Large-Scale Compton Imaging for Wide-Area Surveillance Page: 4 of 16
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3 Background photon model
We assume a total photon flux from the ground, integrated over the energy range of 90 keV to 3 MeV, to
be 2.2 counts/cm2/sec. This is at the high end of the range (0.4 - 2.3 counts/cm2/sec) that is observed for
materials such as soil, concrete and gravel [1]. The photon energy spectra for these materials are crudely
similar. For our study, we use an energy spectrum, shown in Figure 1, from a soil model calculated withNi
C
0
a
0
E
c
N
E
0
z1
0.8
0.6
0.4
0.20
500
1000
1500
2000
2500
Energy (keV)Figure 1: Calculated photon energy spectrum for dirt.
GAMGEN and MCNP from assumed isotopics for generic "dirt" [2]. Note that there is no 662 keV line in
the dirt spectrum, so the background is relatively low in the signal region of our scenario.
Ideally, we would model the production of background photons from a circular region of ground with
a very large radius. This is computationally intensive in a full Monte Carlo simulation. The details of how
we handle this are given in section 5.2. We have analytically calculated the background photon flux into the
detector, to understand how the flux depends on the radius of the circular region of ground, and the detector
height. Neglecting air attenuation and assuming that each patch of ground emits into 27t, one can integrate
over a circular region beneath the detector to calculate the number of photons reaching the detector:Ny= AdetB 1
. 7 Ad
h
r1
hl+1where Ade is the area of a planar detector parallel to the ground, B is the number of gamma rays per second
per unit area emitted by the ground, h is the height of the detector above the ground, and r is the radius
of the circular patch of ground directly below the detector. In the limit as r goes to infinity, the equation
above reduces to N = AdetB. Note that a circular patch of ground with radius ten times the height (r = 10h)
contributes 90% of the total flux.2
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Lange, D J; Manini, H A & Wright, D M. Large-Scale Compton Imaging for Wide-Area Surveillance, report, March 1, 2006; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc880341/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.