Development of a Cerium-Doped Lanthanum Bromide Gamma-Ray Spectrometer for Planetary Missions and Feasibility Studies for Determination of Elemental Abundances of Radioactive Elements (Th, K and U)

We present the development of a cerium-doped lanthanum bromide (LaBr 3 : Ce) gamma-ray spectrometer (GRS) with the primary objective of determining the abundance and distribution of Th, U, K, and other major elements, including Fe on the entire planetary surface by measuring gamma-ray signals produced by radioactive decay, neutron inelastic scattering and neutron capture reactions in the energy region 0.03- 8 MeV. The energy resolution of the LaBr3 : Ce GRS developed in-house using front-end and processing electronics at 511 and 1274 keV is estimated to be 4.1% and 2.5% respectively. The intrinsic activity count rate for our 3 n * 3 n LaBr 3 : Ce GRS is ~61 counts s -1 (i.e. ~0.18 counts s -1 cm -3 ) for the 40K energy window (1400-1520 keV) and ~3.4 counts s -1 for the 232 Th (2550-2700 keV) energy window. Although this large intrinsic activity of the LaBr 3 : Ce crystal inhibits estimation of the concentrations of Th and K, our attempts using a NaI(Tl) GRS (with electronics developed in-house) were more successful. The Th concentration of US-110 was estimated to be ~11.4 ppm and is within 14% of the 13.2 ppm value determined using a HPGe GRS. The K concentration of US-110 was estimated to be 0.87% and is within ~10% of the 0.78% value determined independently using a HPGe GRS.