05-25-2006-0013-High-energyneutron

8/3/2019 05-25-2006-0013-High-energyneutron

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Ref.: EURISOL DS/Task5/TN-06-14

High-energy neutron attenuation in iron and concreteVerification of FLUKA Monte Carlo codeby comparison with HIMAC experimental results

Daniela ENE

NIPNE, Bucharest, Romania

AbstractNeutron energy spectra penetrated through iron and concrete shields were calculated withFLUKA Monte Carlo code to investigate the capability of the program to be applied inthe design of the biological shield of the EURISOL facility. Simulations have beencarried out for the experimental arrangements supplied in benchmark problem HIMAC toestimate the neutron attenuation in the energy range from 20 to 800 MeV. The RQMD &DPMJET-II external event generator coupled with FLUKA was used to model theproblem since the source of the neutron in the calculation was the 400MeV/nucleon 12Cions bombarding a thick Cu target. Neutron fluence profiles and corresponded derivedattenuation lengths are also compared with the experimental data. The calculation resultsare in fairly good agreement with the measurements except some discrepancies found forthe activation detectors set-up that can be reasonably explained. It is concluded from thiscomputational work that FLUKA code is a suitable tool to be used in the accuratedetermination of the high energy radiation field surrounding the powerful drivers of theEURISOL facility and further planning and optimizing of the installation shielding

design concept.

1. Motivation

A major objective of the Task#5/subtask_A "Safety & Radioprotection" of the EURISOLDS project consists in developing and studying of methods for shielding against promptradiation and for the containment of the radioactivity suitable to EURISOL facility design[1-2]. The first step in developing this strategy is the code validation by comparison withexperimental data.There are two major problems connected with the simulation of the radiation transportproduced by high intensity spallation sources through bulk-biological shields:i) Nuclear data and the reaction models of the existing computer codes are notcompletely evaluated in their accuracy because of the lack of experimental data at highenergy range;ii) Sophisticated variance reduction techniques are needed to obtain particle fluxesand energy spectra with good statistics since the radiation flux attenuation spans overseveral orders of magnitude.From these reasons the qualification of the methods and tools by comparison withbenchmark experimental results, especially at deep penetration is strongly necessary.

The aim of this work is to investigate the applicability of the Monte Carlo code FLUKAin simulating high energy neutron deep penetration problem.The reliability of the code has been tested by means the experiment benchmark problem


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HIMAC (Heavy-Ion Medical Accelerator in Chiba, Japan) [4]. The neutron energyspectra behind thick iron and concrete shield layers of the HIMAC experimentalconfigurations were calculated with reasonable statistics in the energy range from thermal

to 800 MeV and compare with the experiment data to quantify the accuracy of the codeestimations.Deeply penetrating high energy neutrons arising from spallation process represents themain shielding problem in the operation of these sources. The secondary protons andphotons which are also generated are less important [6] as they can be more easilyattenuated through the very thick layers involved in the shields. Even though the neutronsemitted directly from the intra-nuclear cascade represents a small fraction [5] they have astrong forward anisotropy and can reach the incident proton energy. The neutrons createdin subsequent evaporation stage and emitted almost isotropically represent a minorshielding concern as they have lower energies and therefore can be attenuated in the mostshielding materials. Do to the considerable contribution of the high energy neutrons the

maximum of the radiation dose will be in the proton beam direction and therefore thedesign of the shield for this direction will be of high importance.Accurate calculations are necessary to design an optimal layout of the shielding underALARA principles reducing consequently the shielding costs that represents thesignificant part of the total cost of the facility.

2. Geometry and Simulation setup

2.1 Geometry set-upThe test problem supplied neutron energy spectra measured values produced by 400

meV/nucleon C ions at HIMAC. The stopping-length Cu target size was 10 X 10cm and5cm thick. In the experiment the measurements have been done using two high energyneutron detectors developed in the Japanese laboratory: Self-TOF detector and Ne213organic liquid scintillator at various depths through thick iron and concrete layer shieldswere neutrons were penetrated.Figure 1 (a and b) shows the experimental arrangements at the HIMAC beam line usingthe Self-TOF detector and NE213 detector. Additionally pairs of 209 Bi(n,xn) and12

C(n,2n)11

C activation detectors have been used only for neutrons penetrated throughconcrete shields, see Figure 1c. The use of the three detector experiment arrangementswas necessary due to the detector characteristics:-The Self-TOF detector was used to measure mainly the primary high-energy neutron

attenuation through the shields because this detector has low sensitivity to scatteredneutrons.-The NE213 organique liquid detector is able to measure both direct and scatteredneutrons in the energy range of tens to 800MeV.-Bi and C activation detectors were used to measure neutron fluence distribution in theshield. Gamma ray measurements from the activation detectors have been done withHPGe detectors.Both iron and concrete shield slab has a size of 100 X 100 cm being centered on the beamaxis. The thickness of the iron shield assembly was changed to: 20, 40, 60, 80 and 100cmwhile the neutron spectra penetrating through concrete shield slabs in stacks of 50cmlayer have been measured up to 200cm thickness, and respectively 250cm for Bi and C


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detectors.

Figure 1: Experimental arrangement at HIMAC of: (a) the Self-TOF detector, (b) anNE213 detector and the Bi and C activation detectors, respectively.


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During the experiments the Self-TOF detector was fixed at the same position (509 cm foriron shields and respectively 506 cm for concrete layers). The iron collimator with anaperture of 10 X 10 cm used in the Self-TOF detector arrangement has the role to shape

the neutron almost normally into detector and to avoid the signals induced by thefragment charge particlesThe NE213 detector was placed in the contact with the shield surface (point A) and faraway from the shielding surface (point B) 5m downstream of the copper target.Measuring point B was selected for comparison with the Self-TOF results.The benchmark supplies neutron energy spectra and attenuation profiles through- iron shields of 0 to 100cm thickness [5];- concrete shields of 0 to 200cm thickness [6].

2.2 Simulation set-up

Attenuation of secondary neutrons produced by 12C heavy ions on a thick (stopping-length) copper target through iron and concrete shields has been calculated withFLUKAMonte Carlo interaction and transport code [3]. Simple slab geometry modelshave been adopted in the simulations to describe the experimental arrangementsmentioned above. The RQMD & DPMJET-II external event generator (nucleus-nucleusinteraction) coupled with FLUKA. code has been used. Transport of theelectromagnetique cascade was switched-off as has been mentioned before photon andproton contributions are a minor concern for the shielding. Neutron were transporteddown to 19.6MeV the limit below FLUKA uses multigroup cross section libraryThe use of a biasing technique was essential to obtain results with reasonable statisticalsignificance. To apply the Russian roullete and splitting at boundary crossing based on

region relative importance the shielding geometry was divided in several layers (of 10 cmthickness). Region-dependent weight window in three energy ranges has been used tominimize weight fluctuations. The densities and atomic compositions of the shieldmaterials i) Iron (Density 7.87 g/cm3), and ii) Concrete (Density 2.27 g/cm3) [Type 02-a,ANL-5800, 660(1963)] were taken from the benchmark problem.The neutron spectra on the front surface of the Self-TOF detector were scored using asurface current estimator usbrdx, while for NE213 and activation detectors a tracklength estimator usrtrx has bee used.

3. Results. Discussions

3.1 Comparison of the results for iron shield.3.1.1 Self-TOF detector arrangementThe neutron energy spectra measured by the Self-TOF detector compared with the resultsof FLUKA calculations are shown in Figure 2. The Figure presents also the sourceneutron spectrum at 0 degree. Experimental results are available only for the energyrange between 100 to 600MeV, except for 100cm thickness. This can be explained by theinsufficient number of neutrons above 600 MeV and respectively by the low efficiency ofthe Self-TOF detector for neutrons below 100 MeV. As can be seen in the Figure the


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Figure 2: Comparison of measured and calculated neutron energy spectra penetratingthrough iron shields for Self-TOF detector configuration

Figure 3: Comparison of measured and calculated neutron energy spectra penetratingthrough iron shields for NE-213 detector configuration


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shapes of the spectra do not change so much with the shield thickness. The spectra have abroad peak around 200 to 300 MeV with the iron thickness up to 80 cm.The softening of the neutron spectra with increasing of the shielding thickness can not be

observed because in this experimental arrangement almost the direct high-energyneutrons penetrated through the shields in the forward direction can be detected.The softening of the spectrum occurs at 100 cm shield layer where the broad peak is notpresent and the neutron component with energy less than 100MeV can be observed.As can be seen in the Figure the calculated results are in fairly good agreement with themeasurements giving harder spectra than the measured results for all cases

3.1.2 NE213 detector arrangement

Figure 3 shows comparison between experimental and calculation results for the case ofNE213 detector, position A. In this case the neutron energy spectra are available over the

whole range (20-800) MeV. The broad peaks around 200 to 300 MeV seen in theFigure 2 have been distributed widely to the lower and higher energy regions. Thecalculations overestimate the measurements with increasing of the shield thickness in theenergy range below 100 MeV and underestimate experimental spectra in the energy rangeabove 400 MeV and are in good agreement with the experiment for (100-400) MeVenergy range. One can conclude that in the entire energy range (20-800) MeV theobtained results are in a reasonable good agreement with experiment values.

Table 1: Attenuation profiles of neutronfluences through iron shields for STOF

detector configuration

x(cm) Experiment FLUKA C/E

0 4.338 6.26 1.44

20 2.329 2.14 0.92

40 0.603 0.627 1.04

60 0.161 0.216 1.35

80 0.045 0.068 1.5

100 0.018 0.027 1.51

Table 2: Attenuation profiles of neutronfluences through iron shields for NE-213detector configuration

x(cm) Experiment FLUKA C/E

20 6.544 5.44 0.83

40 2.084 2.214 1.06

60 0.738 0.845 1.14

80 0.359 0.333 0.93

100 0.122 0.132 1.08

Figure 4: Attenuation profiles of neutronfluences through iron shields.

Table 3: Attenuation length of the

neutron fluence in iron (g cm-2)Configuration Experiment FLUKA C/E

Self-TOF 123.0 136.52 1.1

NE-213 (A) 160.5 168.88 1.05


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3.3.3 Neutron fluence length

The attenuation of the neutron fluence has been obtained by integrating the neutron flux

over the given energy range of both experimental and calculated spectra.Figure 4 and Table 1 and 2 show the comparison of calculated results versus experimentresults of the attenuation profiles of neutron fluence integrated from 100 to 600 MeV forSelf-TOF detector arrangement and respectively from 20 to 800MeV for NE213 detectorarrangement. The calculated neutron fluences are in reasonable agreement in absolutevalues with the experimental results the discrepancy increasing with the thickness of theshields up to a factor 1.5 for Self-TOF detector arrangementThe obtained neutron fluence attenuation lengths of two different energy ranges are

summarized in Table 3. Calculated attenuation lengths values have been derived by 2square method (MINUIT/PAW fitting subroutine).The comparison of attenuation lengths results between experiment and calculations is in

good agreement within 10% although the absolute fluence values show largerdiscrepancies as can been previously discussed.

3.2 Results for concrete shield

3.2.1 Self-TOF detector arrangement

For the NE213 detector arrangement the neutron energy spectra measured by the Self-TOF detector compared with the results of FLUKA calculations are shown in Figure 5.As can been discussed previously the experimental results given by this type of detectorare available only for the energy range between 100 to 600 MeV. Similar to the ironpenetrating neutron spectra the shape of the curves presented in Figure 5 has a broad peakaround 200 to 300 MeV and very slight softening with increasing of the thickness of theshield. As can be seen in the Figure with increasing of the shielding thickness theFLUKA results tend to overestimate the experimental results for energy above 300MeV.At 200 m thickness the difference between experiment and calculation reach one order ofmagnitude in the energy range of 300 t0 400 MeV.

3.2.2 NE213 detector arrangement

Figure 6 shows the comparison between experimental and calculation results for the caseof NE213 detector position A. Over a large energy range (20-300) MeV the neutronspectra show a flat shape as in case of the iron shield attenuation.

The calculations underestimate experimental spectra in the energy range above 400 MeVand overestimate the measurements in the energy range between 150 to 400MeV. A goodagreement with the experiment is given for energy range less than 100 MeV, but largeoverestimation can bee seen at 200 cm thickness. One can conclude that in the entireenergy range (20-800) MeV the obtained results are in a reasonable good agreement withexperiment values.


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Figure 5: Comparison of measured and calculated neutron energy spectra penetratingthrough concrete shields for self-TOF detector configuration.

Figure 6: Comparison of measured and calculated neutron energy spectra penetratingthrough concrete shields for NE213 detector configuration


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3.2.3 Neutron fluence length

The same procedure as for iron shield calculations has been used to obtain the neutron

fluence and corresponding attenuation lengths values.Figure 7 and Table 4 and 5 show the comparison of calculated results against experimentresults of the attenuation profiles of neutron fluence integrated from 100 to 600 MeV forSelf-TOF detector arrangement and respectively from 20 to 800MeV for NE213 detectorarrangement. The calculated neutron fluences are in reasonable agreement in absolutevalues with the experimental results having the tendency to overestimate the experimentwith increase of the shielding thickness. The big discrepancy of a factor 2.5 for Self-TOFdetector arrangement can be explained by the fact that measurements for the 200 cmthickness have been done in the different machine time than for the first 150 cm thicknessshield and by the large errors accompanying these results.The comparison of attenuation lengths results between experiment and calculations is in

good agreement within 20% see Table 6.

Table 4: Attenuation profiles of neutronfluences through concrete shields forSTOF detector configuration

X(cm) Experiment FLUKA C/E

50 2.526 2.142 0.85

100 0.655 0.712 1.09150 0.174 0.302 1.74

200 0.047 0.12 2.50

Table 5: Attenuation profiles of neutronfluences through concrete shields forNE-213 detector configuration

X(cm) Experiment FLUKA C/E

50 4.002 5.02 1.26

100 1.84 2.05 1.12

150 0.68 0.83 1.22

200 0.155 0.3 1.97

Figure 7: Attenuation profiles of neutronfluences through concrete shields.

Table 6: Attenuation length of the

neutron fluence in concrete (g cm-2)Configuration Experiment FLUKA C/E

Self-TOF 86.9 104.71 1.2NE-213 (A) 124.4 121.40 0.98


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3.2.4 C and Bi activation detector arrangement

Neutron energy spectra measured by activation detectors have been obtained by an

unfolding procedure based on a calculated spectrum behind 50 cm thick concrete shield.Due to this initial guess spectrum the shape of the experiment curves are similar with thecalculation results. As can bee seen in the Figure 7 the measured spectra overestimatewith up to 100% at the 50 cm depth in the concrete shield the FLUKA results. This largedifference decreases with shield thickness reaching a factor of 20% at 250cm thickness ofthe shield. This discrepancy can be explained by the charged particles mainly protons, butalso pions, deuterons and tritons generated from the graphite by fragmentation reactionswhich produce in detectors the same radionuclides as neutrons. Attenuation profiles forthis experimental configuration are presented Figure 8 and Table 4. The large differencebetween experiment and calculation reflects the differences found the in energydistributions.

Figure 7: Comparison of measured and calculated neutron energy spectra penetratingthrough concrete shields for Activation detectors configuration

Table 4: Attenuation profiles of neutronfluences through concrete shields for theactivation detector configuration

x(cm) Exp FLUKA C/E

50 93.908 8.216 0.087

100 23.127 2.772 0.12

150 7.794 1.09 0.14

200 2.336 0.389 0.166

250 0.63 0.138 0.218Figure 8: Attenuation profiles of neutronfluences through concrete shields for theactivation detector configuration


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05-25-2006-0013-High-energyneutron