Neutrons are created when galactic cosmic rays strike atoms in the lunar regolith. These neutrons bounce from atom to atom like billiard balls, losing energy with each collision. Along the way, some of these neutrons escape into space, where LEND can detect them.
The presence of hydrogen in the lunar soil reduces the number of neutrons that escape. To map out likely deposits of water ice, LEND scientists look for this deficit of neutrons in the epithermal (medium) energy range.
If the deficit were simply due to random fluctuations, the hydrogen map would never coalesce into a sharp image, but as this animation shows, the map of epithermal neutron deficit at the south pole of the Moon improves over time and converges on particular spots. These include especially strong signals in the permanently shadowed parts of Cabeus and Shoemaker craters, where ice would be completely shielded from the sun. But LEND and other missions have found signs of water in places that aren't permanently shadowed while apparently excluding some places that are, both of which are surprising and exciting discoveries.