Analyzing of the Die Away Curve of the Msl Dynamic Albedo of Neutrons (dan) Instrument

The Dynamic Albedo of Neutrons (DAN) is flown on board Mars Science Laboratory (MSL) to provide measurements of the dynamic albedo of thermal and epithermal neutrons induced by a pulse 14 MeV neutron generator. The DAN instrument consists of pulse neutron generator (DAN/PNG) electrically and logically combined with neutron detection system (DAN/DE). The major science objective of DAN instrument is to detect and provide quantitative estimates of the hydrogen content in the sub-surface layer of Mars. The amplitude and shape of the die-away time profile strongly depends on the content, depth and geometry distribution of water ice/bound layer. Die-away curves of thermal neutrons are simulated using a Monte Carlo transport code (MCNPX) for a homogeneous model of regolith with different contents of water. Tw different cases were considered in the initial simulations: single layered model and double layered model. In the first case, the sub-surface is modelled as a homogeneous single layer with different water contents. The preliminary results show that DAN could measure the water content as low as 0.1-0.2 weight %. The second case includes the sub-surface described by two layers. It is used to evaluate the DAN sensitivity to detect water depth (where the bottom layer is richer in water content than the upper layer).