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

1Nuclear Physics Institut of CAS, 250 68 Řež near Prague ,The CzechRepublic

Tel. +420-220 940 149, Fax. +420-220 941 130, E-mail:

2Joint Institute for Nuclear ResearchDubna, 141980, Dubna, MoscowRegion, Russia

3School 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

Proceedings of WP-ADS-E&T 2006

Jaipur, India, Jan.23-26, 2006

Paper No. XXX

I.INTRODUCTION

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=4.2cm, l=11.4cm) and 30natural uranium rods (r=1.8cm, l=10.4cm) 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 1mm thick cadmium layer. Images of the target-blanket, and the polyethylene box are shown in Figures 1 and2.

Figure 1One section of the target.

Using the EPT setup, several experiments with relativistic protons directed to the target have been performed. Activation analysis was mostly used to gather information on the produced neutron fields and the proton beam.

Figure 2 The polyethylene box.

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 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.THE INFLUENCE OF THE SETUP PARTS

The setup was put in MCNPX code as accurately as possible (See Figure3 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 improvedreplacing 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 < 0.1MeV) 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 3 Front cross-section of the target placed in the polyethylene box (MCNPX plot).

We used five gold foils (2×2×0.005cm3) as control detectors, foils 1 and 2 in the first gap at radial distances 3 and 10.7cm from the target central axis, foils 3 and 4 at the same radial positions but in the third gap and foil 5 in horizontal positionon the top of the second section. Two reactions were simulated in the foils: 197Au(n,)198Au, which has large resonances in lower energy (LE < 0.1MeV) part of the spectrum; and 197Au(n,2n)196Au for which the threshold is 23MeV, so it shows the changes of higher energy (HE23MeV) part of the neutron spectrum.

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 cylinderof similar dimensions as the real blanket (Figure 5).

Figure 5Front cross-sections of target approximations and of the target in detail (MCNPX plot).

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.

2.The influence of the polyethylene box andcadmium layer

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 10-6MeV are stopped in cadmium, and neutrons with energies higher than 10-6MeV cross without problems (Figure 6).The box and cadmium do not change the HE neutron field (from the energy of 1MeV) significantly – up to few percents.

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 framesand shielding material (Al envelope around U rods – 1mm thick, iron holders for lifting the target and holding U rods together – 3mm thick, hexagonal Al plate at the beginning and the end of the section – 5mm 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 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 - 0.5kg/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. 15cm.The simulated curve for the case with the box has the same behavior as experimental data.

Figure 7 The asymmetry in production rates up-down (MCNPX simulation).

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 influencedby the beam profile and its displacement. We assessedthe 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 6mm and 6cm diameters and one with the beam with Gaussian profile, where FWHM was 3cm. There were no differences outside the limits of statisticalerror (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).

2.Beam displacement

In series of calculationsthe centre of the Gaussian beam was displaced for 3, 5, 8, and 10mm. Production rates in control detectors showed a very strong dependency on the beam displacement – the beam displaced for 5mm can change production rates for 20-30% (Figure 8). With the inaccurately known beam displacement for 3mm we can count that our results have systematic error up to 10%.

Figure 8Increase of the production rates for displaced beams in reference to the centered beam (MCNPX simulation). Foil 1 is placed 3cm from the axis in the first gap and foil 5 is on top of the second section of the target.

IV.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 4mm 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 2cm 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 4or 8mm 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 thereare 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 1.5GeV 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 OF SIMULATIONS

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 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.

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.

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 determineonly 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 keffwas 19.3%, what to some point agrees with independent calculations of S.R. Hashemi-Nezhad, who calculated keffto 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.

Figure 12 Dependency of the number of produced neutrons in the whole setupper one proton and MeV on the energy of protons(MCNPX simulation).

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).