1. Project Objectives

Lunar Exploration Neutron Detector (LEND) will fly on the board of NASA interplanetary mission entitled “Lunar Reconnaissance Orbiter” (LRO – 2008). The program includes launch of spacecraft LRO in late 2008. The spacecraft will fly to the Moon, and then will spend at least one year in low polar orbit around the Moon, collecting detailed information about the Lunar environment. LRO’s instrument suite will provide the highest resolution data, and the most comprehensive data set returned from the Moon.

Additional information can be found at:

http://lunar.gsfc.nasa.gov/ 

http://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter

The main objective of the LEND project is to create neutron detector of high sensitivity which is able to detect water ice (presumably of cometary origin) in ever shadowed polar regions of the moon. With the help of LEND data maps of hydrogen distribution in the shallow subsurface will be obtained with the resolution of 5 km at polar regions. The detection limit of 100 ppm of hydrogen which corresponds to 0.1% wt% of water in the regolith will be provided.

In order to fulfill the project requirements LEND has to resolve the following problems:

• To provide high spatial resolution for mapping of epithermal neutrons due to the collimation of detected neutrons, and neutron spectrometry;
• To provide the global mapping of the moon neutron albedo in the broad range of energies from thermal neutrons up to fast neutrons having energy up to 15 MeV.

2. Principle of LEND Operations

LEND detector is based on the registration of secondary neutrons from the Moon, which are born in the subsurface matter with a thickness of 1-2 meters, irradiated by cosmic rays. Neutrons of high energy are born in the regolith and then slowed down and absorbed by nucleus of the main elements in the regolith due to nuclear reactions of 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 hydrogen nucleus 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. Figure 1 shows an example of the dependence of orbital flux of epithermal neutrons upon the hydrogen content of the regolith. Here the axes Y shows counting rate by the gas filled high pressure counter orbiting the Moon, while the axes X shows hydrogen content in different types of regolith (shown by the different color).

Usage of different methods of neutron spectrometry allows us to obtain average hydrogen or water ice content along with the estimates of thickness of the ice cover due to comparison of counting rates of neutrons having different energies. Figure 2 shows how the flux of epithermal neutrons (blue curve) and the flux of fast neutrons (red curve) depend upon the thickness of water ice on the Moon surface.

Neutron spectrometers are detectors which are uniformly directed in different directions. They detect neutrons arriving from one horizon to another. In order to provide the required spatial resolution LEND uses collimation of epithermal neutrons. It allows us to get rid of neutrons arriving under high angles, therefore reducing the field of view of the detector.

Figure 1. Dependence of orbital flux of epithermal neutrons upon the hydrogen content of the regolith.
The axes Y shows counting rate by the gas filled high pressure counter orbiting the Moon, while the axes X shows hydrogen content in different types of regolith (shown by the different color).

Figure 2. Dependence of the flux of epithermal neutrons (blue curve) and the flux of fast neutrons (red curve) upon the thickness of water ice on the Moon surface.

3. LEND Design and On-board Location

LEND instrument consists of nine detectors to measure fluxes of thermal, epithermal, and fast neutrons (see Figure 3).

Four collimated detectors for epithermal neutrons CSETN1 – CSETN4 have maximum sensitivity and minimum field of view (FDV). These detectors are based on big proportional counters filled by ³He and protected by cadmium shield from the open field of view site. Collimators are made from Boron and polyethylene. They absorb epithermal neutrons arriving out of the field of view. Collimating allows mapping from the 50 km orbit with the resolution of 5 km.

Three thermal neutrons detectors STN1 – STN3 are located on the top of a collimator along with one epithermal neutrons detector SETN. These four detectors have similar proportional counters. SETN is different from the rest of 3 detectors by the cadmium cover put around the counter. Detectors STN1 and STN3 form so called “Doppler filter”. The latter compares velocities of thermal neutrons with the orbital velocity of the spacecraft (SC). Detectors STN2 and SETN measure the local velocities of thermal and epithermal neutrons around the SC.

Neutrons recording is conducted by the counters filled with helium is due to the nuclear reaction of a neutron capture.

³He + n = ³H + p + 765 keV

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. This detector has a scintillation detector based on stilbene crystal. It is surrounded by a plastic scintillator to protect it from charged cosmic rays. Fast neutron recording occurs in the organic crystal due to the protons produced by the crystal matter.

Analogous detectors were used in the Russian instrument HEND developed by IKI RAS. It successfully operates on the board of Mars Odyssey interplanetary mission.

Figure 3 shows design of the LEND instrument.

Figure 3. Design of the LEND instrument.

The LEND instrument is located on the LRO science deck. It has a simple mounting with eight pins. It will be connected with SC by a controlled thermal interface and isolated from the deck. Besides, it will be isolated from the space by MLI. The SC systems will be responsible for keeping the operational temperature in the range -20 °C – +50 °C. Electrical interface along with the data and commands interface were designed in accordance with the project requirements. LEND’s location on the board of LRO is shown by Figure 4.

Figure 4. Location of scientific instruments onboard the LRO

4. LEND’s Major Parameters

Listed in Table 1 are the major parameters of the LEND instrument.

Table 1. LEND's parameters

5. Work schedule. List of collaborators

The LEND instrument was ordered by Russian Space Agency. The leading institution is the Institute of Space Research of Russian Academy of Sciences (IKI RAS).

The project PI is Dr. Igor Mitrofanov. LEND is included in the Russian Federal Space Program for the period 2004 – 2008. During this time design, production, testing and deliver to NASA Goddard Space Flight Center will be provided. During the period 2009 – 2010 LEND space operation and data processing will occur.

Listed in Table 2 are the subcontracted Institutions involved in the LEND project and their responsibilities.

Table 2. The LEND cooperation

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