The major goal of the LEND investigation is to measure the spatial variations of neutron emission of the lunar surface, i.e., to map the flux of lunar neutrons. The goal determines the method of observations: the neutron counting rate is measured for elementary surface elements of the lunar map (pixels), and the difference between them characterizes the spatial variations of the lunar emission. There are different methods to identify a count within individual pixel(s) on the surface (see section 9 below), but in all cases each pixel shall accumulate some particular number of counts related with this surface area. To do this, first of all, one needs to separate the counting rate from the spacecraft background.
 There are several similar background components for mapping by omnidirectional detectors of neutrons and for mapping by collimated detector. The first one, BGDGCR, is associated with local neutrons from the body of the instrument and spacecraft, which are produced by energetic particles of GCRs. This component is the only source of neutrons in the cruise stage of flight. It is very easy to estimate in lunar orbit. Indeed, in lunar orbit this component decreases proportionally with increasing solid angle of the Moon, because the Moon shadows a part of isotropic flux of GCR:
where is the value measured in cruise (see Table 2) and ξ is a factor ∼1, which takes into account consumption of the LRO fuel after the lunar orbit insertion. LRO propulsion system uses hydrazine, which is a much stronger moderator of neutrons than the spacecraft structure and payload. We have made preliminary numerical simulation of spacecraft (LRO) with LEND onboard. The numerical simulation of the spacecraft model with variable amount of hydrazine has shown that ξ is close to 0.93.
 It is necessary to mention here that there are two types of GCR induced background. The first one is related with the producing neutrons in the body of spacecraft. The second one is the detection of GCR charged particles in the volume of proportional counters by ionization losses (see section 6.2). The value measured during cruise includes both neutrons and charged particles. These components also show similar behavior as function of altitude above Moon. The only difference is that value ξ is equal to 1 for charged particles. So, the values estimated for orbital observations according equation (3) and presented in Table 2 contain 100% of GCR neutron induced background and 90% of GCR charged particle induced background. It leads to the small systematic error that we underestimate the total GCR induced background during orbital measurements (for example, for collimated detectors CSETN1–4 it may be as high as 0.04 cps).
 The second background component, BGDbs, is due to backscattering of lunar neutrons by the mass of LRO (including the LEND mass itself) to the sensor. This component is approximately proportional to the solid angle of the visible Moon, ΩMoon/4π, but one has also to take into account the limb-darkening effect of neutrons emission [Lawrence et al., 2006]. In the ideal case of two thick target models for the Moon and LRO interacting with the GCR, one would expect that the decrease of local neutrons because of shadow by the Moon (see expression (2)) would be compensated by backscattering of lunar neutrons by the mass of LRO. In reality (because the mass of LRO is not a thick target), this compensation may not take place.
 The third type of background, BGDOther, may be associated with counts of neutrons (or photons, or charged particles; see section 6), which are not associated with the analyzed energy range. For example, sensor SETN of epithermal neutrons also detects a small number of thermal neutrons because of partial transparency of Cd enclosure around the counter. Also, epithermal neutrons are detected by STN3 sensor of thermal neutrons, and one should consider this as a background component for counts of thermal neutrons.
8.1. Background for Sensors STN1–3 of Thermal Neutrons
 Counting rates from sensors STN1–3 should represent the thermal emission for the Moon. Using the difference from the sensors STN1 and STN2 as a Doppler filter, one gets the difference of counts, Cfront – Cback, as the signal for measuring the thermal emission. One might expect that there is practically no background component for this signal, because background components BGDGCR and BGDbs for individual sensors STN1 and STN2 are about the same. These two sensors are installed symmetrically on two sides of LEND. The mass of LRO is located closer to STN1 than to STN2 but it is a secondary effect [see Litvak et al., 2012]. Direct comparison of the thermal neutron map created from the Doppler system STN1 and STN2 with the map for thermal neutrons from LPNS data shows good consistency between them [see Litvak et al., 2012].
 STN3 is the third sensor of thermal neutrons, which is installed at the top edge of the collimator. Counts of this sensor include the signal for thermal neutrons from the Moon together with the local background components BGDGCR and local neutrons due to backscattering BGDbs. One should take into account that the term backscattering is used here in the broad sense: lunar neutrons of all energies may scatter in the material of LRO, and be moderated. This scattering-and-moderation of neutrons is the diffusion process: background componentBGDbs of STN1–3 may be produced by lunar neutrons over the entire energy range from thermal particles up to MeV.
8.2. Background for the Sensor SHEN for Fast Neutrons
 SHEN background, BGDGCR, is about 0.25 cps at the orbital altitude of 50 km, and the total signal of fast neutrons with energy above 0.5 MeV is about 0.5 cps (see Table 2). The component of backscattering should be quite significant for this sensor because scattering of a neutron in the large mass of LRO may change its velocity vector, and scattered particles could penetrate SHEN from the unprotected side of the instrument; the collimator only protects sensors from direct neutrons from the Moon, but not from the structure of LRO itself. Within its FOV, SHEN is measuring fast neutrons starting from ∼1 MeV up to several MeV. The regional variations of these neutrons are mainly determined by the average variations of atomic mass in the Moon subsurface [see, e.g., Maurice et al., 2000]. The backscattering neutrons detected in the SHEN are originally emitted by the lunar surface with even higher energies, but lose some energy due to collisions with spacecraft body and collimator. They also represent variations of atomic mass in the regolith. As a result all components of the Moon's neutron signal measured by SHEN are showing the same regional variations and we may say that LRO amplify the counting statistic in SHEN but worsening its spatial resolution (because backscattering neutrons came to SHEN outside its FOV).
8.3. Background for the Sensor SETN of Epithermal Neutrons
 The average count rate of SETN is about 11 cps, which includes the GCR induced background component ∼1 cps from GCRs (Table 2). BGDbs component is also need to be taken into account but first analysis shows that it does not significantly influence on the regional variation of counting rates measured by SETN. It is confirmed by the good agreement between global map of epithermal neutrons measured by SETN and epithermal neutron map derived from LPNS (see comparison in the work of Litvak et al. ). No excess of counting rate is visible at equatorial latitudes on nearside of the Moon. It indicates an absence or small fraction of high-energy epithermal neutrons (high-energy neutrons backscattered in the collimator [seeLawrence et al., 2011; Litvak et al., 2012]) in the total counting rate.
 SETN has a Cd enclosure to absorb neutrons below the Cd threshold at 0.4 eV, but the thickness of 0.5 mm is not large enough to stop all of them. Therefore, we consider the third background component at this sensor, BGDCd, which is produced by neutrons with energies <0.4 eV. Again, the spatial variation of thermal neutrons is distinguishable from variations of epithermal neutrons. So, to determine the epithermal neutrons in the data of SETN, one has to separate the signal of epithermal neutrons with their spatial variations from two background components, BGDbs and BGDCd, which also have two different spatial variations. To resolve this deconvolution problem, one may also use the data from collimated sensors CSETN1–4, which also include the counts for epithermal neutrons and fast neutrons (see the discussion in papers by Lawrence et al. , Mitrofanov et al. , and Litvak et al. ).
8.4. Background for Collimated Neutrons CSETN1–4
 The average counting rate (corrected for efficiency and GCRs variations) in the LEND collimated detector system is about 5.1 counts per second (cps). It contains the signal counts of lunar neutrons from the collimated FOV, counts of background counts from lunar neutrons outside the FOV and background counts from neutrons produced by the spacecraft itself.
 Estimation of BGDbs is important for understanding the local background of neutron emission onboard the LRO. Especially it is important for collimated sensors where ratio between signal in the FOV and counting rate produced by neutrons outside of FOV can characterize the instrument's ability to observe local spots on the Moon surface. The Moon partially shields GCRs and as a result the spacecraft background induced by GCRs is decreasing by approaching to the Moon. On the other hand, the surface of the Moon produces neutrons, which scatter and diffuse within the body of LRO and produce the background component BGDbs. In the thick target approximation of LRO, one would assume that scattering and transport of neutrons takes place similarly in the mass of LRO, as it does in the subsurface of the Moon. In this case backscattering of lunar neutrons may compensate the loss of local neutrons produced by LRO because of partial obscuration of GCRs. In reality such an approximation does not work and it is necessary to use complicated numerical modeling together with experimental measurements to distinguish value of backscattering component.
 The LEND capability to collimate neutrons has been the subject of several critical papers. They have suggested that such a collimated neutron telescope is not able to detect any significant neutron counts within such a narrow FOV because it will be polluted with high-energy epithermal neutrons penetrating through the collimator walls [Lawrence et al., 2010, 2011; Eke et al., 2012]. These papers have suggested that the LEND collimated signal is as small as 0.05 cps, which is less than 1% from the total counting rate measured by the collimated detectors. The opposite point of view supported by LEND team claims that counting rate in the FOV is much higher and may be estimated as 1.5–1.9 cps [Mitrofanov et al., 2010b; Litvak et al., 2012; Boynton et al., 2012] and agrees with the original numerical modeling of the instrument [Mitrofanov et al., 2008].
 Below we tried to overview several alternative attempts to estimate weight of the background components in the total counting rate measured by LEND collimated detectors.
 The GCR induced background (both neutrons and charged particles) component, BGDGCR, could be easily estimated for CSETN1–4, as well as for other LEND sensors, from cruise measurements (see beginning of section 8 and Table 2). But the contribution from lunar neutrons outside the FOV requires special consideration. Indeed, the CSETN sensors have a narrow FOV, much smaller than the solid angle from the Moon. The CSETN FOV signal is produced by counts from detection of epithermal neutrons within the FOV, and it varies with change of neutron emission from the surface with a spatial resolution of about 10 km. The energy range of these neutrons is determined by the efficiency of the 3He counters and may be identified as 0.4–500 eV (see Figure 2). However, there are also counts from neutrons, which have been scattered and moderated by the mass of LRO, including LEND itself; this is background component BGDbs of CSETN1–4. These neutrons are emitted by the entire visible Moon and their variations correspond to a much larger resolution scale (as seen by omnidirectional detector), and their energy could be higher than 500 eV, because they are moderated in the mass of the spacecraft before detection. After the value of BGDGCR = 2.4 cps (Table 2) is subtracted from the total count rate of CSETN, 5.1 cps, one gets the count rate of 2.7 cps from lunar neutrons, which is produced by superposition of signal counts, CFOV, of directly propagated epithermal neutrons in the energy range 0.4–500 eV from the instrument FOV, with the background counts BGDbsof scattered-and-moderated neutrons from the entire visible Moon.
 Several approaches could be applied to estimate this component of the background. Eke et al.  have used the altitude dependence of different components to distinguish between collimated signal (does not depend on the spacecraft altitude), GCR induced background (decreases with decreasing of the spacecraft altitude) and backscattering background (Moon's neutrons detected by collimated sensors outside the FOV. This signal increases with decreasing of the spacecraft altitude). By results of the analysis, Eke et al. have concluded that collimated signal is very small (<0.05 cps) and overwhelmed by omnidirectional high-energy epithermal neutrons backscattered in the collimator and spacecraft. It means that total counting rate measured in the collimated detectors is dominated by omnidirectional component and cannot resolve local spots of hydrogen variation on the Moon surface. This conclusion looks to be flawed because it contradicts to LEND observations of PSRs and NSRs presented bySanin et al. , Mitrofanov et al. [2010b, 2012], and Boynton et al. . So, Sanin et al.  have shown that LEND collimated detectors (no assumption about backscattering component has been used, just monitoring of regional variations of total counting rate) observe significant depression of neutron flux inside PSR Shoemaker in comparison with sunlit neighboring areas. Moreover regional variations of total counting rate correlates well with local relief (measured by LOLA/LRO) of Shoemaker crater [see Sanin et al., 2012; Boynton et al., 2012]. For the additional illustration we have added Figure 14, which shows how the significance of Shoemaker PSR detection (by LEND collimated detectors) is accumulated as a function of time (during mapping phase). The significance (number of standard statistical deviations σ) was calculated as a difference between counting rate measured inside Shoemaker and counting rate measured outside Shoemaker (in the narrow latitude belt limited by the latitude size of Shoemaker crater) divided by the statistical uncertainty of such measurement (same technique as has been presented by Sanin et al. ). It is seen that collimated detectors can resolve this PSR and show monotonous increase (gradual accumulation of the counting statistic during mission) of the negative significance (counting rate inside Shoemaker area is smaller than counting rate measured outside Shoemaker area) down to ∼−5σ at the end of mapping phase.
Figure 14. The significance (number of standard statistical deviations σ) of PSRs detection inside Shoemaker provided by collimated (CSETNs) detectors and accumulated as a function of time (during mapping phase).
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 Another way to get estimates of CFOV and BGDbsfor the collimated sensors, CSETN1–4, could be based on the simple phenomenological requirement: the large-scale variations of epithermal neutrons in the energy range 0.4–500 eV, as measured by omnidirectional sensor SETN, should be the same as the large-scale variations of epithermal neutrons in the same energy range, as measured by collimated sensors, CSETN1–4 [seeMitrofanov et al., 2011; Litvak et al., 2012]. Lawrence et al. have proposed that high-energy epithermal neutrons may pass through the collimator and be detected by3He tubes. It was based on the analysis of global map of counting rate measured by collimated sensors. This map [see, e.g., Lawrence et al., 2011; Eke et al., 2012; Litvak et al., 2012] demonstrates significant depression of neutron flux at the poles together with significant excess of neutron flux at the nearside mare basins. It may be interpreted as a map of high-energy epithermal neutrons which behave both as low-energy epithermal neutrons at the poles enriched with H and as high-energy neutrons at the nearside mares enriched with Fe. Using LPNS data,Lawrence et al. suggested that detection of high-energy neutrons may add up to 2.25 cps into the total counting rate of the LEND collimated detectors. It leaves only ∼0.45 cps for the collimated signal (just compare 2.25 cps with 2.7 cps defined above as a counting rate for all lunar neutrons). LEND team [Mitrofanov et al., 2011] found this approach reasonable as a concept but flawed in details. First of all, this technique provides still small (10% from the total counting rate) collimated signal which contradicts other LEND observations (see discussion above). Second, Mitrofanov et al.  have repeated this approach using LEND fast neutron data and found that backscattering component is significantly smaller ∼1.1 cps which turns into the ∼1.6 cps for collimated signal (again compare 1.1 cps with 2.7 cps). Third, Mitrofanov et al. suggested to use the global latitude variations (Orbital Phase Profiles) of the averaged counts rate over two lunar hemispheres. The Orbital Phase Profiles can be used for comparison of regional variations seen by SETN and CSETN detectors. It may help to distinguish between collimated low-energy epithermal neutrons and uncollimated high-energy epithermal neutrons.Litvak et al. has farther developed this approach and has shown that high-energy epithermal component could not explain the whole observable picture and fast neutrons regional variations need to be involved to interpret global map of collimated counting rate. This analysis resulted in following values of collimated signal (CFOV) and the backscattering component (BGDbs): CFOV ∼ 1.7 cps (∼30% from the total counting rate) and BGDbs ∼ 1.0 cps.
 The measurements goal of the LEND collimated sensors is the experimental testing of spatial variations of lunar epithermal neutrons with an amplitude of about 5% at the distance scale as small as 10 km. The signal-to-noise ratio for these measurementsSNR(col) (5%) with four collimated sensors could be estimated, as a function of total exposure time for some particular testing surface element
For many tested spots at the lunar poles, the total exposure time is comparable to or larger than 1000 s [see Mitrofanov et al., 2010b, 2011], so expression (4) shows that LEND has SNR ∼1 for the measurements of 5% spatial variation of neutron emission at the scale of 10 km on the lunar surface.