A Combined Gamma-Ray and Neutron Detector for Measuring the Chemical Composition of Comets and Other 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 technique for remotely measuring the chemical composition of airless planetary surfaces. While lunar-ray spectroscopy was first demonstrated with Apollo Gamma-Ray (AGR) 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). Analysis of other elements such as Ca, Mg, O, Al, and U should soon be completed using LP-GRS data. Furthermore, it has been shown that measurements of thermal neutrons provide an important correction (up to a factor of 3) to measurements of neutron-capture gamma-ray lines such as those from Fe and Ti. With the inclusion of a gamma-ray instrument on the NEAR mission, and gamma-ray and neutron instruments on missions to Mars and Mercury, it is clear that gamma-ray and neutron spectroscopy (GR/NS) is becoming a standard technique for planetary science missions. While we are focusing here on using GR/NS for future cometary missions, these techniques will also provide powerful ways of measuring the surface composition on rocky and icy moons and asteroids. Additional information is contained in the original extended abstract.