interest (ROI) was defined, the Compton continuum subtracted, and the net peak area extracted by fitting the peak to a Gaussian function. This approach is suited to our counting statistics and enables consistent treatment of closely spaced or low-intensity peaks. The software also performs background signal reduction and associates each peak with a radionuclide using the LARA database [32], based on a user-defined table of isotopes and emissions.

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After obtaining the net counts with Compton signal and background subtraction, the remaining terms from Equation 1 were applied to determine the activity of the emitting radionuclide.

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As part of the additional corrections, a theoretical correction factor $F_{CORR}$ of 0.58 was applied to the $^{226}$Ra gamma emission to account for interference with the $^{235}$U emission peak. This correction assumes that both radionuclides are in secular equilibrium within their respective decay chains and that the sample contains natural uranium, i.e., with a $^{235}$U/$^{238}$U activity ratio consistent with unprocessed material [33].

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When multiple emissions of the same radionuclide are analysed, a weighted average was calculated, giving greater weight to results with lower uncertainty. The uncertainty budget for the major emissions of the natural radionuclides detected in JSC Mars-1 is shown in Table 1.
<table><caption>Table 1: Standard uncertainty budget of the main emissions of the <sup>238</sup>U natural decay chain radionuclides found in JSC-Mars 1.</caption><thead><tr><th rowspan="2">Components</th><th colspan="5">Relative standard uncertainties (%)</th></tr><tr><th><sup>238</sup>U (<sup>234</sup>Th) (93.3 keV)</th><th><sup>226</sup>Ra</th><th><sup>214</sup>Pb (351.93 keV)</th><th><sup>214</sup>Bi (609.31 keV)</th><th><sup>210</sup>Pb</th></tr></thead><tbody><tr><td>FEP efficiency ε<sub>p</sub> determination (configuration 1)</td><td>2.08</td><td>1.86</td><td>1.74</td><td>1.73</td><td>2.02</td></tr><tr><td>Efficiency transfer T<sub>e</sub> (configuration 2)</td><td>3.8</td><td>1.4</td><td>1.0</td><td>0.8</td><td>15.33</td></tr><tr><td>Coincidence summing correction factor</td><td>0.21</td><td>0</td><td>0.03</td><td>2.49</td><td>0</td></tr><tr><td>External correction factors (<sup>235</sup>U effects)</td><td>0</td><td>4.24</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Net counts peak areas, Compton and background subtraction</td><td>9.24</td><td>7.3</td><td>3.87</td><td>4.87</td><td>13.8</td></tr><tr><td>Gamma ray probability</td><td>2.13</td><td>0.53</td><td>0.2</td><td>0.42</td><td>0.94</td></tr><tr><td>Sample quantity (weight)</td><td>0.93</td><td>0.93</td><td>0.93</td><td>0.93</td><td>0.93</td></tr><tr><td>Sample linear attenuation coefficient (μ)</td><td>12.51</td><td>13.43</td><td>14.31</td><td>15.10</td><td>11.69</td></tr><tr><td>Combined standard uncertainty</td><td>10.3</td><td>8.8</td><td>4.6</td><td>8.5</td><td>20.8</td></tr></tbody></table>

## 2.9. Minimum detectable activity (MDA)

To calculate the MDA, we used $k_{1-\alpha} = k_{1-\beta} = 1.645$, corresponding to a 5% level of false positive and false negative events. The resulting MDA expression is given by:

$$MDA = \frac{7.36\sqrt{B \cdot FWHM}}{t\varepsilon_p T_{\varepsilon} I_{\gamma}}, \qquad (5)$$

where the value B is the averaged counts over 10 channels in the vicinity of the peak signal. However, since the NORMs we are interested in are also present in the background spectrum, we also took the area of the emission peak present in the background within the same ROI and added it to the averaged counts.

Compton scattering of gamma rays from the sample contributes to the surrounding continuum near the peak of interest. Therefore, to obtain representative MDA results, we used a sample with a radionuclide composition similar to that of the sample under study so we could make an a priori assessment of the sensitivity of the setup. This approach allows for a more realistic assessment of the setup's sensitivity to lunar regolith samples by approximating the expected Compton background from their natural radionuclide composition.
In this case, the JSC Mars-1 NORM concentration is comparable to that of a lunar sample returned by the Chinese Chang'E 5 mission [21, 18] and is therefore suitable for MDA analysis.

# 3. Results and discussion

## 3.1. Sample holder geometry and design

The optimal radius values for each energy correspond to a constant volume of 1.25 cm³ of JSC Mars-1, equivalent to 1 g of material (Figure 3). The results indicate that the optimal radius for our measurement system falls within a narrow range of less than 1.2 mm (from 15.7 to 17 mm).

The resulting weighted average gives a radius of 16.18 mm and a thickness of 1.52 mm. For manufacturing and assembly reasons, the final geometry was fixed to a cylinder with a radius of 16 mm and a thickness of 1.5 mm. The selected sample holder design consists of a 16 mm radius cavity machined into the bottom of a CF flange to hold the sample (Figure 4). At the top, a welded piston fits tightly into the cavity, leaving no gaps at the side edges. The depths are set so that the free space between the top and bottom parts is 1.5 mm, ensuring minimal empty space inside the holder.

The bottom thickness of the flange was set to 500 µm to minimise gamma-ray attenuation by the sample holder while remaining at the feasible limit of structural integrity and manufacturability. The attained thickness was $876 \pm 41$ µm.
<figure><img src="image_4.png" /><figcaption>Figure 3: Optimal radius obtained for each energy within the defined detector constraints. The red dashed line corresponds to the average weighted value. Three main emission of the radionuclides $^{210}$Pb, $^{214}$Pb and $^{214}$Bi are marked in red.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4: Final dimensions and design of the CF sample holder.</figcaption></figure>

## 3.2. Auto-attenuation results for JSC Mars

For the self-attenuation measurement, two acquisitions were made: one for 4.62 days with the sample in place, and another for 5.5 days without the sample Figure 5.
<figure><img src="image_6.png" /><figcaption>Figure 5: Linear attenuation results for the JSC Mars-1 sample and the power-law fit applied to them.</figcaption></figure>

### 3.3. Efficiency curves

The efficiency values obtained in configuration 1 (Figure 6) and configuration 2 (Figure 7) show a significant increase in the total and FEP efficiencies of the measurement system and a shift of the maximum efficiency value towards lower energies. This indicates that the system has become significantly more effective at detecting low-energy gamma rays after the removal of the copper screen.

More specifically, for the $^{210}$Pb 46.54 keV emission, the efficiency in configuration 2 increased by a factor of 38.3. A summary of the efficiency gains over the calibration curves obtained using configurations 1 and 2, is given in Table 2.
<figure><img src="image_7.png" /><figcaption>Figure 6: Comparison of the total efficiency curves of the GeHP3 detector before and after removal of the copper screen (configuration 1). Each point corresponds to a measurement made on a point source located 10 cm above the detector with an expanded uncertainty (k=2). Three main emission energy levels of the radionuclides $^{210}$Pb, $^{214}$Pb and $^{214}$Bi are marked in red.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 7: Comparison of the full-energy peak (FEP) efficiency curves of the SG50 geometry in contact with the detector before and after removal of the copper screen (configuration 2). Three main emission energy levels of the radionuclides $^{210}$Pb, $^{214}$Pb and $^{214}$Bi are marked in red.</figcaption></figure>

<table><caption>Table 2: Comparison of efficiencies of the different configurations with and without copper screen for three energy levels. These values correspond to the main emission energies of three radionuclides belonging to the $^{238}$U decay chain. Each shows the efficiency gain in the low, medium and high energy ranges.</caption><thead><tr><th rowspan="2">Geometry</th><th rowspan="2">Setup</th><th colspan="3">Efficiency (%)</th></tr><tr><th>46.54 keV (<sup>210</sup>Pb)</th><th>92.59 keV (<sup>234</sup>Th)</th><th>609.31 keV (<sup>214</sup>Bi)</th></tr></thead><tbody><tr><td rowspan="2">Punctual source @ 10 cm total efficiency (configuration 1)</td><td>With Cu</td><td>0.23</td><td>1.5</td><td>1.8</td></tr><tr><td>Without Cu</td><td>2.2</td><td>2.5</td><td>1.9</td></tr><tr><td rowspan="2">SG 50 in contact FEP efficiency (configuration 2)</td><td>With Cu</td><td>0.22</td><td>4.8</td><td>2.2</td></tr><tr><td>Without Cu</td><td>8.7</td><td>9.3</td><td>2.5</td></tr></tbody></table>

### 3.4. Background characterisation

To better assess the influence of the sample's intrinsic signal compared to the laboratory background, we calculated the ratio between the total counts
measured with the sample to the background-only counts in the same FEP.
Overall, the sample spectrum exceeds the background by approximately 39%.
The observed ratios at given energies are: 1.10 at 46.54 keV ($^{210}$Pb), 1.43 at
63.3 keV ($^{234}$Th), 1.55 at 92.59 keV ($^{234}$Th), 1.68 at 186.21 keV ($^{226}$Ra), 1.25
at 351.93 keV ($^{214}$Pb), and 1.20 at 609.31 keV ($^{214}$Bi).

However, additional background measurements revealed that the sample
holder itself may substantially contribute to the observed signal. Acquisitions
using both a used and pristine CF40 sample holder showed that several emis-
sion peaks exhibit comparable or even higher intensities than those measured
with the holder loaded with the JSC Mars-1 sample. This suggests that a
significant portion of the detected signal may originate from the holder rather
than the sample. Using a pristine flange further rules out radon deposition as
the source of this effect. Consequently, the current measurement setup can-
not distinguish clearly between the sample signal and the container signal,
specially at low energies.

3.5. Gamma-ray analysis

The measured activity values for the <sup>238</sup>U and <sup>232</sup>Th decay chains are
shown in Table 3 and Figure 9. These results correspond to a total acquisition
time of 36.25 days. The activity values and derived concentrations presented
here are based on spectra that have not been corrected for the radiological
contribution of the sample holder.

Assuming secular equilibrium between <sup>238</sup>U and <sup>226</sup>Ra, and between <sup>232</sup>Th and <sup>228</sup>Ac, we obtain <sup>238</sup>U and <sup>232</sup>Th concentrations of 1.59 ± 0.30 ppm and 3.78 ± 0.70 ppm, respectively. These values compare well with a previous study [21] that reported values of 1.05 ± 0.10 ppm (LMRE) and 1.36 ± 0.30 ppm (LESTS) for <sup>238</sup>U, and 3.44±1.00 ppm (LMRE) and 3.29±0.40 ppm (LESTS) for <sup>232</sup>Th.

Additional discrepancies arise when comparing activity values obtained
from different radionuclides within the same decay chain. For instance, us-
ing the activity of <sup>234</sup>Th, the direct decay product of <sup>238</sup>U, we obtain a
concentration of 2.3 ± 0.3 ppm, incompatible with the reported values. This
discrepancy, clearly shown in Figure 9a, appears between the radionuclides
<sup>226</sup>Ra, <sup>214</sup>Pb and <sup>214</sup>Bi, which are in equilibrium, and <sup>234</sup>Th, and <sup>210</sup>Pb, all
at low energies, which show incompatible activity levels.

The case of <sup>210</sup>Pb is particularly significant, as it shows the largest dis-
crepancy. Although the enhanced detection efficiency in the low-energy range
was expected to improve precision, the overestimation remains. While this
may be partly due to unsupported <sup>210</sup>Pb from <sup>222</sup>Rn outgassing, we consider
this effect insufficient to fully explain the difference.

Several emissions attributed to natural radionuclides present in the stain-
less steel flange overlap with key lines such as those from <sup>234</sup>Th (63.3 and
92.6 keV) and <sup>210</sup>Pb (46.54 keV), well within the low-energy region where
overestimation is observed. Background measurements performed with empty
and pristine sample holders revealed a non-negligible signal contribution in
this energy range. While preliminary, this background may explain the over-
estimation of these two isotopes, as well as the slight shift observed in the
<sup>232</sup>Th chain activity values. Due to limited counting statistics, the holder's
contribution could not be fully subtracted from the loaded spectra, but it
is likely a significant factor in the discrepancies observed at low energy. A
more detailed background characterisation of the holder is proposed to im-
prove activity attribution in future measurements.

In addition to structural background, the spectrum below 100 keV also
includes a strong Compton continuum and X-ray fluorescence from elements
present in the shielding and sample matrix. These features complicate the
identification of weak peaks and can lead to count overestimation when defin-
ing the ROI, particularly for low-intensity gamma lines. As shown in Fig-
ure 8, peak ROI selections in this range are heavily affected by the surround-
ing spectral structure, increasing the risk of residual contamination from the
continuum.
<figure><img src="image_9.png" /><figcaption>Figure 8: Raw spectra of JSC Mars-1 for the region below 100 keV, the red areas correspond to the ROIs used to obtain the net counts of the labelled radionuclide.</figcaption></figure>

The detection efficiency itself may also differ from experimental values. For example, in our geometry, the experimental efficiency for the $^{210}$Pb emission differs by 6.5% from the efficiency curve shown in Figure 7. However, this deviation alone does not explain the observed overestimation.

The efficiency transfer is particularly sensitive in the low-energy range, where gamma-ray attenuation becomes more pronounced due to changes in geometry and material composition. In our setup, the primary limiting factor is not the sample itself, but rather the sample holder, which is made of INOX 316L. The stainless steel base of the CF40 flange, despite being thinned to a nominal thickness of 500 µm, introduces significant attenuation in the low-energy regime, much greater than that of the JSC Mars-1 material. This structural constraint contributes substantially to the uncertainty of the efficiency transfer, specially when using a reference configuration, such as the SG50 geometry, whose container is made of plastic. Additionally, while the beam diameter used in the self-attenuation experiment (8 mm) provides a sufficiently large footprint to mitigate the effects of inhomogeneous distribution,
the measurement remains sensitive to local variations in activity. Spatial inhomogeneities, including grain-to-grain variability and imperfect mixing, can also affect the apparent attenuation at low energies and may contribute to discrepancies between small-volume measurements and bulk configurations. To address these limitations, future improvements could involve replacing the current stainless steel base with a lighter, less attenuating material, such as aluminium. This change would significantly reduce low-energy gamma attenuation and improve overall detection sensitivity.

To mitigate these effects, we propose replacing the stainless steel base of the current sample holder with a lighter and less attenuating material, such as low-activity aluminium. Future holders will undergo a detailed analysis of their natural radionuclide content to ensure their contribution can be reliably characterised and subtracted from the sample signal. Overtime, sample holders, including those made of aluminum, can accumulate $^{222}$Rn daughter products on their surfaces. However, electropolishing prior to use can remove these deposits, and storing them under a controlled nitrogen atmosphere can help prevent further accumulation and/or deposition [34]. In addition, the use of an uranium- and thorium-rich sample as a reference could help to quantify the uncertainties in low-activity measurements more precisely, as it would isolate the effect of the sample material on the efficiency transfer between geometries. These improvements are essential to ensure the reliability of future analyses involving low-mass samples and measurements near the detection limit.

Due to the uncorrected contribution of the sample holder, no definitive uncertainty assessment can be made on the measured activities of the JSC Mars-1 sample. As a result, the concentrations presented here should be interpreted as apparent values, subject to background interference in the low-energy region. For the $^{232}$Th decay chain, the relative consistency between radionuclide activities suggests a possible secular equilibrium, except for $^{208}$Tl, which shows a deficit likely linked to its 35.9% branching ratio.

While the activity values of $^{214}$Pb and $^{214}$Bi appear compatible with that of $^{226}$Ra, the potential contribution of the sample holder prevents a firm conclusion regarding the radon-tightness of the configuration in this measurement.
<table><caption>Table 3: Specific activity (activity per unit mass of material) with expanded uncertainties (k=2) obtained for both natural decay chains on 1.07 g of JSC Mars-1 sample (LNHB) and 472 g of the same material (LMRE, LESTS) [21]. The discrepancy for the <sup>234</sup>Th and <sup>210</sup>Pb is discussed in subsection 3.5.</caption><thead><tr><th rowspan="2">Radionuclide</th><th colspan="3">Activity (Bq kg<sup>-1</sup>)</th></tr><tr><th>LNHB (this study)</th><th>LMRE [21]</th><th>LESTS [21]</th></tr></thead><tbody><tr><td><sup>238</sup>U (<sup>234</sup>Th) (Bq kg<sup>-1</sup>)</td><td>28.6 ± 5.7</td><td>13.0 ± 1.7</td><td>16.8 ± 3.2</td></tr><tr><td><sup>226</sup>Ra</td><td>19.8 ± 3.6</td><td>14.6 ± 2.7</td><td>14.1 ± 9.6</td></tr><tr><td><sup>214</sup>Pb</td><td>14.1 ± 5.3</td><td>6.4 ± 0.8</td><td></td></tr><tr><td><sup>214</sup>Bi</td><td>15.6 ± 6.6</td><td>6.7 ± 0.8</td><td></td></tr><tr><td><sup>210</sup>Pb</td><td>43 ± 18</td><td>9.3 ± 1.6</td><td></td></tr><tr><td><sup>232</sup>Th (<sup>228</sup>Ac)</td><td>15.4 ± 2.5</td><td>14.0 ± 4.0</td><td>13.4 ± 1.6</td></tr><tr><td><sup>224</sup>Ra</td><td>14.8 ± 2.7</td><td>13.5 ± 2.3</td><td></td></tr><tr><td><sup>212</sup>Pb</td><td>12.8 ± 1.3</td><td>12.8 ± 1.1</td><td></td></tr><tr><td><sup>212</sup>Bi</td><td>15.2 ± 2.4</td><td>15.0 ± 4.0</td><td></td></tr><tr><td><sup>208</sup>Tl</td><td>11.6 ± 1.6</td><td>4.4 ± 0.5</td><td></td></tr><tr><td><sup>40</sup>K</td><td>146 ± 12</td><td></td><td></td></tr><tr><td><sup>235</sup>U</td><td>0.13 ± 0.03</td><td></td><td></td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>Figure 9: Specific activity (activity per unit mass of material) with expanded uncertainties (k=2) of the radionuclides from the <sup>238</sup>U (a) and <sup>232</sup>Th (b) decay chains obtained in this study on JSC Mars-1 sample (in blue). The obtained values are compared with those reported at LMRE (in orange) and LESTS (in green) facilities [21]. They are shown by order of appearance in the decay chains. The discrepancy for the <sup>234</sup>Th and <sup>210</sup>Pb is discussed in subsection 3.5.</figcaption></figure>
### 3.6. Minimum detectable activity (MDA)

The MDA values presented in Table 4 were calculated without taking into account the background contribution from the sample holder. Under these conditions, the sensitivity requirement of 12 mBq for $^{238}$U appears to be met for most emissions, with the exception of the low-energy lines from $^{234}$Th at 63.3 keV and $^{210}$Pb. These low-energy values are also more susceptible to overlapping peaks and background complexity, which can affect the accuracy of the MDA determination.

A more detailed characterisation of the sample holder background would allow for refined MDA estimates in future studies. The use of a low-activity aluminium holder, as previously proposed, would also improve the reliability of MDA estimates by enabling a more accurate characterisation of the background and at the same time reducing the attenuation of gamma-rays.

<table><caption>Table 4: Preliminary MDA values for each of the main emissions of the $^{238}$U decay chain radionuclides for an acquisition of 36.25 days performed on 1.07 g of JSC Mars-1 sample. These values do not take into account the sample holder's contribution to the background.</caption><thead><tr><td>Radionuclide</td><td>Emission energy (keV)</td><td>MDA (mBq)</td></tr></thead><tbody><tr><td><sup>234</sup>Th</td><td>63.3</td><td>13</td></tr><tr><td><sup>234</sup>Th</td><td>92.59</td><td>4.7</td></tr><tr><td><sup>226</sup>Ra</td><td>186.21</td><td>5.5</td></tr><tr><td><sup>214</sup>Pb</td><td>351.93</td><td>0.71</td></tr><tr><td><sup>214</sup>Bi</td><td>609.31</td><td>0.79</td></tr><tr><td><sup>210</sup>Pb</td><td>46.54</td><td>39</td></tr><tr><td><sup>228</sup>Ac</td><td>911.2</td><td>1.3</td></tr><tr><td><sup>224</sup>Ra</td><td>240.99</td><td>4.6</td></tr><tr><td><sup>212</sup>Pb</td><td>238.63</td><td>0.40</td></tr><tr><td><sup>212</sup>Bi</td><td>727.33</td><td>4.4</td></tr><tr><td><sup>208</sup>Tl</td><td>583.19</td><td>0.94</td></tr><tr><td><sup>40</sup>K</td><td>1460.82</td><td>4.6</td></tr><tr><td><sup>235</sup>U</td><td>185.72</td><td>0.34</td></tr></tbody></table>
553 4. Conclusions

This study demonstrates that the gamma spectrometric analysis of nat-
ural radionuclides in extraterrestrial samples with very low mass and low
NORM content is feasible using a dedicated low-level detection approach.
A customised methodology was developed to meet the specific constraints
of gram-scale regolith samples, combining an adapted sample holder design,
optimised measurement geometry, and improved detector efficiency at low
energies. The re-characterisation of the GeHP3 detector and the generation
of new efficiency curves successfully enhanced sensitivity in the 30 keV to
100 keV range, enabling the detection of key emissions such as those from
$^{234}$Th and $^{210}$Pb.

This study also highlights the importance of careful background assess-
ment when analysing low-mass samples, particularly in the low-energy range.
Based on our results, we identify the contribution of the sample holder as a
potential source of spectral distortion and apparent activity overestimation
for some radionuclides. Although the measurements presented here are pre-
liminary, they underline the value of dedicated holder background character-
isation and radiopure material selection. The complexity of the low-energy
region, particularly due to the yet uncorrected contribution of the sample
holder, prevents us from quantitatively isolating the impact of shielding and
the active veto on detection limits in the current study. However, the re-
duction in the background level—bringing the count rate close to that of the
low-mass sample and its Compton component demonstrates the effectiveness
of the passive shielding in enabling such low-activity measurements. More
broadly, the identification of the container as a possible contributor to sig-
nal bias represents a relevant consideration for other low-level spectrometry
applications.

This methodology now allows gram-scale measurements of regolith sam-
ples, opening the possibility of analyzing statistical dispersion within a larger
sample population. This is an important step in assessing the heterogeneity
of returned lunar samples from different collection sites and comparing local
measurements with orbital data. It provides a framework for characteriz-
ing other planetary analogs and supports future studies aimed at extracting
geochemical and volatile transport information from rare and fragile extrater-
restrial materials.
5. Acknowledgments
___

The contribution of Í.L. Chacartegui Rojo and P.-Y. Meslin was par-
tially supported by CNES, through its PhD and APR programmes focused
on DORN/Chang'E 6. Additional funding was provided by the French Na-
tional Research Agency (ANR) through the program ANR-22-EXOR-0001
Origins of the Institut National des Sciences de l'Univers, CNRS (RADEX
project, led by F. Girault), and by the IdEx Université Paris Cité (ANR-
18-IDEX-0001). This research is performed in the framework of RadonNET
(23IND07), funded by the European Partnership on Metrology, co-financed
by the European Union's Horizon Europe Research and Innovation Pro-
gramme and by the Participating States (Funder ID: 10.13039/100019599).
It also contributes to the SCINTI+ project, funded by the French National
Metrology Institute (LNE).

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