The Neutron Spectrometer System is an instrument designed to measure the leakage flux of low-energy neutrons out of the 
            lunar soil.  A key determinant of leakage flux in the epithermal energy range (>0.5 eV ? 500 keV) is abundance of 
            hydrogen-bearing compounds. NSS gauges the fluxes of neutrons in two energy ranges: a) thermal neutrons, less than 0.5 eV, 
            which respond to both abundance and burial depth of hydrogenous materials, and b) epithermal neutrons, which vary in a 
            different way with hydrogen abundance and burial depth from the thermal neutron population.  Together, these two measurements 
            constrain not only water-equivalent hydrogen (WEH) abundance but permit a simple model of stratigraphy (burial depth and 
            lower-layer WEH abundance, assuming the overburden is hydrogen-free). Water-equivalent hydrogen is the weight percent of 
            water that a material would contain if all the detected hydrogen was in the form of H2O. For example, a material containing 
            1500 ppm H would have a water-equivalent hydrogen content of 1.35 wt.%.
            
            NSS consists of gas-proportional counter neutron detectors with a design based on the Lunar Prospector Neutron Spectrometer. 
            NSS will measure any temporal variations of at least 0.5 wt.% water-equivalent hydrogen (WEH) at the lander's location. NSS 
            measurements are also used to infer the probable burial depth and WEH wt.% of a buried layer below otherwise desiccated 
            regolith.