The variations of neutron component of lunar radiation background from LEND/LRO observations

• The estimation and discussion of LEND/LRO collimated count rate is presented.
• The spectral density of the lunar neutron flux is shown for the selected landing sites.
• The variations of the lunar epithermal neutron flux during 24th solar cycle are presented.

Lunar neutron flux data measured by the Lunar Exploration Neutron Detector (LEND) onboard NASA׳s Lunar Reconnaissance Orbiter (LRO) were analyzed for the period 2009–2014. We have re-evaluated the instrument’s collimation capability and re-estimated the neutron counting rate measured in the Field of View (FOV) of the LEND collimated detectors, and found it to be 1.0±0.1 counts per second. We derived the spectral density of the neutron flux for various lunar regions using our comprehensive numerical model of orbital measurements. This model takes into account the location of the LEND instrument onboard LRO to calculate the surface leakage neutron flux and its propagation to the instrument detectors. Based on this we have determined the lunar neutron flux at the surface to be ~2 neutrons/[cm2 sec] in the epithermal energy range, 0.4 eV to 1 keV. We have also found variations of the lunar neutron leakage flux with amplitude as large as a factor of two, by using multi-year observations to explore variations in the Galactic Cosmic Ray (GCR) flux during the 23rd–24th solar cycles.