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64
Journal of Science and Technology in the Tropics (2010) 6: 59-65
by nitrogen indicates that the occluded material has
a high concentration of nitrogen. This indication
not only differentiates rubber with C4 from rubber
without C4, but also depicts the ratio of nitrogen in
each phantom in order to determine the presence of
plastic explosives [20].
Furthermore, Figure 3 and Figure 5 show that
incident neutrons with MB distribution energy will
only trigger slightly more extra γ energy spectra
compared to mono energetic neutrons. This outcome
is plausible as: (1) more types of elements in the
chemical composition and MB neutron energy
(more random energy) will give rise to more
possibilities of inelastic scattering; (2) although
the more random of the incident energy, the more
transitions incurred by inelastic scattering and
capture processes are expected, the difference
of randomized neutron energies is small. Hence,
small range of randomized neutron energy does not
impact much to the prompt γ energy spectra.
Figure 4 does not show an obvious difference of γ
energy spectra between MB and mono energetic.
Obviously, there are not many types of element in
the case of neutron-rubber scattering. The targets
of these simulations are in static form, Doppler
broadening does not affect the outcome of γ
energy spectra. Overall, the γ energy spectra
calculated by GEANT4.8.2.p01 shows good
agreement with MCNP in terms of the ratio of γ
energy spectra counts in histograms.
CONCLUSION
The present study on PGNAA computational
simulation which focuses on the comparison of
the corresponding characteristic γ rays is able to
show the differences among explosive and nonexplosive
occluded materials. Besides, we
demonstrate that γ rays generated from s-wave
neutron captured are not the only energy
spectrum to show the differences, non s-wave
neutron capture can also yield γ rays, which are
able to provide some significant energy peaks
for identifying hydrocarbon materials, e.g.
N (5.53 MeV) and N (5.56 MeV) which have
almost the same count as N (10.83 MeV) and
higher than C (4.95 MeV). Although the incident
neutron energies are randomly generated according
to Maxwell-Boltzmann distribution in mode 0.025
eV, the prompt γ spectra can still be able to show
Figure 5. Prompt γ energy spectrum of C4 hidden by Rubber generated by GEANT4.8.2.p01
compared with MCNP.
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