W. C. Feldman, Fluxes of fast and epithermal neutrons from Lunar Prospector: evidence for water ice at the lunar poles, Science, 281(5382): 1496-1501. September 5, 1998.

NASA's Lunar Prospector left Earth for the Moon in January, 1998. Among its remote sensors was a neutron spectrometer which measured epithermal and fast-neutron counts from January 6 through 27 June, 1998. For most of its mission the spacecraft circled the moon in a polar orbit at 100 km altitude with a 118 minute period. A precise 90 degree orbit was necessary to conduct scientific observations above the north and south poles.

The neutron spectrometer gathered data from the surface to determine its chemical composition, specifically, the concentration of hydrogen. This hydrogen is a great indicator of water ice, the only phase that water can last at in the lunar surface. Fast neutrons are formed at high energies as the result of interactions between galactic cosmic rays and the nuclear constituents of the lunar soil.

The neutron spectrometer detected particles with two Helium gas-filled proportional counters. One detector was covered by a cadmium sheet sensitive only to epithermal neutrons. The second was covered by a sheet of tin. Fast neutrons were measured with the anticoincidence shield of the gamma-ray spectrometer. The energy level of these particles is between 0.5 and 8.0 MeV.

Overlying dry layers of regolith (lunar soil) hide deep ice from the spectrometer, so the quantities of ice detected are very near to the surface. Visual observations from the Clementine mission corroborate the polar locations where Lunar Prospector detected large quantities of hydrogen (water ice). The .75um visual measurements from Clementine show the specific areas are all in permanent shadow.

Water ice near the lunar surface would quickly disappear in any area exposed to the sunlight.