Indian Journal of Pure & Applied Physics
Vol. 50, July 2012, pp. 465-468
Scattering of neutrons in a high energy proton accelerator enclosure and the
production of 41Ar and 14C activity using FLUKA
K Biju*, C Sunil & P K Sarkar
Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 094
*E-mail: [email protected]; [email protected]
Received 20 April 2012; accepted 10 May 2012
The neutron field inside high energy and high intensity proton accelerators can produce activation of air. The neutrons
backscattered by the concrete walls contribute largely to production of argon-41 and carbon-14 concentration. Detailed
knowledge on the neutron spectrum is necessary for the accurate estimation as the neutrons lying above thermal energy up
to around 1.5 MeV, can also produce the 41Ar considerably large amount in addition to the thermal neutrons. Hence, the
reflection of neutrons in a typical proton accelerator facility for proton energies in the range 100-500 MeV by concrete walls
and the concentration of 41Ar and 14C has been studied in detail in using FLUKA Monte Carlo code. The neutron spectra
inside the facility clearly indicate the dominance of the neutrons of energies within 1.5 MeV. The estimated neutron
albedo/backscatter factors for the secondary generated by the 100 MeV proton beam falling on the copper target have been
estimated around 11%. The build up of 41Ar and 14C concentration for a 24 hr proton beam irradiation and the decay of the
activity for the next 24 hours after the beam shut down are estimated. The Monte Carlo computed saturated 41Ar
concentration values are 2.5 times higher than that of the estimations based on analytical calculations, which assume the
activation only by thermal neutrons.
Keywords: Proton accelerator, Argon-14, Carbon-14, FLUKA, Monte Carlo method
1 Introduction
The activation of air in high energy accelerators
becomes significantly high as they are expected to
operate at very high current in the order of few mAs.
41
Ar is one of the major long lived isotopes produced
by the activation of air. The radioactive isotope, 14C,
produced by the activation of 14N is also of interest
being very long lived. The major parameters of these
isotopes are presented in Table 1. The activation is
mainly by the slow reflected neutrons by the concrete.
The considerable cross-section is observed for the
production of these radionuclides for the neutrons of
energies above thermal neutrons up to about 1.5 MeV.
The neutron activation cross-section as a function of
neutron energy for the 41Ar production1 is shown in
Fig. 1. Hence, the accurate evaluation of the
activation requires the detailed information on the
neutron energy spectrum inside the enclosure. Monte
Carlo simulations can accurately model this scenario
to arrive at the distribution of the neutron energies
inside the enclosure. Lee et al.2 reports the Monte
Carlo evaluation of the concentration of 41Ar
produced in a 100 MeV Korean proton linear
accelerator facility and the cooling time satisfying the
regulations is reported. Similarly the study of 41Ar
activation of air in a proton medical cyclotron using
FLUKA Monte Carlo code is reported by Bezshyyko
et al3.
In this study, efforts are made to analyse the
scattering and the energy spectra of neutrons inside a
typical high energy proton accelerator room after the
proton beam is stopped by a copper target using
FLUKA Monte Carlo simulations. The production of
41
Ar and 14C radioactive isotopes by the activation of
air by secondary neutrons is also quantified using
FLUKA. The results obtained using Monte Carlo
simulations have been compared with the analytical
calculations assuming the activation is only by
thermal neutrons.
2 Monte Carlo simulations
The simulations are carried out using Monte Carlo
based computer code4,5 FLUKA to analyse the
scattered neutron spectrum and quantify the 41Ar and
14
C activity.
2.1 Geometry of the simulation
Cross-sectional view of the simulated geometry is
shown in Fig. 2. A typical accelerator room of
15 m × 15 m × 15 m with 50 cm thick concrete wall is
simulated. The source is a pencil beam of protons
falling on Copper stopping target. The thickness of
INDIAN J PURE & APPL PHYS, VOL 50, JULY 2012
the target is chosen as more than the range of the
protons. The range of the protons for various energies
is calculated6 using SRIM 2008.04 and is presented in
Table 2 along with the thickness of the stopping
targets used in the simulation. A copper beam tube is
also modeled to avoid proton activation of the air.
3 Results and Discussion
3.1 Scattering of neutrons
The secondary neutron spectra produced by the
proton beam on copper target incident on the walls
Table 1 — Major parameters of the activation products
under study
Radio nuclide
41
14
Ar
C
Reaction
40
Ar(n,Ȗ)41Ar
N(n,p)14C
14
Thermal neutron
Cross-section (mb)
Half life
660
1810
110 min.
5730 Y
466
and reflected from the walls to inside volume are
estimated by scoring the neutron current using the
boundary crossing estimator. The ratio of the neutron
fluence reflected per steradian to the incident fluence
is called here as albedo/reflection factor. Fig. 3 shows
the albedo factors for mono-energetic neutrons
incident on a concrete as a function of neutron energy
computed using FLUKA. This is obtained by
simulating a pencil beam of mono-energetic neutrons
falling on concrete slab and estimating the particles
reflecting back (0-2 ʌ solid angle) from the concrete
using the boundary crossing estimator (USRBDX). At
lower neutron energies the albedo factor is found high
and at 1 MeV the value is around 0.12 and the value
falls down for higher energy neutrons. Table 3
presents the albedo factors in the actual geometry of
the proton accelerator enclosure towards the front,
Table 2 — Range of the protons and the thickness of the
stopping target used in the simulations
Proton energy
(MeV)
Calculated range
using SRIM (cm)
Thickness and radius
of cylindrical target
100
200
300
400
500
1.32
4.35
8.53
13.58
19.2
2.0
5.0
9.0
14.0
20
Table 3 — Albedo factors (The ratio of the neutron fluence
reflected per steradian to the incident fluence) in the proton
accelerator room of 15 m × 15 m × 15 m
Fig. 1 — Neutron activation cross-sections as a function of
neutron energy for the 40Ar(n,Ȗ)41Ar reaction (ENDF/B-VII.0)
Fig. 2 — Cross-sectional view of the simulated geometry
using FLUKA
Energy of protons
(MeV)
100
200
300
400
500
Front Wall
0.111
0.107
0.110
0.111
0.113
Albedo factors
Back Wall
Side Walls
0.118
0.115
0.118
0.116
0.119
0.118
0.121
0.119
0.121
0.119
Fig. 3 — Albedo factors with mono-energetic neutrons
467
BIJU et al.: SCATTERING OF NEUTRONS IN A HIGH ENERGY PROTON ACCELERATOR
Fig. 4 — Neutron energy spectrum produced by 100 MeV protons
back and side walls. It is found that the albedo factors
are almost constant for the neutron spectrum produced
by the protons of energy 100 MeV-500 MeV of about
0.11. This value is around that of equivalent to 1 MeV
mono-energetic neutrons. This is well explained by
the dominance of low energy neutrons in the neutron
energy spectrum around 1 MeV produced by the
protons and reflected from the walls as shown in
Fig. 4. Also the presence of neutrons of energy range
up to 1.5 MeV that contribute to the 41Ar production
in addition to the thermal neutrons is clearly
noticeable in the spectra.
Fig. 5 — Estimated 41Ar activity concentration in a
15 m × 15 m × 15 m proton accelerator enclosure
3.2 Induced activity
The build up of 41Ar and 14C activity for a 24 hour
proton beam irradiation and the decay of the activity
for the next 24 hours after the beam shut down in a
15 m × 15 m × 15 m accelerator room are estimated
using the RESNUCLEI option. Figs 5 and 6 show the
production and decay of 41Ar and 14C in the nonventilated proton accelerator enclosure, respectively.
In the case of 41Ar, the activity builds up during the
irradiation and saturates at around 4 hour irradiation
whereas the 14C concentrations builds up even after
24 hours of irradiation. However, the 14C activity is
found to be lower when compared to that of the 41Ar
by four orders of magnitude.
41
The
analytically
calculated
Ar
activity
7
concentration using the factors provided by the
NCRP Report No. 144 is presented in Table 4. These
factors take care only the activation produced by
thermal neutrons. From Table 4, it is seen that the
analytical calculations estimate the 41Ar by about 2.5
times less as compared to the values calculated by the
Fig. 6 — Estimated 14C activity concentration in a
15 m × 15 m × 15 m proton accelerator enclosure
Table 4 — Comparison of the evaluated 41Ar concentration
using analytical method and Monte Carlo simulation
Proton
Energy
(MeV)
Saturated 41Ar
conc. (Bq/m3)
(Analytical)
Saturated 41Ar
conc. (Bq/m3)
(FLUKA)
Ratio
(FLUKA/
Analytical)
100
200
500
1.43E+07
5.48E+07
2.96E+08
3.60E+07
1.36E+08
7.75E+08
2.53
2.48
2.61
Monte Carlo simulations. This confirms the necessity
of using Monte Carlo simulations to calculate the
activation of air in the accelerators.
4 Conclusions
The Monte Carlo simulated 41Ar activity
concentration in the high energy proton accelerators
operating with high current of the order of mAs, can
INDIAN J PURE & APPL PHYS, VOL 50, JULY 2012
be higher than the acceptable limits. The analytical
based 41Ar calculations, assuming the activation by
thermal neutrons underestimates the values around 2.5
times as compared to the values obtained using Monte
Carlo method. The amount of 14C activity is also
estimated in this study and found very much less than
41
Ar activity.
References
1 http://www.nndc.bnl.gov/exfor/servlet/E4sSearch2.
Last
visited on Dec. 15, 2011.
2 Lee C W, Lee Y O & Cho Y S, Nucl Instru & Methods in Phys
Res A, 580 (2005) 656-659.
468
3 Bezshyyko O A, Golinka-Bezshyyko L O & Kadenko I M,
Argon activation in air at medical cyclotron rds eclipse
during production of 18F, problems of atomic science and
technology, N5. Series: Nuclear Physics Investigations (52)
(2009) 37.
4 A. Fasso, A. Ferrari, J. Ranft, P.R. Sala, CERN-2005-10.
INFN/TC_05/11, SLAC-R-773 (2005).
5 Battistoni G, Muraro S, SalaP R, Cerutti F, Ferrari A, Roesler
S, Fasso A & Ranft J, in: Proceedings of the Hadronic Shower
Simulation,Workshop 2006. Fermilab, 6-8 September 2006.
AIP Conference Proceeding, 896 (2007) 31.
6 http://www.srim.org/SRIM/SRIMLEGL.htm. Last visited on
Dec. 15, 2011.
7 NCRP Report No. 144, 2003.