A Gamma Ray and Neutron Spectrometer (GRNS) for mapping lunar surface composition and water abundance on the SER3NE mission

Selene’s Explorer for Roughness, Regolith, Resources, Neutrons and Elements (SER3NE) is a lunar orbiter mission designed to map the topmost composition of the lunar surface, including elemental composition and water abundance. Planned instruments include a Gamma Ray and Neutron Spectrometer (GRNS) for elemental composition, including hydrogen indicating water, a Laser Altimeter (LA) for surface roughness and albedo observations, and a near-infrared spectrometer (LIPS) to determine water forms.

The GRNS detector is designed for both in situ utilization as well as remote sensing. It has a core of CLLBC and LaBr₃ crystal scintillators in a chessboard pattern for high-resolution gamma-ray detection (30 keV-8MeV) and thermal to epithermal neutron sensitivity. Gd foil on CLLBC allows separation of thermal and epithermal neutrons, while LaB3 and CLLBC enable advanced neutron detection analysis. Encapsulated by EJ-248M plastic scintillators, the detector includes anti-coincidence detector for charged particle rejection. With gamma-ray spectroscopy, rock-forming elements as well as KREEP and trace elements can be detected in the shallow surface of the moon. The local count rates of thermal and epithermal neutrons allow for the analysis of the distribution of hydrogen on the lunar surface, as well as for estimation of neutron lifetime from the lunar orbit.

A demonstrator of the GRNS instrument has been successfully tested in the lab. A prototype of this lunar GRNS instrument will fly on the CENSSat-1 Bifrost CubeSat mission, scheduled for launch 2027.

In this presentation, the GRNS instrument concept will be presented, focusing on the detector design and suitability for elemental composition analysis on a lunar orbiter.