Proceedings of WP-ADS-E&T 2006
Jaipur, India, Jan. 23-26, 2006
Paper No. XXX
Simulations on the “Energy plus Transmutation” Setup
M. MAJERLE1, J. ADAM1,2, S.R. HASHEMI-NEZHAD3, M.I. KRIVOPUSTOV2, A. KRÁSA1, F. KŘÍŽEK1, A. KUGLER1,
V.M. TSOUPKO-SITNIKOV2, V. WAGNER1
Nuclear Physics Institut of CAS, 250 68 Řež near Prague ,The Czech Republic
1
Tel. +420-220 940 149, Fax. +420-220 941 130, E-mail: majerle @ujf.cas.cz
2
Joint Institute for Nuclear Research Dubna, 141980, Dubna, Moscow Region, Russia
3
School of Physics, University of Sydney, Sydney, Australia
ABSTRACT: “Energy plus Transmutation” setup is a thick, lead target surrounded with an uranium blanket and placed
in a polyethylene shielding. Relativistic protons are directed to the target and produced neutron field is studied
experimentally. MCNPX simulations of the setup were performed to determine the influence of the setup parts on the
results, to estimate the systematic error due to not-accurately known experimental conditions, and to see to which point
we can rely on our calculations. Parameters that we cannot determine experimentally were calculated – the criticality,
and the number of produced neutrons per one incident proton.
KEYWORDS: Energy plus Transmutation, spallation, MCNPX
I.
INTRODUCTION
Activation analysis was mostly used to gather information on
the produced neutron fields and the proton beam.
The target uranium blanket of the “Energy plus
Transmutation (EPT)” setup [1] is composed of four identical
sections. Each section contains a cylindrical lead target
(r=4.2 cm, l=11.4 cm) and 30 natural uranium rods (r=1.8 cm,
l=10.4 cm) distributed in hexagonal lattice around the lead.
The target-blanket is placed inside a polyethylene box. The
inner walls of the box are dressed with 1 mm thick cadmium
layer. Images of the target-blanket, and the polyethylene box
are shown in Figures 1 and 2.
Figure 2 The polyethylene box.
Figure 1 One section of the target.
Using the EPT setup, several experiments with relativistic
protons directed to the target have been performed.
The main aim of these and similar experiments is to obtain
experimental data for testing computer calculation codes.
One of the suitable codes for this purpose is MCNPX [2].
The setup was accurately described in MCNPX 2.4.0 and
simulations were performed. One of the quantities that can be
calculated is the transmutation rate of a specific activation
material, B(A) – the number of produced nuclei of isotope A
per one incident proton and per 1g of the detector - (which
can be compared directly with experimental results) or
neutron fields. Production rates are calculated with the
convolution of the neutron spectrum with cross-sections for
the reaction (F4 and FM cards in MCNPX). Also, we used
Proceedings of WP-ADS-E&T 2006
Jaipur, India, Jan. 23-26, 2006
Paper No. XXX
SSW card, which writes all the particles that cross a specific
surface, and HTAPE3X code to count the neutrons.
It is possible to simulate our setup leaving out parts of it (what
is experimentally not possible) to get information about how
different parts of the setup influence the results.
With sets of simulations, where we slightly changed one input
parameter, we determined how these parameters influence
our setup.
Two of the parameters that can affect the obtained results are
physics models and cross-section libraries used in
calculations. We carried out calculations using different
libraires and models, and compared the results.
At the end, MCNPX was used to calculate the criticality of
our experimental setup, as well as the number of produced
neutrons per one incident proton. These two parameters are
very important when comparing our setup with similar
setups.
II.
simulated in the foils: 197Au(n,)198Au, which has large
resonances in lower energy (LE < 0.1 MeV) part of the
spectrum; and 197Au(n,2n)196Au for which the threshold is
23 MeV, so it shows the changes of higher energy (HE >
23 MeV) part of the neutron spectrum.
THE INFLUENCE OF THE SETUP PARTS
The setup was put in MCNPX code as accurately as possible
(See Figure 3 and 4). As seen from comparison with the setup
schematics (Figures 1 and 2), the description in MCNPX is
close to the reality, but still can be improved replacing some
of the assumptions with precisely measured data.
The placement and the dimensions of polyethylene box
should have no effect on higher energy neutrons. The field of
lower energy neutrons (En < 0.1 MeV) should be homogenous
inside the whole box, due to moderations and reflection of
fast neutrons in polyethylene. Generally, the placement of the
target in the box and the properties of the box should not
influence the neutron field inside the box very much. These
assumptions were all confirmed with simulations and some of
them will be discussed in the article.
Figure 4 Side cross-section of the target placed in the
polyethylene box (MCNPX plot) with control detectors.
These two reactions on the control foils were monitored,
while we were changing parameters of the setup. From the
changes of production rates, we made conclusions on the
influence of the currently changed parameter.
1. Simplifications of the setup
In earlier simulations we approximated the uranium rods with
the uranium, homogenously distributed inside a hexagonal
block and cylinder of similar dimensions as the real blanket
(Figure 5).
Figure 5 Front cross-sections of target approximations
and of the target in detail (MCNPX plot).
Figure 3 Front cross-section of the target placed in the
polyethylene box (MCNPX plot).
We used five gold foils (2×2×0.005 cm3) as control detectors,
foils 1 and 2 in the first gap at radial distances 3 and 10.7 cm
from the target central axis, foils 3 and 4 at the same radial
positions but in the third gap and foil 5 in horizontal position
on the top of the second section. Two reactions were
With sets of simulations we found out that calculations with
these simplifications give the similar results for reactions
with HE neutrons and up to 40% different values for reactions
with LE neutrons. However, including the polyethylene box
into the setup decreases the differences in the LE range to
10% while leaving HE region unchanged.
Proceedings of WP-ADS-E&T 2006
Jaipur, India, Jan. 23-26, 2006
Paper No. XXX
8
7
-1
-1
9
6
5
B(
198
1E-1
10
-5
For safety reasons, a polyethylene box was constructed inside
which we put the target-blanket assembly. Its function was to
reduce the number of high energy neutrons emitted to the
surrounding environment. As polyethylene moderates
neutrons, it returns part of them back inside the box, where
they make homogenous LE neutron field. This field is some
orders higher than the field of LE neutrons from the target
(Figure 6). Because we wanted to reduce the number of LE
neutrons, we covered inner walls of the polyethylene
container with 1 mm thick cadmium layer. Cadmium should
absorb neutrons scattered back to the target-blanket. A set of
simulations (without box, with box and without cadmium,
and with both - box and cadmium) showed that only neutrons
with energies lower than 10-6 MeV are stopped in cadmium,
and neutrons with energies higher than 10-6 MeV cross
without problems (Figure 6). The box and cadmium do not
change the HE neutron field (from the energy of 1 MeV)
significantly – up to few percents.
data [3] showed that at the bottom part of the target there are
more low energy neutrons than at the top. A simulation with
wooden plates (approximated with water - 0.5 kg/l) under the
target but without the box was done and another simulation
where also the polyethylene box was included. The detectors
were placed from the top to the bottom of the target in the first
gap. The asymmetry because of wooden plates was ca. 20%
for LE neutrons, and 5% for HE neutrons, on the other hand
when including the polyethylene box the real asymmetry is
seen (Figure 7). This asymmetry is due to the target-blanket
placement in the box: it lies on the bottom polyethylene wall
while the top wall is at distance ca. 15 cm. The simulated
curve for the case with the box has the same behavior as
experimental data.
Au) [10 g proton ]
2. The influence of the polyethylene box and cadmium
layer
1E-2
with box
only wood
4
3
2
1
Nneutrons
0
1E-3
-9
-6
-3
0
3
6
9
12
Distance on Y axis [cm]
without box
1E-4
whole setup
Figure 7 The asymmetry in production rates up-down
(MCNPX simulation).
1E-5
1E-6
1E-10
-12
without Cd
1E-7
1E-4
1E-1
1E+2
1E+5
Neutron energy [MeV]
Figure 6 Neutron spectra in the first gap 3 cm from the
central axis for the target without box; with box and no
cadmium; and with both, box and cadmium (MCNPX
simulation).
3. The influence of the setup parts (metal parts, wood)
Another parts of the setup are: metal frames and shielding
material (Al envelope around U rods – 1 mm thick, iron
holders for lifting the target and holding U rods together –
3 mm thick, hexagonal Al plate at the beginning and the end
of the section – 5 mm thick), as well as the wooden rack on
which are mounted all four sections of the target.
There were no significant differences between the results of
calculations with and without metal parts, both production
rates in all five control detectors were the same inside the
limits of the 3% statistical error. The metal parts could
eventually scatter HE neutrons, without influencing LE
neutrons, but calculations showed that we can neglect this
effect.
The wooden rack, where the target is mounted, together with
textolite, where the rack is placed, could eventually influence
LE neutrons in the same way as polyethylene. Experimental
III.
THE INFLUENCE OF EXPERIMENTAL
CONDITIONS
The parameters of the beam have the biggest influence on the
production rates. Beam profiles are usually approximated
with Gaussian distributions in X and Y directions. We can
determine experimentally the parameters of the distribution,
but with limited accuracy (ca. 3mm). With MCNPX
simulations we could estimate how production rates in our
detectors are influenced by the beam profile and its
displacement. We assessed the systematic error we can count
with because of not precisely known beam parameters.
1.
Beam profile
Three beam profiles were used and production rates in control
detectors were compared. Calculations were performed on
the setup without the box (the box adds LE neutrons and
spoils the distribution from the target), where two simulations
were with homogenous beams with 6 mm and 6 cm diameters
and one with the beam with Gaussian profile, where FWHM
was 3cm. There were no differences outside the limits of
statistical error (5%) in control detectors, showing that the
beam profile is not of great importance for our calculations,
as long as it is symmetric (in two exceptional cases, the
profile has influence on experimental results: if the detectors
are placed in the beam, and if the part of the beam misses the
target).
Proceedings of WP-ADS-E&T 2006
Jaipur, India, Jan. 23-26, 2006
Paper No. XXX
1E-1
Beam displacement
Displaced b./center b. - 1 [%]
In series of calculationsthe centre of the Gaussian beam was
displaced for 3, 5, 8, and 10 mm. Production rates in control
detectors showed a very strong dependency on the beam
displacement – the beam displaced for 5 mm can change
production rates for 20-30% (Figure 8). With the inaccurately
known beam displacement for 3 mm we can count that our
results have systematic error up to 10%.
70
1E-2
Nneutrons
2.
1E-4
1E-5
1E-6
1E-10
60
1E-7
1E-4
1E-1
1E+2
1E+5
Energy [MeV]
50
3 mm
5 mm
8 mm
10 mm
40
30
20
10
0
foil 1 (n,2n) foil 1 (n,g) foil 5 (n,2n) foil 5 (n,g)
Foil and reaction
Figure 8 Increase of the production rates for displaced
beams in reference to the centered beam (MCNPX
simulation). Foil 1 is placed 3 cm from the axis in the first
gap and foil 5 is on top of the second section of the target.
IV.
nothing
4mm foil
8 mm foil
1E-3
THE INFLUENCE OF THE DETECTORS
We need to know how we change neutron field in our
experimental setup, when we insert in it our detectors.
Detectors are of small dimensions and are not supposed to
change the experimental conditions a lot – metal holders are
much more massive and have no effect on our control
detectors. However, there are some cases, which could
change experimental conditions and which were studied:
We did simulations with 2 and 4 mm thick gold foils in the
first gap, looking if it changes production rates in our control
foils in the third gap and on top of the target. There were no
changes outside the limits of 3% statistical error.
Inside a gap, detectors could shield the neutrons. A gold strap
0.1 mm thick and 2 cm wide, stretching from top to bottom of
the target was added in front of the control foils in the first
gap. The simulation with that foil showed that it decreases
production rates for reaction 197Au(n,)198Au for up to 15%,
while the field of HE neutrons is not changed.
Plastic foils, which hold our detectors in the gap could
thermalize neutrons. This is well seen if we put in one of the
gaps 4 or 8 mm thick, plastic (polyethylene) foil, and are
looking at the neutron spectra in the same gap (Figure 9).
However, there is no effect on HE part of the spectrum.
Figure 9 The neutron spectra inside the gap with 4 and 8
mm thick polyethylene foil. The spectrum without the foil
is plotted for control (MCNPX simulation).
With simulations we checked if detectors put one after
another in sandwich do not have some influence on each other.
We could not see any such influence.
As there is not always enough space for different detectors to
measure radial distribution in the same direction, some
detectors are placed in other directions, profiting from the
symmetry of the target. Simulations showed that there are no
significant differences between the production rates in the
two directions from Figure 10, at least for HE energy
neutrons. The differences in the foils closer to the target
center arise, if the beam is displaced - for 1.5 GeV experiment
with the elliptical and displaced beam (parameters of the
beam were taken from [4]) the differences in closer foils were
up to 15%.
Figure 10 The detectors for measuring radial
distributions in the gap in two directions (MCNPX plot).
V.
PHYSICAL
UNCERTAINTIES
SIMULATIONS
OF
1. Cross-section libraries and Intra-Nuclear Cascade
model
Mostly, we use the method of direct convolution of the
calculated neutron spectrum with cross-section libraries
Proceedings of WP-ADS-E&T 2006
Jaipur, India, Jan. 23-26, 2006
Paper No. XXX
included in MCNPX (ENDF-B/VI) to get the production rates
in the detector – F4 tally card method. Another way is to
calculate the neutron field – HTAPE3X method – and
convolute it with experimental or other cross-section libraries.
The results were differing for up to 15% for a similar setup, as
described in [5].
Simulations described so far were calculated using BERTINI
Intra-Nuclear Cascade model. There are more models
included in MCNPX, and we tried if they give different
results. We found out that ISABEL INC is giving mostly the
same or very similar results as BERTINI INC – differences
inside 10%, while on the other hand, CEM INC calculations
do not agree that well with other two - mostly within 20%
(Figure 11).
For the moment, we do not have enough experimental data to
qualify the INC model which describes the best our
experiments, and these differences are setting the limits to
which we can trust our calculations.
25
15
0,04
10
5
0
-5
1
2
3
4
5
6
7
8
9
10
cem
isabel
-10
-15
-20
-25
Foil and reaction
Neutrons/proton/MeV
model/bertini-1 (in %)
20
determine only with simulation are : its criticality, and the
number of produced neutrons per one incident proton.
MCNPX offers a tool (KCODE) to calculate the criticality of
the system (keff). For our setup keff was 19.3%, what to some
point agrees with independent calculations of S.R.
Hashemi-Nezhad, who calculated keff to be 22% [3].
At the energy 1.5 GeV, per one incident proton the setup
produces 54 neutrons. If we calculate without the box, this
number is 49 neutrons.
We were also interested in the dependency of the number of
produced neutrons on the energy of the beam. Figure 12
shows the dependency of produced neutrons per one proton
and per one MeV of its energy. At ca. 1.5 GeV we reach the
optimum energy for our setup, after which the number of
produced neutrons slowly decreases with energy. If we would
like to increase the number of produced neutrons per one
proton, we can try different things. We extended the setup
with six sections, and we tried the beam of deuterons. Both
changes only slightly increased the number of produced
neutrons per one proton (few neutrons). At energies higher
than 5 GeV, the number of produced neutrons starts to
decrease rapidly.
0,035
0,03
0,025
no_box
box
0,02
0,015
0,01
0,005
0
Figure 11 The differences in production rates in control
foils when calculating with different INC models
(MCNPX simulation). Number 1 means (n,) reaction in
foil 1, number 2 - (n,2n) in the foil 1, number 3 - (n,) in
the foil 2... see Figure 4.
0
1000
2000
3000
Beam energy [MeV]
Figure 12 Dependency of the number of produced
neutrons in the whole setup per one proton and MeV on
the energy of protons (MCNPX simulation).
2. Reactions with protons
MCNPX calculates production rates by convoluting only
neutron spectra in the detector cells with cross-sections for
the specific reaction. In the experiment, the nuclear reactions
can be caused by neutrons or by protons (primary – from the
beam, and secondary – from reactions with nuclei). As the
cross-sections for reaction with protons that we want to study
are not included in MCNPX, we are limited to rough
approximations.
For one of our previous experiments [6], we tried to
convolute the proton field with experimental cross-sections
for reactions with protons. We found out that in some cases up
to 10% of radioactive nuclei can be produced by protons.
These are another percents that we have to add to the
systematic error.
VI.
SETUP PARAMETERS
Two interesting parameters of our setup that we can
CONCLUSION
The comparison with the experimental data showed that
MCNPX simulations describe our setup enough faithfully [4].
Varying the parameters of the setup, we were able to study the
systematic error of our experimental data, and to determine
the limit to which we can trust our simulations.
The systematic error is mostly dependent on the beam
displacement – with the accuracy we know the displacement
(3 mm), our experimental results are in 10% limits of
systematic error.
The detectors we use nowadays have negligible influence on
our results. When there is not enough space for the detectors
for measuring HE neutrons, we can profit from the symmetry
of the setup and place them in other radial directions (LE
neutron field is not symetric because of the placement of the
box).
Calculated results strongly depend on the choice of
cross-section libraries and Intra-Nuclear Cascade model.
Proceedings of WP-ADS-E&T 2006
Jaipur, India, Jan. 23-26, 2006
Paper No. XXX
Trying different combinations of these parameters, we get
results which agree inside 50% - this is the limit of the
accuracy of our calculations.
The setup criticality was calculated to be ca. 20% and at the
energy 1.5 GeV it produces ca. 50 neutrons per one incident
proton.
These simulations helped us to inquire in discrepancies
between experimental and calculated production rates – these
discrepancies are partly caused by systematic error and partly
by the code deficiencies. In next experiments we will try to
lower important inaccuracies and with more experimental
data we will be able to qualify which models and libraries are
the most reliable when simulating ADS systems.
ACKNOWLEDGMENTS
The authors are grateful to the staff of the Dubna Nuclotron
accelerator for providing good proton beams for our
experiments.
The experiments were supported by the Czech Committee for
collaboration with JINR Dubna. This work was carried out
partly under support of the Grant Agency of the Czech
Republic (grant No. 202/03/H043) and ASCR K1048102 (the
Czech Republic).
REFERENCES
1.
2.
3.
4.
5.
6.
M.I. KRIVOPUSTOV et al., “Investigation of Neutron
Spectra and Transmutation of 129I, 237Np and Other
Nuclides with 1.5 GeV Protons from the Dubna
Nuclotron Using the Electronuclear Setup "Energy plus
Transmutation", Preprint JINR Dubna, El-2004-79
http://mcnpx.lanl.gov/
I. ZHUK, S.R. HASHEMI-NEZHAD, personal
discussion
V. WAGNER et al., Proceedings of WP-ADS-E&T 2006,
Jaipur, India, Jan. 23-25, 2006 (this proceedings)
M. MAJERLE et al., “MCNPX Benchmark Tests of
Neutron Production in Massive Lead Target”,
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12-15, 2005
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