Neutron Electron Water Tomography (NEWT): A Potential New Way to Search for Water at the Moon

The search for water at the surface of the Moon, in the form of ice mixed with or buried under regolith, is a major focus of lunar exploration, since availability of this vital resource in situ would be a great improvement over lifting it out of Earth’s gravity well. Subsurface water has been sought via remote sensing using “albedo” neutrons, which are secondary energetic particles that result from nuclear interactions below the lunar surface when it is struck by cosmic-ray ions, and which can escape to space and be detected from orbit. Their flux is modulated by the presence of water in the regolith through which they pass on their way to free space, and so orbital sensors can be used to infer the presence of water by mapping neutron variations across the lunar surface.However, neutron spectrometers are large and complex instruments, and calibrating their observations is likewise complicated. We explore here the possibility to sense variations in neutrons, and thus in water near the lunar surface, by measuring instead the electrons that result from beta decay of the albedo neutrons above the lunar surface. The mapping from neutron flux to electron flux adds another layer of indirectness to the detection of water, but the potentially much lesser size and complexity of an electron spectrometer compared to a neutron spectrometer may be enough of an advantage to make it worth pursuing.For this feasibility study, we calculate the flux of electrons that would be present at and above the lunar surface from representative albedo neutron distributions. We quantify variations in the electron flux that would result from variations in regolith water content and examine the degree to which regolith temperature variations or interplanetary electron background might confuse the detection of such variations. We also briefly address the possibility of using electron observations at the Moon to obtain a value for the neutron decay lifetime via a technique independent of those presently used for this measurement.