On gamma-ray spectrometry of rare earth elements on the moon: Reference data from proton accelerator experiment

Abstract

The laboratory experiment was conducted at Joint Institute for Nuclear Research with a prototype of planetary gamma-ray spectrometer (GRS) based on a High Purity Germanium (HPGe) detector and a proton detector (PD) configured with the GRS in the coincidence mode. The instrument set-up allows to detect gamma rays produced in the target irradiated by protons using integrated gamma-ray spectra (all detected gamma rays) as well as tagged gamma-ray spectra that select only photons synchronized with the proton detection. The latter option improves signal-to-noise ratio by selecting prompt photons produced in the irradiated target. The main objective was to conduct experiment with an irradiation of 11 samples of Rare Earth Elements (REEs) with high energy protons (170 MeV) to find a set of uniquely characterized gamma-ray lines that confidently indicate the presence of the given REE. These measurements are acquired to understand the conditions of a hypothetical space experiment on the lunar surface aimed for the reconnaissance of lunar resources. In this experiment, ambient gamma radiation is produced by charged particles of Galactic Cosmic Rays (GCRs).45 significant gamma-ray lines were found for 11 tested samples of REEs in total. This list includes 7 gamma-ray lines which have already been identified in the existing catalog of characteristic gamma-ray lines produced in the inelastic scattering reactions. 18 gamma-ray lines are unique and they were detected for an individual REE only. Other 27 detected gamma-ray lines are associated with several REEs and therefore were combined into 10 joint groups. Some groups are rather large and consists of 4 REEs.The proposed method of spectrometry with the options of time-integrated and tagged spectra allowed to determine the fraction of the gamma-ray lines, which occurs promptly at moment of proton-nuclei interaction. For some gamma-ray lines such a fraction is rather large and approaching 50%, and for some other gamma-ray lines it could be as small as 10–15%.The obtained experimental results were used to predict the expected intensities of the REEs gamma-ray lines in a future gamma-ray spectrometric experiment on the lunar surface. In particular, for a hypothetical lunar ore field it is estimated that a gamma-ray spectrometer with 10 times larger total effective area, than one used for the experimental set-up, may detect Cerium gamma-ray line at 553 keV after 1.6 h of signal accumulations.