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abundance values by retrieving the default values in an
instantiated object of G4StableIsotope. These isotope
abundance values may not suit the user need. Hence,
the method G4HadronicProcess:ChooseAandZ
had been altered to suit the user defined isotope
abundance values [14].
Besides, the simulations have been separately run
with two sets of electromagnetic physics models: (1)
Standard Electromagnetic physics model; and (2)
Low Energy Electromagnetic physics models. Both
electromagnetic physics models consist of e - e + pair
production, photoelectric, γ conversion and Compton
scattering. Extra physics models have been attached
to e - and e + : (1) Multiple Scattering; (2) Low Energy
Ionization and Low Energy Bremsstrahlung; or
Standard Ionization and Standard Bremsstrahlung;
whereas Low Energy Rayleigh has been attached
to γ. However, we find that both sets of physics
model – Standard Electromagnetic and Low Energy
Electromagnetic produce the same outcomes.
Therefore, only one set of results is shown in this
paper. Furthermore, the registered processes for
proton and ions (deuteron, triton, 3 He, alpha, generic
ion) are Multiple Scattering, Low Energy Ionization
and respective hadronic routines e.g. low energy
proton/deuteron/triton/alpha inelastic and elastic
processes.
We assume the process of neutron thermalization
will eventually produce neutrons with energy
distribution described by Maxwell-Boltzman (MB)
distribution,
(1)
Instead of integrating the distribution function to
yield the randomized incident neutron energies, the
function can be treated in the form of a histogram
with 10000 or more bins. Then the histogram can
be normalized to its maximum value. The output
of this function f(E) can be cumulated as a discrete
cumulative distribution [15]. Even random numbers
between 0 and 1, which are generated via a Class
Library for High Energy Physics (CLHEP-2.0.3.1),
are distributed on the ∑f(E)-axis. The corresponding
x-axis values are the MB randomized incident
neutron energies. The MB distribution code has
been embedded into a new class inherited from
the class G4GeneralParticleSource. The highest
probability energy of the random distribution is
0.025eV within the energy range of 0.001eV < E <
0.050eV.
Virtual experimental setup
The listed isotopes in Table 1 and materials in
Table 2 were constructed in a user defined detector
construction class and registered into the GEANT4
kernel. Figure 2 shows the experimental setup, which
consisted of two spherical volumes (phantoms), i.e.
the internally occluded sphere (black, with radius
0.62 cm) and the exterior occluding hollow sphere
(grey, with radius 1.05 cm). The setup was similar
to Nunes et al. [12] and Sohrabpour et al. [16]. The
hollow sphere and the inner spheres could be either
set as the same material or set as different materials.
The total physical volume of both spheres was 1
cm 3 . Therrmalized neutrons would be directed in
Table 1. List of registered isotopes.
Registered isotopes Natural abundance (%)
1 H 99.985
2 H 0.015
12 C 98.80
14 C 1.20
16 O 99.757
17 O 0.038
18 O 0.205
14 N 99.640
15 N 0.360
Table 2. List of registered materials.
Registered Chemical Density
materials composition (g/cm3 Temperature
) (K)
C4
Rubber
C H O N 4 6 6 6
C H 5 8
1.83
0.94
273.15
273.15
Figure 2. A typical run sample (snapshot) with
prompt γ rays.
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61
Journal of Science and Technology in the Tropics (2010) 6: 59-65