Combined gamma-ray and neutron detector for measuring the chemical composition of airless planetary bodies

Galactic cosmic rays (GCR) constantly impinge all planetary bodies and produce characteristic gamma-ray lines and leakage neutrons as reaction products. Together with gamma-ray lines produced by radioactive decay, these nuclear emissions provide a powerful tool for remotely measuring the chemical composition of planetary surfaces having little or no atmospheres. While lunar gamma-ray spectroscopy was first demonstrated with Apollo Gamma-Ray measurements, the full value of combined gamma-ray and neutron spectroscopy was shown for the first time with the Lunar Prospector Gamma-Ray (LP-GRS) and Neutron Spectrometers (LP-NS). Any new planetary mission will likely have the requirement that instrument mass and power be kept to a minimum. To satisfy such requirements, we have been designing a GR/NS instrument which combines all the functionality of the LP-GRS and LP-NS for a fraction of the mass and power. Specifically, our design uses a BGO scintillator crystal to measure gamma-rays from 0.5 - 10 MeV. A borated plastic scintillator and a lithium glass scintillator are used to separately measure thermal, epithermal, and fast neutrons as well as serve as an anticoincidence shield for the BGO. All three scintillators are packaged together in a compact phoswich design. Modifications to this design could include a CdZnTe gamma-ray detector for enhanced energy resolution at low energies (0.5 - 3 MeV). While care needs to be taken to ensure that an adequate count rate and background suppression is achieved for specific mission designs, previous mission successes demonstrate that a combined GR/NS provides essential information about planetary surfaces.