Probing Planetary Bodies for the Structure of Subsurface Vola7les: Geant4 Models of Fast, Epithermal, and Thermal Neutron Emission of Varying Stra=graphy of Water Bearing Regoliths. Gordon Chin NASA Goddard Space Flight Center Greenbelt, Maryland, USA, ([email protected]) Roald Sagdeev, Jao Jang Su, Joseph Murray Department of Physics University of Maryland College Park, Maryland, USA Annual Mee7ng of the Lunar Explora7on Analysis Group Oct. 20-­‐22, 2015 Hydrogen/H2O Layer Monte Carlo Simula7ons can model the genera7on and transport of neutrons in planetary regolith Ini$al'velocity'and'
posi$on'of'neutron'
Length'of'free'flight'
Crossing of Material
Boundary
Length'of'free'flight'
in'new'material'
Crossing of
Material
Boundary
Collision
new'velocity,''
γ energy
Monte Carlo (MC) technique was ini=ally developed by von Neumann, Metropolis, Rosenbluth and others for the ManhaPan Project at the Los Alamos Na=onal Lab (Metropolis et al. 1953). • 
MC Codes such as MCNPX (Pelowitz, 2005) and FLUKA (Fasso et al. 2000) are available to simulate gamma ray and neutron produc=on through the spalla=on process and the leakage spectrum from planetary surfaces. • 
Geant4 is an open source toolkit developed by CERN, Fermilab, SLAC, KEK, ESA and others for the simula=on of the passage of par=cles through maPer. It is widely used for high energy, nuclear and accelerator physics, as well as medical and space science. hPp://geant4.cern.ch • 
Geant4 output is a Root object therefore has powerful built-­‐in graphics, sta=s=cal, and data analysis methods Collision
Type'of'collision'
Elastic/inelastic
scattering
• 
Fissio
Absorptio
n
n
#,'veloci$es'of'new'
terminated,'
neutrons,'fragments,'
secondary'
secondary'par$cles'
par$cles'
page4 Geant4 Models of 3 Types of Subsurface Water Structure Dry FAN 2 gm/cm3 200 cm Water Structure Single Uniform Layer 200 cm thick Water Abundance 0.1 to 10 % Wt FAN Single Layer 200 cm thick 0.01 -­‐ 10% Wet Leakage Neutron (LIS June 2009) versus Energy
Single Layer 0-200 cm
3
FAN 2 g/cm Uniform Dry
0.01% Water
0.02% Water
0.05% Water
0.1% Water
0.2% Water
0.5% Water
1% Water
2% Water
5% Water
10% Water
Leakage Flux (n cm-2 s-1)
10-1
10
-2
10
-9
10
-8
10-7
10
-6
10
-5
10-4
10
-3
E (MeV)
10-2
10-1
1
10
102
10
3
FAN Single Layer 200 cm thick .01-­‐10% Wet Ra=o of Neutron Emission Neutron Emission Ra=o from Single Layer 200 cm thick .01-­‐10% Wet rela=ve to Dry FAN Thermal Epithermal Energy (MeV) Fast Thermal, Epithermal and Fast Neutron emission ra=os from Single Layer 200 cm thick .01-­‐10% Wet rela=ve to Dry FAN Ra=o of Neutron Emission Thermal Fast Epithermal 1/(%Wet/1.91 + 1) % Wet Geant4 Models of 3 Types of Subsurface Water Structure Dry FAN 2 gm/cm3 Water Structure Single Uniform Layer Varying Depth Water Abundance 1, 2, 5 and 10 % Wt 200 cm 20, 40, 60, 80, 100, 120 cm Dry FAN over 10% Wet Leakage Neutron (LIS June 2009) versus Energy
Dry Layer over 10% Wet
3
FAN 2 g/cm Uniform Dry Layer
Uniform 10% Water
20 cm Dry Layer Over 10% Wet
40 cm Dry Layer Over 10% Wet
60 cm Dry Layer Over 10% Wet
80 cm Dry Layer Over 10% Wet
10-1
Leakage Flux (n cm-2 s-1)
100 cm Dry Layer Over 10% Wet
120 cm Dry Layer Over 10% Wet
140 cm Dry Layer Over 10% Wet
10
-2
10
-9
10
-8
10-7
10
-6
10
-5
10-4
10
-3
E (MeV)
10-2
10-1
1
10
102
10
3
Ra=o of Neutron Emission to Dry FAN Neutron Emission Ra=o from a dry layer 0-­‐140 cm thick over 10% Wet Thermal Epithermal Energy (MeV) Fast Ra=o of Neutron Emissions to Dry FAN Neutron Emission Ra=o from a dry layer 0-­‐140 cm thick over 10% Wet 10% Wet Thermal Fast 10% Wet Epithermal 10% Wet Thickness of Dry Layer (cm) Geant4 Models of 3 Types of Subsurface Water Structure Dry FAN 2 gm/cm3 Water Structure Single Uniform Layer 25 cm Varying Depth Water Abundance 1, 2, 5 and 10 % Wt 25 cm 200 cm 25, 50, 75, 100, 125, 150 cm FAN Single Layer 25 cm thick 10% Wet Leakage Neutron (LIS June 2009) versus Energy
Single Layer 10% Water
3
FAN 2 g/cm Uniform Dry
000 - 025 cm
025 - 050 cm
10-1
050 - 075 cm
075 - 100 cm
Leakage Flux (n cm-2 s-1)
100 - 125 cm
125 - 150 cm
10
-2
10
-9
10
-8
10-7
10
-6
10
-5
10-4
10
-3
E (MeV)
10-2
10-1
1
10
102
10
3
Ra=o of Neutron Emission to Dry FAN Neutron Emission Ra=o from a layer 25 cm thick At 10% Wet located 0 to 125 cm within regolith Thermal Epithermal Energy (MeV) Fast Ra=o of Neutron Emissions to Dry FAN Thermal, Epithermal and Fast Neutron emission ra=os from Single Layer 25cm thick 10% Wet rela=ve to Dry FAN Thermal 10% Wet Fast 10% Wet Epithermal 10% Wet Depth (cm) Conclusions •  Neutron energy can be binned into Thermal, Epithermal and Fast neutron ranges. •  Abundance of hydrogen/water can be inferred by the suppression of Epithermal neutrons. •  Thermal neutron emission is enhanced by the presence of hydrogen/water, and is highly sensi=ve its abundances but at abundances >2% the curve flaPens out. •  Burial depth of wet soil layer modifies the emergent neutron spectrum. –  Emergent flux compared to flux from dry soil in Thermal, Epithermal, and Fast energy bins show different behavior as a func=on of depth –  Thermal neutron flux decreases monotonically with burial depth to about 60 cm, whereas epithermal neutron flux increases monotonically as burial depth increases. –  Is there diurnal varia=on of the ver=cal structure layering (thermal ver=cal pumping) of lunar vola=les?