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62
Journal of Science and Technology in the Tropics (2010) 6: 59-65
parallel to the both spheres. The source of neutrons
was confined in a plane square (2.5 × 2.5) cm 2 , and
located 5 cm (arbitrary) from the spheres. As long
as the yield γ ray of inelastic (n, n'γ) or capture
(n,γ) reaction propagated from spherical volumes to
atmosphere in 4π directions, energy of the prompt γ
ray would be cumulated for further analysis. There
was no variance reduction applied on the analysis.
During the simulation, random neutron incident
energy and energy of prompt γ ray results were kept
in ASCII and Abstract Interfaces for Data Analysis
(AIDA) 9170 format files for further analysis which
was respectively based on Grace and Java Analysis
Studio (JAS3).
Figure 2 shows a typical run of the simulation.
Both spherical volumes (inner and outer) were set as
explosive C4, and they were placed in the ambience of
atmospheric gas (70% N 2 , 30 % O 2 ), alternatively, the
ambience could be set as vacuum too. The spherical
volumes were exposed to 100 incident neutrons with
MB energy distribution. Light grey lines represent
the neutron tracks (incident neutrons or scattered
neutrons). This typical run produced two γ tracks
(grey lines) as labeled in Figure 2. The chemical
composition of C4 is given in Table 2.
RESULTS AND DISCUSSION
In this PGNAA simulation, γ energy spectra of
a typical plastic explosive (C4), rubber, and C4
occluded by rubber were calculated and compared.
Figures 3-5 show the comparison of γ energy spectra
produced by GEANT4.8.2.p01 and MCNP (analyzed
by Nunes et al.) [12]. The γ energy spectra represent
the interactions between the thermal neutrons and
the nucleus of the constituent elements of C4 and
rubber, and represent also γ interactions in the sample
material itself. The ratio of non-neutron-capture
(i.e. ionization, Bremsstrahlung, transportation,
annihilation, radioactive decay and multiplescattering)
processes, which are related to γ yield, to
neutron capture process is ~ 0.08. It may be infered
that thermal neutron capture process plays a significant
role in γ production. The identity of the constituents
of the particular material can then be adduced. There
are three configurations of virtual experimental
setup: (1) both inner and outer spherical phantoms are
placed in atmospheric environment and are irradiated
by MB distributed energy neutrons (the upper most
histogram); (2) both spherical phantoms are located
in vacuum and are irradiated by MB distributed
Figure 3. Prompt γ energy spectra of C4 generated by GEANT4.8.2.p01 compared with MCNP.
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