Measuring planetary hydrogen by remote gamma-ray sensing

Abstract

A γ-ray spectrometer (GRS) orbiting about an airless or nearly airless planetary body may be used to detect and measure hydrogen concentration and the neutron leakage flux. The H concentration affects both the magnitude and energy spectrum of the neutron flux, while the neutron flux, in turn, determines the intensity of the observed H γ-ray line for a given concentration. Because of this interconnection, the dual measurement of both H and neutron flux enhances the accuracy of each, and assists in the measurement of other elements. Hydrogen is detected by means of its 2.223 MeV γ ray which arises from the capture of thermal neutrons producing deuterium. The 2.223 MeV H signal is observed in the γ-ray spectrum against an interference spectrum consisting of cosmic γ rays, planetary background emission, and a variety of γ rays arising from cosmic-ray particle interactions with the γ-ray spectrometer and spacecraft (SC). In addition, line interferences are generated by the interactions of neutrons with hydrogenous materials in the GRS and SC. In this paper the expected signal levels and the sources of continuum and line interference in the detection and measurement of H are assessed in terms of two possible missions, a lunar orbiter and a comet nucleus rendezvous. In lunar orbit, a 100 h observation at an altitude of 100 km should enable the detection of H at a level of 0.06% with an uncertainty of ∼ 0.02%. At a distance equal to the radius of a comet's nucleus, in 100 h a GRS can detect H at a level of 0.07% and can measure H at expected cometary levels (∼ 6%) with an uncertainty of 0.06%.