Speaker
Description
One of the primary goals of The Mars Science Laboratory (MSL) rover is to understand current and historical water abundance on Mars (Grotzinger et al., 2012). A key instrument on MSL for accomplishing this goal is the Dynamic Albedo of Neutrons (DAN; Mitrofanov et al., 2012). The DAN instrument is sensitive to hydrogen (H) from the surface down to roughly 60 cm depth in the regolith. DAN cannot discriminate between different host molecules for the H it detects, but for convenience, all H abundances are provided in terms of Water Equivalent Hydrogen (WEH), which is the amount of water that would exist if all of the H was bound in water.
DAN has two modes of operation: passive and active. In passive mode, spallated neutrons in the regolith are sourced by galactic cosmic rays and by the rover’s Multi Mission Radio-isotope Thermoelectric Generator (MMRTG). In active mode, a Pulsing Neutron Generator (PNG) is employed to produce roughly 107 high-energy neutrons per pulse at a frequency of 10 Hz (Sanin et al., 2015). In both modes neutrons are detected by two 3He proportional detectors, one that records neutrons with energies below ~ 1keV (Counter of Total Neutron: CTN), and another that records neutrons with energies between 0.4 keV and 1 keV (Counter of Epithermal Neutrons: CETN). After the PNG emits fast neutrons in active mode, the detector portion of DAN counts epithermal and thermal neutrons in 64 logarithmic time-dependent bins. The resultant plot of neutron counts versus time is called a ‘die-away’ curve. By analyzing the shape of a die-away curve, we determine the WEH abundance and depth distribution in the regolith directly beneath the rover (Sanin et al., 2015). It has been shown that varying chlorine (Cl) abundances are the biggest source of variability in neutron absorption for the Martian regolith (Hardgrove et al., 2011), however there are other absorbers (e.g., iron). Neutron absorbers are collectively referred to as Absorption Equivalent Chlorine (AEC), following the convention of (Tate et al., 2015).
By comparing measurements to simulations, hydrogen abundance is determined. Simulations of the DAN instrument are typically done with MCNPX/MCNP6 (McKinney et al., 2006), but this nuclear transport code subject to export control, making it difficult to run on open-architecture computing facilities. Thus, our goal is to be the first to use Geant4 (Agostinelli et al., 2003) for DAN modeling. We will use MCNPX/MCNP6 simulations, as well as DAN-active measurements to determine the viability of our Geant4 application.
We will present our ongoing work to use Geant4 as a replacement for MCNPX/MCNP6. Our Geant4 application is designed to mimic the DAN instrument and the Martian environment. We have included a PNG as the primary event, and the detectors are the CTN and CETN. The neutronHP package processes used include neutronElastic, neutronInelastic, nCapture, and nFission. With the use of a .g4mac macro simulations will be run for a variety of WEH and AEC values, and regolith geometries. Simulations are run with the University of Tennessee’s Advanced Computing Facility. We hope to implement the multithread function (G4Threading) to take full advantage of running on a computer cluster.
References
Agostinelli et al. (2003) Nuc Instruments and Methods in Phys. Res. A, 506(3), 250–303. Grotzinger et al. (2012) Space Sci. Rev., 170, 5-56. Hardgrove et al. (2011) Nucl. Instruments Methods Phys. Res. A, 659, 442-455. McSween, et al. (2010) J. Geophys. Res., 115. Mitrofanov et al. (2012) Space Sci. Rev., 170, 559-582. Sanin et al. (2015) Nucl. Instruments Methods Phys. Res. A, 789, 114-127. Tate et al. (2015) Icarus, 262, 102-123.
