Instrument Overview
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The Lunar Exploration Neutron Detector, LEND, was designed and built by P.I. Igor Mitrofanov at the Russian Space Institute for incorporation into NASA's Lunar Reconnaissance Orbiter (LRO) as a large orbital neutron telescope for mapping the Moon's neutron albedo. LEND is the follow-on instrument from the High Energy Neutron Detector (HEND) onboard Mars Odyssey, although LEND incorporates a set of collimated detectors to improve spatial resolution. The methods and procedures of LEND data processing and analysis are based on existing procedures that have beendeveloped for analysis of HEND data.

The LEND instrument is based on the registration of secondary neutrons from the Moon, which are created in the upper 1-2 meters of lunar regolith which is irradiated by cosmic rays. Neutrons of high energy are born in the regolith and then slowed down and absorbed by nuclei of the major elements in the regolith through inelastic scattering and capture. The neutron flux coming out of the regolith depends upon the composition of the regolith, and is strongly affected by the presence of hydrogen or its compounds. When colliding with a hydrogen nucleus, a neutron losses half of its energy which leads to fast thermalization, thus to a significant increase of the flux of thermal neutrons and reduction in the flux of the epithermal neutrons. The orbital flux of epithermal neutrons depeds uponthe hydrogen content of the regolith.

Usage of different methods of neutron spectrometry allows us to obtain the average hydrogen or water ice content along with estimates of thickness of the ice cover due to comparison of counting rates of neutrons having different energies. The flux of epithermal neutrons and the flux of fast neutrons depend upon thethickness of water ice on the Lunar surface.

LEND has eight 3He sensors for detecting thermal and epithermal neutrons. Four of the 3He sensors are un-collimated and four are collimated. The external sensors STN1-3 (Sensor Thermal Neutrons) and SETN (Sensor EpiThermal Neutrons) are the un-collimated sensors that detect thermal and epithermal neutrons. STN-3 and SETN both have open fields of view. The combination of these two sensors will allow the measurement of local density of thermal and epithermal neutrons around the spacecraft. STN 1 and 2 are located near the midpoint of the instrument and have the thick mass of the collimation module just between them. For sensors on the +X and -X sides, the collimation material absorbs all external particles coming from directions -X and +X, respectively. The velocity vector of LRO will correspond to one of these directions: therefore, sensors STN 1 and 2 will detect neutrons with velocities (Vn + Vorb) and (Vn - Vorb), respectively, and will operate as a Doppler filter to separate the flux of external neutrons from the Moon from the local spacecraft background. The full set of four sensors, SETN and STN 1-3, will provide the data to characterize the neutron component of lunar radiation background at the altitude of LRO separately from the neutroncomponent of local background produced by LRO itself.

The collimated sensors of epithermal neutrons, CSETN1-4, are also 3He counters, that are installed inside the collimating module, which effectively absorbs external neutrons outside of instrument Field of View. Absorbing neutrons is very difficult; one of the best absorbing materials is 10B, and its absorption efficiency becomes much higher when neutrons are slower. The LEND collimators have external layers of polyethylene for moderation of impacting neutrons and internal layers of 10B for their absorption. These sensors are also enclosed by Cd shields that absorb all neutrons with energies below ~0.4 eV, which mainly correspond to thermal neutrons. The neutron collimator provides the instrument Field of View (FOV): epithermal neutrons of direct flux inside the FOV are recorded by the detector, while the majority of neutrons outside the FOV are absorbed by the collimator. This collimation technique provides mapping of epithermal neutrons from the Moon's surface with the horizontalresolution of 5 km for LRO at an altitude of 50 km.

The final LEND sensor is the Sensor for High Energy Neutrons (SHEN), which is a stilbene scintillator that produces a flash of light each time a high energy neutron in the energy range 0.3 to 15.0 MeV collides with a hydrogen nucleus and creates a recoil proton. Special shape-sensitive electronics distinguish proton counts from electron counts, and an activeanti-coincidence shield eliminates external charged particles.
This sensor is installed inside the central hole of the collimation unit, and its Field of View corresponds to 40-degree (HWHM). The SHEN sensor outputs data in 16 energy channels from 300 keV to more than 15 MeV, and is the sourceof the SC1-3 spectra.

For further information about the LEND instrument and theLEND experiment please see [MITROFANOVETAL2008].


Scientific Objectives
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The science objectives of neutron mapping of the Moon are outlined by three investigation tasks developed by theLRO team.

Task (1) is to explain the origin of the neutron flux depressions in the lunar polar regions, which could be produced by enhancement of implanted hydrogen from the solar wind, or by accumulateddeposits of water ice from comet impacts in shadowed craters.

Task (2) focuses on the variation of hydrogen content at moderate latitudes of the Moon in comparison to surface variations ofminerals, thermal conductivity and soil maturity.

Task (3) corresponds to the measurement of neutrons over a broad energy range from thermal energies up to tens of MeV to determine the neutron contribution to the total radiation dose at an altitudeof 50 km above the Moon.

The Lunar Exploration Neutron Detector (LEND) will sddress these tasks by:
Creating high resolution hydrogen distribution maps with sensitivity of about 100 ppm of hydrogen weight and horizontalspatial resolution of 5 km.

Characterizing surface distribution and column density of possible near-surface water ice deposits in the Moon's polarcold traps.

Creating a global model of neutron component of space radiation at altitude of 30-50 km above the Moon's surface with spatial resolution of 20-50 km at the spectral range fromthermal energies up to 15 MeV.


Calibration
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Calibration information for the LEND instrument is provided in the LEND Calibration Report(CALIB/LEND_calibration_report.pdf).


Operational Modes
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LEND operates autonomously, collecting data throughout the lunar orbit. LEND generates approximately 0.26 Gbits of measurement data per day. In order to perform early calibration measurements, LEND became active shortly after the first mid course correction (MCC) burn. Operationally, LEND is simple and has only three instrument modes: MEASUREMENTS, STAND-BY, and OFF. While in MEASUREMENTS mode, instrument electronics and detector high voltage are both 'on' and the instrument generates measurement and housekeeping data. In STAND-BY mode, instrument electronics are 'on', detector high voltage is 'off', and only housekeeping data are generated. While in OFF mode, the instrument is 'off', the instrument external heater is'on', and only external temperature data are generated.