DOWSER: Detecting Objects' Water from Spatial Epithermal-neutron Response

THE GOAL TO BE ADDRESSED is discovery of subsurface water on the Moon and Mars, in both orbital and lander missions. THE APPROACH AND METHODOLOGY TO BE USED TO ADDRESS THE GOALS is the development of a compact, robust, low-power and low-voltage neutron detector with velocity and energy resolution. Neutron detection is a proven technique for identifying planetary subsurface water. Due to their 15-minute lifetime, neutrons are absent as background in cosmic rays or solar neutron wind. Any neutron found near a planetary body has been produced by cosmic-ray spallation of planetary matter and thereby carries signatures of its birthplace. The energy spectrum of planetary neutrons reveals the presence of hydrogen. Spallation neutrons are born at MeV energies; those found with meV energies have been thermalized by hydrogenous planetary matter. Neutrons pass easily through most materials, so subsurface water can be a source of thermal neutrons. Indeed, thermal neutron detection is the only generally applicable technique for finding water within a solid matrix. Neutron detectors (NDs) have been incorporated in planetary missions for this very purpose. Based on helium-3 proportional counters, these detectors have demanding operational overheads, such as high voltage, high gas pressure, large mass and bulky shielding. They also have limited resolution of neutron energies and directions, which are critical to locating an unknown hydrogenous deposit. We have brought to TRL 2 a novel ND that overcomes these limitations. It is a network of compact low-overhead detector cells with diverse direction and energy resolution; it replaces the high-voltage proportional-counter electronics of present NDs with modern silicon photomultipliers operating at 28-volts. Optical detection also has a higher intrinsic bandwidth than the electric-discharge detection using proportional counters. The high-pressure helium-3 of present NDs is replaced by a micron-thick boron-10 thin film deposited on a silicon chip, embedded in atmospheric-pressure xenon. We have demonstrated neutron detection with a protoype in a sealed cubical physics package of volume 5 cm^3 and mass of 10 g. All power, control and readout electronics are contained in a single control module powered by the USB bus. Power consumption in steady operation is about 30 mW. Experimental tests to date, supplemented by detailed modeling, suggest that this detector has neutron detection sensitivity and gamma rejection comparable to that of the LEND instrument, and superior energy and directional resolution. We propose to bring to TRL 4 a detector based on a cellular network of compact physics packages operated by a single control module. With appropriate moderators and cell alignment, correlation of signals across the network will sample the angular distribution of planetary leakage neutrons. OUR PROPOSED WORK IS WITHIN THE SCOPE OF THE PLANETARY SCIENCE RESEARCH PROGRAM, because orbital remote sensing and in situ measurement of neutrons have proven to be important tools for planetary geochemistry research. Determining the quantity and vertical distribution of volatile species on and below the surface of planetary bodies is vital for understanding the primordial chemical inventory and subsequent evolution of planets. Due to unavoidable practical limitations, all early lunar mission neutron instruments could only deliver integrated data on detected fluxes, averaged over large energy ranges and escape angles from lunar surface. The first orbital neutron telescope (LEND - CSETN) had a low signal/noise ratio because background ions, electrons, photons, and scattered neutrons triggered neutron detector signals. Our instrument can provide detailed directionality and energy information of planetary leakage neutrons with high detection efficiency and low background signals. Its simple cellular architecture can be adapted to both orbital and lander missions.