Remote Chemical Sensing Using Nuclear Spectroscopy 769
FIGURE 4 (a) Model of the martian surface at high latitudes; (b) The current of neutrons leaking
away from Mars for three different solid surface compositions. Neutron energy ranges are
indicated. (See text for details.)
Under the steady bombardment of cosmic rays, the pop-
ulation of neutrons slowing down in the surface is, on aver-
age, constant with time. The steady-state neutron energy-,
angle-, and spatial-distributions depend on the composition
and stratigraphy of the surface and atmosphere. An impor-
tant property of the neutron population is the scalar flux
(φ), which depends on depth and is given by the product of
the speed of the neutrons,v(cm/s), and the number density
of neutrons slowing down in the medium (nneutrons per
cm^3 ):φ=nv, with units of neutrons per cm^2 per s. The
rate at which neutrons interact with nuclei is given by the
product of the flux of neutrons, the density of the target nu-
clei (Nnuclei per cm^3 ), and the microscopic cross section:
R=φNσ(interactions per cm^3 per s).
Cosmic ray showers can be modeled using Monte Carlo
methods, in which the random processes of particle pro-
duction and transport are simulated. The number of times
something interesting happens, such as a particle crossing
a surface, is tallied. Statistical averages of these interesting
events are used to determine different aspects of the par-
ticle population such as fluxes andcurrents. Monte Carlo
transport simulations generally provide for the following: a
description of the cosmic ray source and the target medium
(including geometry, composition, and density); detailed
physical models of interaction mechanisms and transport
processes (including tabulated data for interaction cross
sections); and a system of tallies.
The general purpose code Monte Carlo N-Particle eX-
tended (MCNPX) developed by Los Alamos National Lab-
oratory provides a detailed model of cosmic ray showers,
including the intranuclear cascade and subsequent interac-
tions of particles within the surface and atmosphere. For ex-
ample, a model of the martian surface used to calculate neu-
tron leakage spectra is shown in Fig. 4a, and includes several
layers, representative of the high latitude surface, which is
seasonally covered by CO 2 ice due to condensation of atmo-
spheric CO 2 in the polar night, and whose frost-free surface
consists of a dry lag deposit covering ice rich soil. The curva-
ture of Mars was included in the MCNPX calculations along
with details of the incident galactic proton energy distribu-
tion. The goal was to determine the effect of surface param-
eters on neutron output, including thecolumn abundance
(g/cm^2 ) of the layers, their water abundance, and major el-
ement composition. The variation of the density of the at-
mosphere with altitude (the scale height is roughly 11 km)
and atmospheric mass were modeled. An accurate treat-
ment of the atmosphere is needed in order to account for
variations in neutron production with density by particles