LEND is a Russian collimated neutron detector, a part of the scientific payload of Lunar Reconnaissance Orbiter (LRO, NASA), which explores Moon from its orbit.

The LEND neutron detector records the flux of secondary neutrons escaping from the lunar surface — so-called neutron albedo. These neutrons are born in the upper subsurface layer 1–2 meters thick, in the interactions with cosmic rays and during the decay of natural radioactive elements. On their way to the surface, they are slowed down and absorbed by the nuclei of the main rock-forming elements. Thus, the neutron flux coming out of ground depends on the substance composition and first of all on the presence of hydrogen or hydrogen-bearing compounds. Knowing hydrogen distribution in the ground, one may judge about the presence of water ice or bound water in minerals there.

First maps showing the distribution of water in the lunar regolith at south and north poles were made, using the data of neutron radiation measurements from the polar regions of the Moon. The Moon turned out to be quite a "wet" place: in some regions hydrogen-enhanced content can reach up to 5 % by mass, which is more than in Sahara desert on the Earth.

Just two months of orbiting the Moon were sufficient to get fundamentally important data and, subsequently, to draw maps of water distribution in the lunar circumpolar regions. They showed that regions with enhanced hydrogen content in the regolith do not match the previously known permanently shadowed regions in the bottom of lunar polar craters. This was an unexpected result, since it had long been assumed that probable local regions with high hydrogen content should coincide with the permanently shadowed regions (PSRs) in the vicinity of the Moon poles. Thus, LEND data forced a new look at the "water cycle" on the Moon.

Moreover, LEND provided basic data, which (in combination with other instruments' output) were used by NASA when choosing a region for lunar surface guided bombardment in the following LCROSS experiment (short for Lunar CRater Observation and Sensing Satellite). LCROSS, a spacecraft, which had been launched into space together with LRO, followed Centaur upper stage of the launcher. This pair was directed to the lunar surface, and namely to the Cabeus crater near the pole. After Centaur had hit the surface, a cloud of debris was released and studied directly by instruments onboard Shepherding Spacecraft, which followed Centaur and, later, by LRO instruments.

The results of the LEND experiment are used for planning future scientific and exploration missions to the lunar poles: Luna-25, Luna-27, and following.

The predecessor of LEND was the HEND instrument, also made at the Space Gamma-Spectroscopy Laboratory of IKI (now the Nuclear Planetology Department) for the 2001 Mars Odyssey orbiter (NASA, launched in 2001). But lunar instrument required very high spatial resolution. To achieve it, a conceptually new device — a collimator made of boron and polyethylene — was developed. In fact, it is a very thick shell, installed in such a way that it absorbs epithermal neutrons coming from outside the instrument's field of view. Collimation technique makes it possible to map the flux of epithermal neutrons coming from the lunar surface with a resolution of 10 km from 50-km-high orbit.

Main parameters

• Detected neutrons energies: from thermal to 15 MeV
• Temporal resolution: variable, >1 s
• Spatial resolution on the surface: ~10 km from the 50 km orbit
• Vertical resolution in the subsurface: ~1–2 m