Indian Journal of Pure & Applied Physics
Vol. 50, November 2012, pp. 863-866
Simulations of photoneutron spectra due to incident high energy electrons on
tungsten target using FLUKA
P K Sahani1*, G Haridas2 & P K Sarkar2
1
Indus Operation & Accelerator Physics Design Division, RRCAT, Indore 452 013, India
2
Health Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
*E-mail: [email protected]
Received 23 August 2012; accepted 28 September 2012
Bremsstrahlung photons followed by photoneutrons are the major radiation hazards in high energy electron
accelerators. These neutrons in high energy electron accelerator are produced by Bremsstrahlung photons through photo
nuclear reactions. Monte Carlo code FLUKA was used to generate the photoneutron spectra for 20 MeV, 450 MeV and
2.5 GeV electron (electron energies in Indus facility) hitting 4 mm of tungsten target. The total photoneutron yield was
calculated from the generated spectrum for different target thicknesses. The results have indicated the isotropic Giant Dipole
Resonance (GDR) neutrons dominating the spectra for the above three incident electron energies. The peak energy of the
neutron spectra lies between 0.1-1 MeV for these electron energies. The average neutron energy was calculated and found to
be 0.82 MeV, 4.09 MeV and 11.6 MeV for 20 MeV, 450 MeV and 2.5 GeV incident electron energy, respectively. The
details of the simulation and results are discussed in the paper.
Keywords: Photoneutron, Bremsstrahlung radiation, Giant dipole resonance, Radiation length, FLUKA
1 Introduction
Neutrons in high energy electron accelerator are
produced by bremsstrahlung photons through photo
nuclear reactions. The bremsstrahlung photons in
electron accelerators are generated when the high
energy electrons interact with accelerator components
or residual gas molecules in the vacuum chamber.
When these bremsstrahlung photons interact with the
target medium, they produce photo neutrons via photo
nuclear interactions with the target nuclei. The photon
interacts with nuclei through three basic processes
viz. giant dipole resonance, quasi-deuteron effect and
photo-pion decay1. Theoretical evaluation of
photoneutron yields are performed by several
researchers and reported in literature2-5. Monte Carlo
code FLUKA6 was used to generate the photoneutron
spectra for 20 MeV, 450 MeV and 2.5 GeV electron
hitting Tungsten target. These are the three electron
energies up to which electrons are accelerated at
Indus accelerator complex at Raja Ramanna Centre
for Advanced Technology (RRCAT), Indore. Both the
bremsstrahlung and photoneutron cross-section
increase with atomic number (Z) of the material.
Therefore, Tungsten target (Z=74) was used for the
present simulations. The photoneutron fluence inside
and around the target and photoneutron yield were
generated for different target thicknesses. The results
have indicated the dominance of Giant Dipole
Resonance (GDR) neutrons in photoneutron spectra
for the three incident electron energies, 20 MeV,
450 MeV and 2.5 GeV.
2 Simulation Details
Monte Carlo simulation with FLUKA for 20 MeV,
450 MeV and 2.5 GeV electron beam hitting
Tungsten target were carried out for obtaining the
neutron spectra. A pencil beam of electron is allowed
to fall normally on the Tungsten target and the
photoneutron fluence and yield are scored. The
geometry is shown in Fig. 1.
2.1 Neutron fluence and spectra for Tungsten target with
different incident electron energies
Pencil beam of 20 MeV, 450 MeV and 2.5 GeV
electron are normally incident on 4 mm (1.14 X0)
Tungsten target of 10 mm radius (X0 stands for
radiation length i.e. the thickness required to reduce
the electron energy by 1/e). Neutron fluence inside
and around the target was scored using USRBIN
scoring card. The neutron fluence (neutrons/cm2-e)
obtained for the three energies are shown in Figs 2-4.
Fig. 1 — Geometry used in the simulation
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INDIAN J PURE & APPL PHYS, VOL 50, NOVEMBER 2012
Fig. 2 — Neutron fluence around the 4 mm W target on 20 MeV
electron incidence
Fig. 4 — Neutron fluence around the 4 mm W target on 2.5 GeV
electron incidence
Fig. 3 — Neutron fluence around the 4 mm W target on
450 MeV electron incidence
The neutron fluence distribution in and around the
target for 20 MeV incident electron clearly indicates
that the neutron distribution is isotropic in nature. The
isotropic nature of the neutron1,4,7,8 is due to the
dominance of GDR neutrons. The neutrons emitted by
GDR mechanism is like the evaporation neutrons
from a compound nucleus. This suggests that the
GDR neutrons dominate the neutron radiation in
accelerator like Indus. However, deviation from
isotropic behaviour can be seen from the fluence
distribution of neutrons for the 450 MeV and 2.5 GeV
case. This is due to the anisotropic emission of
neutrons from the other two processes like quasideuteron effect and photo-pion decay1 than the GDR
neutrons.
Fig. 5 — Neutron spectra from 4 mm W target for 20 MeV,
450 MeV and 2.5 GeV incident electron energies
The photoneutron spectrum from 4 mm Tungsten
target in forward direction due to the normal
incidence of 20 MeV, 450 MeV and 2.5 GeV electron
energy was generated using FLUKA and is shown in
Fig. 5. Because of smaller photo nuclear cross-section
of photons compared to cross-section for
electromagnetic interactions with atoms and electrons,
the interaction length for nuclear inelastic interactions
of photons is reduced by a factor of 50 in Tungsten
using LAMBIAS card for all the three incident
SAHANI et al.: SIMULATIONS OF photoneutron SPECTRA USING FLUKA
865
Table 1 — Average energy of emerging neutron spectrum
from 4 mm Tungsten target
Incident Electron energy (MeV)
Average Neutron energy (MeV)
20
0.82
450
4.09
2500
11.6
Table 2 — Neutron yield from 450 MeV and 2.5 GeV electron
beam for Tungsten target
Target thickness
in mm (X0)
Neutron Yield (neutrons/GeV-Sr-electron)
450 MeV electron
2.5 GeV electron
04 (1.14)
6.565E-5 ± 0.3%
6.548E-5 ± 0.3%
10 (2.86)
1.574E-4 ± 0.3%
2.191E-4 ± 0.1%
15 (4.28)
1.886E-4 ± 0.4%
3.077E-4 ± 0.1%
20 (5.71)
2.005E-4 ± 0.2%
3.586E-4 ± 0.1%
30 (8.57)
2.068E-4 ± 0.5%
3.973E-4 ± 0.1%
40 (11.43)
2.073E-4 ± 0.5%
4.052E-4 ± 0.1%
X0 – radiation length (1X0 for Tungsten =3.5 mm)
electron energies. The yield of neutrons was scored in
0-90 degree with respect to the beam direction using
USRYIELD scoring card for 107 histories in 5 cycles
run.
For a series of discrete energies Ei and particle
yield Y(Ei) in the energy range Ei < E < (Ei+¨Ei), the
average energy8 Eavg is given in Eq. (1) as follows:
n
¦ Y ( E ) E ∆E
i
Eavg =
i
i =1
n
i
…(1)
¦ Y ( Ei )∆Ei
i =1
The calculated average energies from the above
expression are given Table 1.
It has been observed that the average energy of the
neutrons spectrum rises with increase in incident
electron energy. The bremsstrahlung radiation yield is
the fraction of incident electron energy converted to
bremsstrahlung radiation. As the energy of the
electron is increased, the fraction of energy converted
to bremsstrahlung photons increases and as a result
the spectrum of bremsstrahlung photons will be
harder (towards high energy). Due to the incident
harder bremsstrahlung spectrum, the average
photoneutron energy increases as the electron energy
is increased from 20 MeV to 2.5 GeV.
2.2 Neutron yield for Tungsten target with different target
thicknesses
The energy integrated photoneutron yield in
forward direction from the Tungsten target of varying
thickness from 4 to 40 mm was scored using
USRYIELD card. For this 450 MeV (Indus-1 electron
Fig. 6 — Neutron yield from Tungsten target (1X0 =3.5 mm) for
450 MeV and 2.5 GeV incident electron
energy) and 2.5 GeV (Indus-2 electron energy)
incident electron beam are used in simulation. Total
forward photoneutron yield in tungsten target due to
450 MeV and 2.5 GeV electron beam is listed in
Table 2. The variation in the photoneutron yield for
different target thicknesses for these two electron
energies is shown in Fig. 6.
It can be seen from Table 2 that the photoneutron
yield increases with increase in target thickness for
both the energies. At smaller target thickness, both the
yields are almost similar. As the target thickness is
increased yield due to 2.5 GeV incident electron is
found to be significantly higher than the 450 MeV
electron. Also it is observed that the yield saturates
beyond certain thickness for both the energies (6X0 for
450 MeV and 8.5X0 for 2.5 GeV) though the
saturation depths are different. This may be attributed
to the change in yield and energy of bremsstrahlung
photons with respect to the thickness of the tungsten
target.
3 Conclusions
Photo neutrons are generated using Monte Carlo
code FLUKA for 20 MeV, 450 MeV and 2.5 GeV
electron beam. The results have indicated the
isotropic emission of GDR neutrons in the emerging
spectra for 20 MeV and the variation from isotropic
emission is seen for 450 MeV and 2.5 GeV incident
energies. The peak energy of the neutron spectra lies
between 0.1-1 MeV for these electron energies. The
average neutron energy was calculated and found to
be 0.82 MeV, 4.09 MeV and 11.6 MeV for 20 MeV,
450 MeV and 2.5 GeV incident electron energy,
respectively. The neutron yield increases with
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INDIAN J PURE & APPL PHYS, VOL 50, NOVEMBER 2012
increase in target thickness and finally saturates at
larger thickness.
Acknowledgement
The authors would like to thank Dr P D Gupta,
Director, RRCAT and Shri Gurnam Singh, Head,
Indus Operation and Accelerator Physics Design
Division (IOAPDD) for their constant source of
inspiration and encouragement for this work.
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