Element abundance mapping with the SER3NE Gamma-ray and neutron spectrometer

Selene’s Explorer for Roughness, Regolith, Resources, Neutrons and Elements (SER3NE) is a proposed small satellite mission in lunar orbit, aiming to characterize the lunar surface to unravel its volatile origin and delivery processes, in addition to uncovering the geological processes that shaped the Moon. Global element abundance mapping will prospect lunar resources for ISRU at future landing sites. Further, the mission is aiming to determine a more precise space-based neutron lifetime and the orbital evolution of the Earth-Moon system. To achieve these goals, SER3NE will host three instruments: a gamma-ray and neutron spectrometer (GRiNS, by UiO), the Lunar Infrared Point spectrometer (LIPS, by ROB/BIRA-IASB), and the SER3NE Laser Altimeter (S3LA, by DLR).

Here, we present the current Gamma-Ray-including-Neutrons Spectrometer (GRiNS) design, performance specifications and preliminary results of the GRiNS laboratory demonstrator. GRiNS is part of the CLUNGAS family, that was reported on earlier.¹

By detecting gamma-ray and neutron radiation moderated by the lunar surface after GCR impact, GRiNS will be able to create global compositional maps and their abundances of different elements, such as Al, Ca, Fe, K, Mg, Si, Ti, Th, as well as hydrogen. In previous missions² this was demonstrated with LaBr3 as main gamma-ray detector, proving the suitability of this technology. The default approach for hydrogen mapping are He3 pressurized tubes³, however, recently mission approaches with scintillator-based neutron detectors were launched⁴. In our approach we combine gamma-ray and neutron detection in one hybrid detector for reducing weight, cost, and space, and enabling deployment on small satellite missions.   

GRiNS is a compact detector  capable of gamma-ray detection in the range of 100 keV to 8 MeV as well as the detection of thermal to epithermal neutrons. There shall be two detector units, facing nadir (for mapping) and zenith (in addition for neutron lifetime), as depicted in fig.1. The comparison of the ascending and descending gravitational bound thermal neutrons allow for the determination of the neutron lifetime along their flight path. The primary detecting material is a CLLBC (Cs₂LiLa(Br,Cl)6:Ce), a dual-mode scintillator capable of both high-resolution gamma-ray spectroscopy and thermal to epi-thermal neutron detection. The preliminary detector layout aims for a 2x2 array where CLLBC is accompanied by lanthanum bromide (LaBr₃), a crystal scintillator used in previous lunar mission² for high-resolution gamma-ray spectroscopy, for improved gamma-ray – neutron discrimination. CLLBC and LaBr₃ provide a good spectral resolution of <4% FWHM at 662keV, sufficient for detecting the targeted elements. Parts of the crystal scintillators are covered in gadolinium foil, blocking thermal neutrons from reaching the crystal scintillators, allowing for thermal-epithermal neutron separation. The detector units will be enclosed in an anti-coincidence shielding (ACS) made of plastic scintillators. The advantages of plastic scintillators are the customisation of both the shape and radiation detection capabilities. In the default mode, the ACS will work in veto-mode. The rejection rate of events, however, disclose the high-energy particle flux at the given time.

_{Figure 1: Concept of the detector units, nadir and zenith pointing. In green and red the crystal scintillators, in grey the Gd shield, in yellow the ACS, in blue the SiPM array placement.}

The scintillators interface silicon photomultiplier arrays attached to application specified front-end electronics, the IDE3380 ASIC by IDEAS: the ASIC is radiation tested ⁵ and has flight heritage ⁶. The backend electronics will be a space-suitable version of the IDEAS ROSSPAD module.

During pre-phase A of the SER3NE project, we developed the GRiNS instrument design further, defined science and technical requirements, and updated the instrument budgets. Characterisation measurements with the lab demonstrator confirmed the neutron detection capabilities as predicted by simulations, as well as confirmed the spectral resolution of the instrument of 4% FWHM at 662keV. Further, we are working for on a mobile field demonstrator to be deployed in 2025/26.

Acknowledgements  

This work supported by the Research Council of Norway, grant no. 309835, Centre for Space Sensors and Systems (CENSSS), through their SFI Centre for Research-based  Innovation program. We would like to thank our partners and colleagues at Integrated Detector Electronics AS (IDEAS) for their support.

We would like to thank our colleagues Konstantin Herbst and Agata Krzesinska at UiO/PHAB for their discussions and reviews, as well as our partners and colleagues at Integrated Detector Electronics AS (IDEAS) for their support.

 

References

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