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MCNPX Benchmark Tests of Neutron Production in Massive Lead Target
Mitja Majerle1*, J. Adam1, P. Čaloun1, S. A. Gustov2, V. Henzl1, D. Henzlová1, V. G. Kalinnikov2, A. Krása1,
M. I. Krivopustov2, F. Křížek1, A. Kugler1, I. V. Mirokhin2, A. A. Solnyshkin2, V. M. Tsoupko-Sitnikov2, V. Wagner1
Physics Institute of the Academy of Science of the Czech Republic, Prague, The Czech Republic
Institute for Nuclear Research, Dubna, Russia
We used continuos, intensive (1013 protons/s), stable beam of
protons with the energy 660 MeV from the Dubna Phasotron.
Motivation
The protons were directed to a lead target (cylinder with r=4.8 cm
and d=4x12cm). The target was placed in a long, narrow corridor,
bounded with concrete walls.
ADS are future technology, the combination of a classical reactor with an accelerator. The basic principle is to
produce a large number of neutrons in the spallation process (relativistic ions + heavy metal target), and to
introduce them into a sub-critical reactor assembly. Extra neutrons are used to produce fuel from 232Th, and/or
to transmute long-lived nuclear waste to short-lived isotopes.
A set of monitor detectors (few mm thick Al and Cu foils) was
placed directly in the beam in front of the target. Neutron detectors
(Al, Au, and Bi foils) were placed on top of the target, along its
whole length.
At the JINR, series of experiments with different targets (lead, tungsten, bismuth cylinders, surrounded with
polyethylene or with natural uranium and polyethylene) were performed. The physical aim of the experiments
was to study nuclear processes that occur in the target, to measure the transmutation rates for higher actinides
and fission products, to measure the heat production, etc. The NAA (Neutron Activation Analysis) was mostly
used to measure the neutron field and the transmutation rates.
Schematics of the setup
After the irradiation, the activity of the detectors was measured in
HPGe detectors, and the production rates of transmuted elements
were calculated.
Experimental results (Al – left, Au – right)
The experimental results are used to test two calculation codes: DCM (Dubna Cascade Model) and MCNPX
(LAHET in combination with MCNP). This paper is focused on the calculations with MCNPX 2.4.0 of the the
Phasotron experiment.
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Au production rates
Na production rates
1E-04
B [g-1 proton-1]
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Calculation principles
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2) Joint
The PHASOTRON experiment
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Au-198
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Au-196
Au-194
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1) Nuclear
address: [email protected]
B [g proton ]
*) Electronic
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1E-08
Our calculations were done with MCNPX 2.4.0. We described our setup as a cylindrical lead target, to which a
proton beam is directed. To compare the calculations with the experimental data, we had to calculate the
production rates. We tried two different methods.
Au-193
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Au-191
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Distance along the target [cm]
Distance along the target [cm]
Convolution of the neutron spectra with cross-sections
Results with HTAPE3X method, simple target,
beam in the center
We record the neutrons that cross a plane of interest - SSW (Surface Source Write)
card.
We classify the neutrons at detector positions in energy bins - HTAPE3X.
The result is the neutron spectra on the top of the target along its lenght.
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B [g proton ]
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Au-198
Au-196
Au-194
Na-24
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The production rates in the detectors are calculated from the neutron spectrum as
follows:
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Fn (E) is the energy dependent neutron flux, s(E) is the microscopic cross-section at
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Distance along the target [cm]
energy E for a specific reaction, C=NA/M is the normalization constant. For neutrons
in energy bins we use :
Calculated neutron spectra along the target
Calculated production rates
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MCNPX can directly multiply each neutron that crosses the detector with the
microscopic cross-section for a given reaction – F4 card with FM multiplier card.
The direct method is faster, the cross-sections are taken from “la150n” library. The
results have to be multiplied with the normalization constant C=NA/M, where NA is
Avogrado’s number and M is the molecular mass of the detector material.
Bi-206 (exp)
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Bi-206 (sim)
Bi-205 (exp)
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Bi-205 (sim)
Bi-204 (exp)
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Bi-204 (sim)
Bi-203 (exp)
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Bi-203 (sim)
Ration f4/HTAPE3X
Direct calculation of production rates
B [g -1 proton -1]
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Cross-sections were taken from ENDF library, or were calculated from the
experimental data (EXFOR).
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Au-196
Na-24
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Problem : the results from two methods are not always the same !
Experimental and calculated (HTAPE3X) rates for Bi
Ratio between results from HTAPE3x and from F4 card methods.
The influence of the calculation parameters
The influence of the beam geometry
The influence of the setup parts
The accelerator beam had a Gaussian shape and was displaced. Extensive simulations on this and other setups taught us that
the beam, displaced for 3 mm results in a change of neutron field for ca. 5%.
The influence of the different intra-cascade models
The simulations were done using the intra-nuclear cascade model BERTINI INC. The calculations with two other models CEM INC and ISABEL INC - showed that the choice of the model does not influence the results. The models describe the
nuclear reactions the same in the range of few hundreds MeV.
Comparison with experimental data
We found out that we can simplify our simulations to a bare lead target inside concrete corridor, a simple simulation with
BERTINI INC is sufficient. Holders, beam tubes, etc. do not need to be taken in account. Knowing the exact position of the
beam and its geometry is important.
Calculated neutron spectra for a bare target (left)
and for a target inside the corridor (right)
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Production rates for 198Au
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1,6
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experimental
F4,walls
HTAPE3X,walls
HTAPE3X
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Ratio exp/sim
The iron components do not influence the results significantly. This is expected, iron
causes only scattering of neutrons, but our detectors lie on place where these effects are
minimal.
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B [g-1 proton-1]
We included concrete walls in our calculations. They function as a neutron moderator and
reflect a significant part of neutrons back. The refleced neutrons are homogeneously
distributed along the target lenght. These neutrons produce 198Au in a non-threshold
reaction 197Au(n,g)198Au.
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Au-196
Au-194
Na-24
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Calculated prduction rates for 198Au (left)
Comparison of calculations and experimental data (right) –
the agreement is somewhere inside 30%
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We are testing the capabilities of MCNPX, exploring which calculation methods
are the best for our setups and consequently ADS system. A lot of experimental
data from experiments with similar setups waits to be simulated. With MCNPX,
we are able to describe the systems similar to ADS with the accuracy of ca. 30%.
Calculating in parallel with MCNPX gives very good results for our setups – the
speed almost linearly rises with the number of used processors. Calculating in
parallel with PVM is now used at all our calculations.
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Conclusions
The dependancy of the speed of calcualtions on the number
of used processors for a heterogenous cluster (up) and on
2 two-processor machines (left)
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Histories simulated in 1 s
To calculate in parallel, a software package that distributes the load on other processors
has to be used. MCNPX 2.4.0 has built-in support for PVM (Parallel Virtual Machine).
PVM was installed on hosts, MCNPX was compiled with the PVM support, and the
efficiency of the parallel computing was tested.
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Four computers from our office with preinstalled systems can be temporarily changed to
computing workstations. We used Slackware Linux 10.1 on the server, the Etherboot
method to boot hosts, NFS (Network File System) for their file system, directories which
need writting permissions were mounted as ramdisks. Hard-drives of hosts were not used.
Using this method, we can easily extend the cluster to a random number of computers.
The information on how to build such a cluster was found on internet.
Histories simulated in 1 s
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Distance along the target [cm]
MCNPX in parallel
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We have two two-processor computers (processors Xeon 2 GHz) with preinstalled
Linux, intended for calculations (MCNPX, ROOT,...). In setting up the cluster, we
gained experience that helped us to set the machines intended for calculations to work
in parallel. They calculated faster than our cluster.
We tried MCNPX, compiled with different compilers (PGF90, G95, GFORTRAN, Intel
Fortran Compiler). Intel Fortran Compiler did the best job, MCNPX compiled with it
calculates 40% faster than with PGF90.
We want to test the dependency of the speed of simulations on the number of used
processors and the number of events to find an optimal number of computers in
case of building a computational farm for ADS simulations.
Acknowledgments
The authors are grateful to the staff of the Dubna Phasotron accelerator for providing a good proton beam for our
experiment.
These 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
On the field of ADS, the author most profited from the articles of K.D. Tolstov, C.D. Bowman, C. Rubbia. Data about the
experiment comes from many publications of co-authors and my work. A great guide on how to build the cluster was
found on Echelon Beowulf Cluster homepage, and in Linux how-to’s. MCNPX manual and internet forums on RSICC
were the main resources when trying to get MCNPX with PVM work and when discovering MCNPX capabilities.