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Model-dependent deconvolution of HEND neutron data by MCNPX code

Model-dependent deconvolution of HEND neutron data by MCNPX code. M. Litvak, I. Mitrofanov, Charyshnikov, V. Grinkov, A. Kozyrev, A. Sanin V. Tret’yakov Institute for Space Research D. Drake Santa Fe, USA. MODEL. Atmosphere. CO 2 deposit. Dry regolith. Wet regolith. MODEL PARAMETERS.

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Model-dependent deconvolution of HEND neutron data by MCNPX code

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  1. Model-dependent deconvolution of HEND neutron data by MCNPX code M. Litvak, I. Mitrofanov, Charyshnikov, V. Grinkov, A. Kozyrev, A. Sanin V. Tret’yakov Institute for Space Research D. Drake Santa Fe, USA

  2. MODEL Atmosphere CO2 deposit Dry regolith Wet regolith

  3. MODEL PARAMETERS • Flux of CGR • Atmosphere thickness at different times • Chemical composition of atmosphere • Thickness of CO2 frost • Presence of water (%) in CO2 frost • Thickness of dry regolith • Chemical composition of regolith • Presence of water (%) in dry regolith • Presence of water (%) in wet regolith

  4. MODEL PARAMETERS Martian Soil “Chemical analyses of Martian soil and rocks obtained by the Pathfinder Alpha Proton X-ray spectrometer” Radiation Physics and Chemistry 61 (2001)191 –197

  5. MODEL PARAMETERS Martian Atmosphere: Chemical composition and thickness Major gases: Carbon Dioxide (CO2) - 95.32% ; Nitrogen (N2) - 2.7% Argon (Ar) - 1.6%; Oxygen (O2) - 0.13%; Carbon Monoxide (CO) - 0.08% http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html Thickness of atmosphere for given Mars region and season is taken from the Ames global climate model

  6. HOW TO CALCULATE: GEOMETRY The layer produced the neutrons

  7. HOW TO CALCULATE: DATA SELECTION • The Surface of Mars may be divided into the set of regions of interest. • The properties of surface should be homogenous inside selected region • The size of each region should be more than footprint.

  8. HOW TO CALCULATE: ALGORITHM 1. Calculation of neutron flux for model of homogeneous sphere with some initial parameters 2. Convolution of model spectra with efficiency functions for different detectors to get model counts 3. Comparison of model count rate and real count rate of HEND sensors for this Martian area of interest and for the same seasons 4. Fitting of parameters of geometry, water percentageinmodel of homogeneous sphere hCO2 %H20dry hdry %H2Owet

  9. HOW TO CALCULATE: MINIMIZATION Search of model best fit parameters based on 2 minimization: • Where: • Ci is detector counts, i=1,5 (SD,MD,LD,SC1,SC2). • Mi is model counts predicted by MCNPX for (SD,MD,LD,SC1,SC2). • i is statistical error for Ci, Mi • SD, MD, LD are integral counts detected in proportional counters • SC1 is fast neutron counts in energy range (0.8-2.5 MeV) registered in stilben • SC2 is fast neutron counts in energy range >2.5 MeV registered in stilben

  10. BEST FIT PARAMETERS: CGR flux • The selection of HEND data accumulated above large region near Solis Planum. It is driest place on Mars where variation of neutron flux explained only by variations of CGR flux or by Solar activity. • The selection of different time intervals: Ls={ [330o-360o] , [0o -30o ] ,[30o -60o] , [60o-90o], [90o-120o] , [120o-150o] • Applying of the homogenous model to estimate flux of CGR: {Atmosphere +homogenous regolith layer with 2% of water}

  11. BEST FIT PARAMETERS: FROST FREE AREAS • The selection of data for summer surface of Mars (season when CO2 frost is absent) • First step:Applying of the homogeneous regolith model with one free parameter describing content of water in the regolith. • Second step: If homogeneous model was rejected two layers regolith model should be tested with 2% of water in upper layer and two free parameters: thickness of upper layer and water content in bottom layer.

  12. The best fitting parameters describing summer surface are fixed for given region. The data for given region are taken for different winter periods of time. The real data are fitted by model of summer surface with added layer of CO2 frost with variable thickness. BEST FIT PARAMETERS: CO2 FROST THICKNESS

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