Field Tests and Ground Truthing of a Surface-based Neutron Detector in the Atacama Desert, Chile

Introduction: Many photogeologic studies and very recent results from orbital geochemical mapping experiments are consistent with the presence of shallow subsurface ice deposits on Mars. The next step in studying these deposits will be to visit them on the surface, probably with mobile robotic explorers. In order for such rover missions to be successful, they must carry instruments capable of identifying and mapping the deposits detected from above. One approach to detecting subsurface hydrogen is to use a neutron detector capable of discriminating between epithermal and thermal energy neutrons. On planetary bodies with thin or nonexistant atmospheres, fast neutrons produced in the shallow subsurface by galactic cosmic ray interactions are moderated through collisions with nuclei in the regolith. The leakage spectrum of neutrons from a hydrogen-rich regolith will have a higher thermal to epithermal neutron flux ratio than an H-poor regolith. The task we wish to address is the optimization of a neutron detector for a Mars rover platform. A prototype instrument has been developed and preliminary control tests have been conducted in order to characterize the response of the detector. This prototype instrument was used in conjuction with the Zoe rover for a blind science field test as part of the Life in the Atacama (LitA) Project [1]. In this test, a science team with no prior knowledge of a remote field site commanded the rover in a Mars mission simulation. Using the suite of instruments on the rover, including the neutron detector developed in this project, the science team attempted to characterize the geology of the remote site, including the presence or absence of subsurface hydrogen (e.g., in water). This test provided an opportunity to learn how neutron data taken at the martian surface can be used in conjunction with other types of data in a mission-like scenario to provide a more complete understanding of the planet's surface and shallow subsurface. The preliminary results from this test are presented here. Data returned from the neutron detector, as well as correlations between neutron detector data, satellite imagery, and data from instruments on-board the Zoe rover are examined. In addition, results from models using Monte Carlo Neutral Particle eXtended code (MCNPX) are used to interpret detector counts returned from the field. Future, non-blind interpretations are also discussed. These tests will allow for groundtruthing of neutron detector data based upon soil moisture content analysis and more precise spectral correlations from orbital satellite imagery. Instrument Design: The prototype detector consists of a dual He proportional counter tube design. In order to operate the detector, signal processing electronics, spectrum analysis software, and a power supply are also required. On Earth, the atmosphere shields the surface from cosmic rays which would produce fast neutrons in the surface of Mars. Therefore, for testing our instrument on Earth, our design also includes a small neutron source to simulate cosmic ray production of fast neutrons. This source will be in the form of an encapsulated 0.5 microgram pellet of Cf, which emits about 10 fast neutrons per second via spontaneous fission. Methodology: Given a generic soil composition containing variable amounts of hydrogen, MCNPX was used to generate simulated detector counts. The resulting look-up table allows detector counts from the field to be compared to modeled detector counts from MCNPX. Expected count ratios (thermal over epithermal neutron counts) for various degrees of hydration are shown in Figure 1.