Experimental Determination of the Spatial Sensitivity of a He-3 Neutron Spectrometer for Field Analog Experiments to Reunite Planetary Science and Terrestrial Hydrology

Neutron Spectroscopy is a non-invasive way to estimate the hydrogen content (reported as ‘Water-Equivalent-Hydrogen’ or WEH, an indirect measurement of H2O) of the top 1 m of the soil. Terrestrial hydrologists use passive neutron spectroscopy (PNS) to estimate soil moisture (water found between the grains) over time. Utilizing stationary instruments enables changes in the neutron count rates to be used as a proxy for changes in soil moisture caused by fluctuations in groundwater, precipitation, and vegetation, while soil and rock composition remain constant. Planetary science missions use PNS to analyze the WEH at points along a traverse via a mobile platform on rocky worlds with thin atmospheres without precipitation and vegetation. Detected changes in neutron counts on mobile platforms are the result of geochemistry.Herein, a new, portable PNS instrument, referred to as ‘rovers’ (RVs), is utilized to experimentally characterize the radial sensitivity (footprint), orientation and vegetation sensitivities, and moderator thickness, to characterize the differences between moderated and bare detectors. PNS instruments measure neutron counts from moderated and bare neutron detectors, each sensitive to different energy ranges and used (together) as a proxy for variations in abundances of neutron absorbing and neutron scattering elements. Terrestrial hydrologists implement a method to remove counts from neutrons emitted and moderated in the atmosphere to calculate ground WEH from the moderated detector, but they note that the bare detector likely has a smaller footprint and needs additional characterization. Changing the environmental factors within the radial sensitivity (by removing vegetation, or increasing the distance from a body of moderators), determines how each factor affects the count rates. Experimentation in the field: provided maximum radial sensitivities for the new, smaller, portable moderated and bare detectors (31 m and 2 m, respectively), which increases further from the surface or in the vertical orientation; determined adjustments are needed for vegetation (~10% increase in epithermal, and ~15% decrease in thermal counts); and increased portability (removing a layer of moderator).