A comparative study of LaBr₃(Ce³⁺) and CeBr₃ based gamma-ray spectrometers for planetary remote sensing applications

The recent availability of large volume cerium bromide crystals raises the possibility of substantially improving gamma-ray spectrometer limiting flux sensitivities over current systems based on the lanthanum tri-halides, e.g., lanthanum bromide and lanthanum chloride, especially for remote sensing, low-level counting applications or any type of measurement characterized by poor signal to noise ratios. The Russian Space Research Institute has developed and manufactured a highly sensitive gamma-ray spectrometer for remote sensing observations of the planet Mercury from the Mercury Polar Orbiter (MPO), which forms part of ESA's BepiColombo mission. The Flight Model (FM) gamma-ray spectrometer is based on a 3-in. single crystal of LaBr₃(Ce³⁺) produced in a separate crystal development programme specifically for this mission. During the spectrometers development, manufacturing, and qualification phases, large crystals of CeBr₃ became available in a subsequent phase of the same crystal development programme. Consequently, the Flight Spare Model (FSM) gamma-ray spectrometer was retrofitted with a 3-in. CeBr₃ crystal and qualified for space. Except for the crystals, the two systems are essentially identical. In this paper, we report on a comparative assessment of the two systems, in terms of their respective spectral properties, as well as their suitability for use in planetary mission with respect to radiation tolerance and their propensity for activation. We also contrast their performance with a Ge detector representative of that flown on MESSENGER and show that: (a) both LaBr₃(Ce³⁺) and CeBr₃ provide superior detection systems over HPGe in the context of minimally resourced spacecraft and (b) CeBr₃ is a more attractive system than LaBr₃(Ce³⁺) in terms of sensitivities at lower gamma fluxes. Based on the tests, the FM has now been replaced by the FSM on the BepiColombo spacecraft. Thus, CeBr₃ now forms the central gamma-ray detection element on the MPO spacecraft.