1
MCNPX simulations of the experiments with relativistic protons
directed to thick, lead targets
Mitja Majerlea*, V. Wagnera, A. Krásaa, J. Adama,b, S.R. Hashemi-Nezhadc, M.I.
Krivopustovb, F. Křížeka, A. Kuglera, V.M. Tsoupko-Sitnikovb, I.V. Zhukd
Nuclear Physics Institute of CAS, 250 68 Řež near Prague, The Czech Republic
a
b
Joint Institute for Nuclear Research, 141980, Dubna, Russia
c
School of Physics, A28, University of Sydney, NSW 2006, Australia
d
Joint Institute of Power and Nuclear Research, Sosny, 220109 Minsk, Belarus
Do not use this line: Received date here; revised date here; accepted date here
Abstract
For testing the basic principles of accelerator driven systems, the experiments with relativistic protons directed on thick, lead
targets were performed in the JINR Dubna. Produced neutron and proton fluxes were studied at different locations of the
experimental setup using activation detectors. In this paper, experimental results obtained using the “Phasotron setup”, a bare
lead target, and the “Energy plus Transmutation setup”, composed of a lead target, surrounded with a uranium blanket, are
presented. The MCNPX code was used to study the possible sources of systematic uncertainties and to estimate the influence
of different parts of the experimental setups. Simulated results were compared with experimental findings. Observed trends of
disagreement show that MCNPX predictions at high primary particle energies are underestimated.
Keywords: spallation reactions, neutron production, neutron transport, MCNPX
PACS: 25.40.Sc, 24.10.Lx
1. Introduction
The basic principle of the Accelerator Driven
Systems (ADS) is to produce a large number of
neutrons in the spallation process (relativistic protons
+ heavy metal target), and to introduce these neutrons
into a sub-critical nuclear assembly. In an ADS, the
energy is produced by the fission of the fissionable
nuclei in the core and the excess neutrons are used to
breed fuel from e.g. 232Th, and/or to transmute longlived nuclear waste to short-lived or stable isotopes.
Experiments with simplified ADS setups were
carried out at the JINR using incident protons of
energies of 660 MeV up to 2000 MeV. Thick targets
———
*
Corresponding author. Tel.: +420 266 173 170; fax: +420 220 941 130; e-mail: majerle@ujf.cas.cz
2
of lead were investigated and activation detectors
were used to investigate the neutron fields in the
experimental setups.
The spallation process and transport of secondary
particles can be simulated by MCNPX and other
simulation codes. The results from the experiments at
the JINR are a good tool for testing the codes.
cm from the beginning of the target, and then
decrease slowly up to the L = 30 cm, beyond which
B(A) decreases in much faster rate with increasing L.
The range of protons of energy 660 MeV in lead is 30
cm and obviously beyond that no spallation neutrons
are produced (Fig. 2).
2.1. Phasotron experiment
In the Phasotron experiment [1], a cylindrical lead
target of diameter 4.8 cm and length 48 cm was
irradiated along the target axis with protons of energy
660 MeV. The setup was placed in a long and narrow
concrete corridor. The activation detectors in the
form of thin metallic foils (Au, Al, Bi) were placed
every 2 cm on top of the target (Fig. 1). During the
irradiation, secondary neutrons and protons induce
nuclear reactions in the activation foils (e.g.:
197
Au(n,2n)196Au, 27Al(n,)24Na,…) and produce
radioactive isotopes. After the irradiation, the
activities of the foils were measured using HPGe
detectors and number of the produced radioactive
isotopes was determined from their characteristic
gamma spectrum. The final production rate of an
isotope A is expressed by B(A), which is the number
of produced isotope A per in one gram of material
and per incident primary proton.
Figure 1 : The layout of the “Phasotron experimental setup”.
Longitudinal and transverse cross-sections.
The production rates for 198Au, which is produced
mainly by low-energy neutrons (E < 0.1 MeV),
indicate a constant flux of low-energy neutrons along
the whole target. The production rates for threshold
reactions reach a maximum at distance of about L = 5
1E-05
Au-198
Au-196
Au-194
Na-24
1E-06
-1
-1
2. Experiments
B [g proton ]
1E-04
1E-07
1E-08
0
10
20
30
40
50
Distance along the target [cm]
Figure 2 : Variations of the experimental B(A) values for
different isotopes in Au and Al detectors with the distance
along the target.
2.2. Experiments with the “Energy plus
Transmutation” setup
The target-blanket part of the “Energy plus
Transmutation” (EPT) setup [2] is composed of four
identical sections. Each section contains a cylindrical
lead target (diameter 8.4 cm, length 11.4 cm) and 30
natural uranium rods (diameter 3.6 cm, length 10.4
cm, weight 1.72 kg) distributed in a hexagonal lattice
around the lead target. Between the sections there are
0.8 cm gaps which are used for placement of
activation detectors. The whole assembly is mounted
on a wooden plate (thickness 6.8 cm) and placed
inside a box with ca. 20 cm thick walls, filled with
granulated polyethylene (density 0.8 kg/l). The inner
walls of the polyethylene box are dressed with 1 mm
thick cadmium layer. On the floor wall of the
polyethylene box there is a textolite plate of thickness
3.8 cm (Fig. 3).
The activation detectors (Au, Al, Bi foils) were
placed in front and behind of the target and in
between the three gaps between the blanket sections.
There was a set of foils at a distance of 3 cm from the
3
target axis, and in the 1st gap there were three more
sets of detectors at distances 6, 8.5 and 13.5 cm from
the target axis. Radial and longitudinal distributions
of production rates were studied.
Four experiments were performed at proton beam
energies of 0.7, 1, 1.5, and 2 GeV. At all four
experiments, the maximum of the production rates
was in the detectors in the 1st gap. The production
rates in the detectors in radial direction in the 1st gap
decreased with the distance from the target axis.
3. Simulations
3.1. Systematic uncertainties
Both setups were put in MCNPX 2.5.0. Neutron,
proton, (pion, and photon) fluxes at the places of the
activation detectors were simulated and convoluted
with appropriate cross-sections to obtain the
production rates. The required cross-sections were
previously simulated using the MCNPX. Satisfactory
general description of the experimental results was
obtained. Therefore, the MCNPX code was used to
study the influence of the different parts of the setup,
and systematic uncertainties of the experimental
results were estimated.
For the Phasotron experiment the simulations
showed that the concrete walls around the setup
moderate fast neutrons and reflect part of them back
into the setup place, creating a homogenous lowenergy (E < 0.1 MeV) neutron field around the target.
Concrete walls do not have influence on the highenergy (E > 1 MeV) neutron field. Other parts of the
setup (beam tubes, iron table) do not have any
significant effect on the experimental results [1].
The Energy plus Transmutation setup includes
more parts that can influence the results. The
Figure 3: Longitudinal and transverse cross-section of the
“Energy plus Transmutation setup”.
polyethylene box has the same effect on the neutron
spectrum as the concrete walls in the Phasotron
experiment: it moderates fast neutrons and reflects
moderated neutrons back into the place with the
target/blanket. The cadmium layer on the inner side
of the box absorbs the reflected neutrons with
energies less than 1 eV. Again, the box does not
influence the high-energy neutrons at the place of the
setup.
The detectors used at the experiments were in the
form of thin and small foils, and have negligible
influence on the results as was confirmed by the
simulations.
The parameters of the proton beam directed to the
target can influence the results. The shape of the
beam is not important, as long as the beam is directed
to the center of the target. However, it was calculated
that in certain detectors 2 mm inaccuracy in the beam
center coordinates can result in up to 15% inaccuracy
in the B(A) values of the activation detectors.
The activation detectors react also with protons,
pions, photons. The protons can produce up to 30%
of the radioactive isotopes, and need to be taken in
account in simulations. Pions and photons have
negligible influence.
3.2. Comparison of experimental and simulated
production rates
For the comparison of the simulation results with
those of the experiments, the production rates were
calculated with CEM2k model and LA150N +
LA150H libraries [3]. The comparison for the
Phasotron setup shows that after the 40th cm the ratio
between experimental and calculated production rates
rises, see Fig. 4.
Ratio exp/sim
8
6
Au-196
Au-194
Au-193
Au-192
Au-191
4
2
0
4
0
5
10
15
Radial distance [cm]
1,6
Figure 5 : The comparison between experimental and
simulated production rates for the 1.5 GeV experiment on the
“Energy plus Transmutation” setup. The detectors were placed
in the first gap at different radial distances.
Ratio exp/sim
1,4
1,2
Au-198
Au-196
Au-194
Na-24
1
0,8
0,6
Czech Committee for collaboration with JINR
Dubna. This work was carried out partly under
support of the GACR (grant No. 202/03/H043) and
IRP AV0Z10480505 (the Czech Republic).
0,4
0
10
20
30
40
50
References
Distance along the target [cm]
Figure 4 : The comparison between experimental and
simulated production rates along the target for the Phasotron
experiment.
The EPT experiments with lower beam energies
are well described by MCNPX. At the experiment
with 1.5 GeV protons [4], the ratios (threshold
reactions) between experimental and simulated
production rates rise with the longitudinal and radial
distance, see Fig. 5. The same behavior was observed
for the experiment with 2 GeV protons.
4. Conclusion
One of the aims of described experiments was to
provide experimental data on the aspects of ADS for
the comparison with the simulation codes. MCNPX
predicts correct results in some parts of the setups,
but at larger distances from the beam entrance it has
problems with higher energy experiments – the
disagreement between experiment and simulations is
outside the experimental and simulation uncertainties.
Future experiments will probably confirm these
results, and maybe help to improve the MCNPX
program.
Acknowledgments
The authors are grateful to the LHE of the JINR
Dubna for offering the Nuclotron accelerator for the
experiments. The experiments were supported by the
[1]
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[4]
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