The Lunar Exploration Neutron Detector (LEND) scientific instrument is designed for NASA interplanetary mission Lunar Reconnaissance Orbiter 2009 (LRO). The program of the instrument includes launch of the LRO spacecraft in May 2009, cruise to the Moon and a year-long global mapping of the surface of our satellite (see NASA's press release № 04-407 from Dec. 22, 2004 NASA Selects Investigations for Lunar Reconnaissance Orbiter http://www.nasa.gov/).
LEND project's main objective is to develop a highly sensitive scientific instrument that shall be used to search for water ice (presumably of a comet origin) in permanently shadowed lunar polar regions. LEND shall provide high-resolution hydrogen and hydrogen-bearing compounds (water ice in the first place) distribution map with spatial resolution 10 km in diameter (for polar regions) and sensitivity not less than 0.01% mass fraction. It shall also measure lunar neutron albedo as part of space radiation environment at altitude 30-60 km above lunar surface.
LEND uses registration of secondary neutrons coming from the Moon, which are born in the shallow layer of lunar subsurface within the depth of 1-2 m, that is constantly bombarded by space rays. High energy neutrons born in the subsurface are moderated and captured by main rockforming nuclei through the reactions of inelastic scattering and capture. The netron flux emanating from the lunar soil depends on composition of the latter, and first of all, on presence of hydrogen or hydrogen-bearing compounds. Having collided with a hydrogen nucleus, a neutron immediately loses half of its energy, thus leading to quick thermalization and hence to significant increase of thermal neutron and decrease of epithermal neutron fluxes.
LEND incorporates nine detectors designed to measure thermal, epithermal, and fast neutron fluxes. Four collimated epithermal neutron detectors CSETN1 — CSETN4 have maximum sensitivity and minimal field of view (FOV). These detectors are based on large proportional counters filled with 3He and protected by cadmium shield from the side of instrument's open FOV. Boron and polyethylene collimators absorb thermal and epithermal neutrons coming outside FOV. Collimating method allows to map epithermal flux coming from the lunar surface with resolution of 10 km from the orbit altitude of 50 km.
Three thermal neutron detectors STN1 — STN3 (image 1) and one epithermal neutrons detector SETN are placed on the upper side of the collimator. These four detectors include similar proportional counters. The only difference of SETN detector is cadmium shell around the counter. STN1 and STN3 detectors are united in so-called 'Dopler filter', which compares velocities of thermal neutrons with that of the orbiter. STN2 and SETN detectors measure the local field of thermal and epithermal neutrons in orbiter's vicinity. Neutrons are registered by gas-filled counters through nuclear reactions of neutron capture.
SHEN detector is located in the inner space between collimators, and measures the neutron flux in the energy range from 300 keV to 15 MeV. It includes a scintillation detector based on stilbene crystal, surrounded by plastic scintillator to protect it from charged cosmic rays (image 2). Fast neutron are registered in the organic crystal through recoil protons.
During cruise phase and at near-lunar orbit LEND measures physical values as follows:
LEND is located on the LRO science deck and has simple mounting with eight pins. It shall be connected with the spacecraft by special controlled thermal interface, sufficiently isolated from the open space by multilayer thermal isolation. Spacecraft systems will maintain the operational temperature in the range -20 °C – +50 °C. Electrical interface along with the data and commands interfaces were designed in accordance with project requirements.
Figure 4 shows LEND instrument without multilayer thermal isolation.
Figure 5 shows LEND instrument with multilayer thermal isolation.
LEND flight units were delivered to NASA's Goddard Space Fligth Center (Flight Unit-1 in March 2008, Flight Unit-2 in December 2008) and were successfully tested as the part of LRO spacecraft. On June 18, 2009 LEND was launched to the Moon. On June 20, 2009 LEND was switched to the Scientific mode.
Even the very first LEND data obtained in the initial two months of the work aboard LRO allowed to study water distribution in polar regions of the Moon, deemed to be potentially important for future lunar exploration. Local regions with weak neutron emission were discovered at the lunar south pole, with very probable hydrogen-enhanced content in the regolith. Somewhat unexpectedly, these regions does not match the permanently shadowed regions (PSR) in the bottom of lunar polar craters, although it was assumed earlier that probable local regions with enhanced hydrogen content should coincide with PSRs in the poles' vicinity.
Using LEND data as primary source of information along with the results of other LRO instruments, NASA chose Cabeus crater as a location for lunar surface bombardment by LCROSS spacecraft. This experiment was aimed to study composition of lunar surface to look for water ice by measuring the parameters of debris plume.
On September 15, 2009, after commissioning phase was officially completed, LEND began the main phase of the flight, that is to map neutron emission from the Moon with spatial resolution around 10 km.
LEND results and plans for its further operation are discussed at regular workshops of Russian-American science group, which are held at IKI RAS and NASA's GSFC. Members of LEND project also take part in mission planning and meeting of LRO science team.
As for now, LEND is functioning normally. Telemetry and science data are received at IKI RAS and processed by LEND science team, as well as are sent to Planetary Data System http://starbrite.jpl.nasa.gov/pds, thus becoming available for science community. The results of the experiment were published in several journals (> 7 papers) and presented at scientific conferences (>10 presentations).
Funding organization ― Federal Space Agency of Russian Federation
Primary contractor ― Space Research Institute of the Russian Academy of Sciences (IKI RAS)
Principal Investigator ― Dr. Igor Mitrofanov
Works on LEND project are led on the basis of the State contract №025-5452/04 from February 27, 2004 (R&D theme MSP-2001) and are included in the Federal Space Program for 2006–2016.
Works on the LEND project are planned for 2004–2008 (development, tests, assembling, and instrument delivery) and 2009–2011 (operating and data processing).
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