The long (3ms) proton pulse of the European Spallation Source (ESS) gives
rise to unique and potentially high backgrounds for the instrument suite. In
such a source an instrument capabilities will be limited by it's Signal to
Noise (S/N) ratio. The instruments with a direct view of the moderator, which
do not use a bender to help mitigate the fast neutron background, are the most
challenging. For these beam lines we propose the innovative shielding of
placing blocks of material directly into the guide system, which allow a
minimum attenuation of the cold and thermal fluxes relative to the background
suppression. This shielding configuration has been worked into a beam line
model using Geant4. We study particularly the advantages of single crystal
sapphire and silicon blocks .Comment: 12 pages, 8 figures, proceeding of NDS 2015, 4th International
  Workshop on Neutron Delivery Systems, 28 -30 September 2015, ILL Grenoble,
  Franc

Ansell, S.

Bentley, P. M.

Cai, X. X.

DiJulio, D. D.

Santoro, V.

Journal of Neutron Research

English

arXiv

The long (3 ms) proton pulse of the European Spallation Source (ESS) gives rise to unique and potentially high backgrounds for the instrument suite. In such a source an instrument's capabilities will be limited by its Signal to Noise (S/N) ratio. The instruments with a direct view of the moderator, which do not use a bender to help mitigate the fast neutron background, are the most challenging. For these beam lines we propose the innovative shielding of placing blocks of material directly into the guide system, which allow a minimum attenuation of the cold and thermal fluxes relative to the background suppression. This shielding configuration has been worked into a beam line model using Geant4. We study particularly the advantages of single crystal sapphire and silicon blocks

Cai, Xiao Xiao

Online Research Database In Technology

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M.Published in:Journal of Neutron ResearchLink to article, DOI:10.3233/JNR-160034Publication date:2015Document VersionPeer reviewed versionLink back to DTU OrbitCitation (APA):Santoro, V., Cai, X. X., DiJulio, D. D., Ansell, S., & Bentley, P. M. (2015). In-beam background suppressionshield. Journal of Neutron Research, 18(4), 135-144. DOI: 10.3233/JNR-160034December 2015In-Beam Background Suppression ShieldV. Santoro, a X. X. Cai, a,b D. D. DiJulio, a,c S. Ansell a and P. M. Bentley a,daEuropean Spallation Source ERIC, SE-221 00 Lund, SwedenbDTU Nutech, Technical University of Denmark, Roskilde, DenmarkcDepartment of Physics, Lund University, SE-221 00 Lund, SwedendDepartment of Physics and Astronomy, Uppsala University, 751 05 Uppsala, SwedenAbstract. The long (3ms) proton pulse of the European Spallation Source (ESS)gives rise to unique and potentially high backgrounds for the instrument suite. Insuch a source an instrument capabilities will be limited by it’s Signal to Noise (S/N)ratio. The instruments with a direct view of the moderator, which do not use abender to help mitigate the fast neutron background, are the most challenging. Forthese beam lines we propose the innovative shielding of placing blocks of materialdirectly into the guide system, which allow a minimum attenuation of the cold andthermal fluxes relative to the background suppression. This shielding configurationhas been worked into a beam line model using Geant4. We study particularly theadvantages of single crystal sapphire and silicon blocks .Keywords. background, sapphire, filter, neutrons, spallation source1. IntroductionThe European Spallation Source ESS [1] is being constructed in Lund, Sweden, and isplanned to be the world brightest pulsed neutron source. The facility uses a 2 GeV protonbeam hitting a tungsten target to produce neutrons (predominately evaporation neutronsat about 2 MeV and spallation product that are considered background). The neutronsare then moderated in moderators consisting of both liquid hydrogen and water com-partments. Surrounding the moderators are multiple beamports that use neutron guidesto transport cold ( < 20 meV [millielectron volt]) and thermal neutrons to the sampleposition of an instrument via reflection, over a distance of several tens of meters for ashort instrument and a hundreds meters for a long instrument. As well as the desiredneutron flux, many higher energy neutrons (> eV, up to 2 GeV) and other particles alsoscatter down the beamlines, via scattering with the guides, shielding, choppers and otherthermal neutron optical components. This gives rise to potentially high backgrounds forthe instruments, limiting their capabilities by their Signal to Noise (S/N) ratio. One ofthe methods of reducing the fast particle background is a “T0 chopper”, which is a largerotating mass that passes through and obscures the neutron beam during the acceleratorproton pulse. T0 choppers are expensive, and mechanical points of failure that requiremaintenance, so we are exploring steady-state alternatives.One possibility is to place blocks of material directly into the guide system, whichallows for a minimum attenuation of the cold and thermal fluxes relative to the back-ground suppression. These devices are called filters since they have a wavelength depen-arXiv:1512.04251v1  [physics.ins-det]  14 Dec 2015December 2015dent cross-section in such a way that is low for thermal but strongly increases at epither-mal and high energies. The choice of material and its dimension are critical parametersaffecting the performance of the filter. In this work, we studied sapphire (Al2O3) andSilicon (Si), two materials that are usually employed at neutron facilities since they arecharacterised by having a total cross section capable of attenuating the unwanted epither-mal neutrons and gamma radiation in the incident spectrum [2,3]. In this study, thesematerials have been tested using the ESS spectrum at the beam port entrance, which isroughly located 2 m after the moderator. The target and the moderator design used to cal-culate this spectrum corresponds to the ESS TDR design [1,4]. The spectrum is shownin Fig. 1. As can be seen by the different energy bands, the spectrum has a large energyrange that goes from cold and thermal (blue band) to high energy neutrons (yellow band).The red band (from 100 keV to 100 MeV ) corresponds to the fast neutron componentwhile the green bands corresponds to neutrons from 1 eV to 100 keV, which are calledepithermal neutrons and this is the region where nuclear resonances occur in nuclei.For the purpose of our study we classify as “background” all the neutrons with en-ergy greater than 1 eV and as “Signal” all the neutrons with energy less than 1 eV, sincethe instruments at ESS will not use any neutrons with energy > 0.5 eV. In this work wesimulated with Geant4 [5] different thicknesses and different geometrical configurationsfor Al2O3and Silicon filter in order to optimise the S/N ratio.Figure 1. ESS TDR [1] moderator spectrum. The blue band corresponds to neutrons of energy below 1 eV,the green one to neutrons from 1 eV to 100 keV, the red one to 100 keV to 10 MeV, while the yellow band toneutrons with energy greater than 10 MeV.December 20152. Geant4 and NCrystal packageGeant4 is a Monte-Carlo toolkit for the simulation of the passage of particles throughmatter. It is one of the codes used at ESS for shielding [6,7,8,9,10] and detector simula-tions. The toolkit is developed by a worldwide collaboration of physicists, implementedin C++ and has an open source license. The code is used across a number of different sci-entific fields, including high-energy physics, accelerator physics and medical and spacescience. The physics processes offered cover a comprehensive range, from meV and ex-tending to TeV energies, and include electromagnetic, hadronic and optical processeswith a large set of long-lived particles, materials and elements.The composition of a material is flexible since it is possible to specify the atomicand isotopic content of a given volume. However, in Geant4 the effects on the interactioncross-section for thermal neutrons due to inter-atomic bindings is only present for com-monly used materials in nuclear engineering. For this reason, at ESS a package calledNCrystal has been developed in order to simulate slow neutron transport in polycrystalsand imperfect single crystals. The crystal structure can be supplied by the user or bythe nxs library [11,12,13]. The cross section is then calculated and applied for coherentand incoherent neutron scattering. This package is a key element of this work since thewanted filter properties (attenuation of epithermal or higher energy neutrons with almostno effect on thermal and cold neutrons) are described by this tool included in the ESSdetector framework [14]. In this current work, NCrystal is combined with the NeutronHP models for neutrons with an energy range from 4 eV to 20 MeV, which are basedon evaluated neutron data libraries, and also with the Bertini [15,16] model for neutronswith energies greater than 20MeV.Figure 2. Schematic view of the simulation setup.December 2015(a) Neutron spectrum before andafter the filter for 1 block of 2 cm.(b) Neutron spectrum before andafter the filter for 1 block of 5 cm.(c) Neutron spectrum before andafter the filter for 1 block of 10 cm.(d) Neutron spectrum before andafter the filter for 2 blocks of 10cm.(e) Neutron spectrum before andafter the filter for 3 blocks of 10cm.(f) Neutron spectrum before andafter the filter for 3 blocks of 15cm.Figure 3. Neutrons energy spectra before and after the silicon filters for different geometries.December 2015(a) Neutron spectrum before andafter the filter for 1 block of 2 cm.(b) Neutron spectrum before andafter the filter for 1 block of 5 cm.(c) Neutron spectrum before andafter the filter for 1 block of 10 cm.(d) Neutron spectrum before andafter the filter for 2 blocks of 10cm.(e) Neutron spectrum before andafter the filter for 3 blocks of 10cm.(f) Neutron spectrum before andafter the filter for 3 blocks of 15cm.Figure 4. Neutrons energy spectra before and after the sapphire filters for different geometries.December 2015Table 1. Attenuation Coefficients and Signal to Noise ratios for different configurations of Silicon filters.Configuration Attenuation Coefficient Attenuation Coefficient S/N ratiofor High Energy Neutrons for Low Energy NeutronsNo Si - - 0.32 cm 2.5 1.7 0.445 cm 2.5 1.7 0.4410 cm 3.9 1.9 0.622 × 10 cm 9 2.4 1.23 × 10 cm 18. 3 1.93 × 15 cm 40 3.9 3.13 × 18 cm 54 4.6 3.63 × 24 cm 82 6 43 × 27 cm 99 7.3 4.13 × 30 cm 117 8.6 43 × 40 cm 179 14 3.743 × 50 cm 251 23 3.2Table 2. Attenuation Coefficients and Signal to Noise ratios for different configurations of Sapphire filters.Configuration Attenuation Coefficient Attenuation Coefficient S/N ratiofor High Energy Neutrons for Low Energy NeutronsNo Al2O3 - - 0.32 cm 2.99 1.79 0.515 cm 8.28 2.24 1.1310 cm 40.90 3.18 3.912 × 10 cm 479 6.02 24.243 × 10 cm 1632 10.9 45.543 × 15 cm 4206 25.5 50.033 × 18 cm 6160 41.81 44.593 × 30 cm 12483 278 13.523. Simulation of single crystal filterTo find the optimum geometrical layout and the best material choice for ESS we simu-lated a straight beam line using different single crystal block configurations. A neutronbeam with the spectrum shown in Fig. 1 is used as a source and the filter is representedby different shielding blocks as shown in Fig. 2. These blocks are surrounded by a thinlayer (2cm) of B4C to absorb the thermal neutrons and to avoid activation of the material.At the end of the beam line is a detector to record the spectrum that has crossed the fil-ters. We tested several different configurations of the filters (1 block, 2 blocks, 3 blocks)with different thickness of Si and Al2O3. The Si is cooled to 50K while the Al2O3 is atroom temperature. In table 1 and 2 is a summary of the obtained results, while Fig. 3and 4 show the comparison of the ESS spectrum before and after the filter, respectivelyfor Si and Al2O3. The thickness necessary to have a good attenuation of the high energycomponent is larger for Silicon than compared to sapphire, but even in the case of thesapphire the best configuration corresponds to 3 blocks of 15 cm, which is quite large ifcompared to what is currently used at neutron facilities [2]. This is due to the fact thatDecember 2015the ESS TDR spectrum has a large energy range that is difficult to attenuate with onesingle crystal block. Nevertheless, for some configurations like the 3 blocks of 15 cm ofAl2O3 the S/N ratio improved significantly, see Fig. 8.As stated before, the intrinsic property of the filters is acting in a different way fordifferent neutron energies and this feature can be seen clearly in Fig. 5 and Fig. 6 wherethe attenuation coefficient versus neutron energy is shown for Si and Al2O3. A clear dropin the attenuation coefficient is seen in the energy range of interest of the ESS neutroninstruments followed by a rise at the energy range of the background neutrons. The sap-phire filter seems to work better than the Silicon filter, but it is interesting to note forthe last one its behaviour in the region of the neutron resonance around 100 keV wherethe interference between resonance and potential scattering create a window in the crosssection followed by a peak (Fig. 3d 3e 3f).Fig. 7 and Fig. 8 show the attenuation coefficient for “Signal” neutrons (< 1 eV) and“Background” ( neutrons with energy>1 eV) and the S/N ratio for different thickness forSilicon and Al2O3 filters. As can be seen for the first, the best ratio is the configurationof 3 blocks of 27 cm while for the second one it is 3 blocks of 15 cm.4. ConclusionsWe showed the capabilities of Geant4 with the additional package NCrystal to simulatesingle crystal filters. We studied the optimum filter for an ESS beam line using the ESSTDR moderator spectrum. The results of our studies show that single crystal filters canimprove the S/N ratio by 10 to 100 times at a reduced cost of absolute flux. Combinationsof Al2O3 with a Silicon layer could be advantageous and need additional studies. A filteris a viable component on both curved and straight instruments but optimization of thefilter and beam line geometry must be done separately for each beam line taking intoaccount the specific requirements of the instrument in question.December 2015(a) Attenuation Coefficient versusneutron energy for a filter com-posed by 1 block of 2 cm.(b) Attenuation Coefficient versusneutron energy for a filter com-posed by 1 block of 5 cm.(c) Attenuation Coefficient versusneutron energy for a filter com-posed by 1 block of 10 cm.(d) Attenuation Coefficient versusneutron energy for a filter com-posed by 2 blocks of 10 cm.(e) Attenuation Coefficient versusneutron energy for a filter com-posed by 3 blocks of 10 cm.(f) Attenuation Coefficient versusneutron energy for a filter com-posed by 3 blocks of 15 cm.Figure 5. Attenuation Coefficient versus neutron energy for silicon filters for different geometries.December 2015(a) Attenuation Coefficient versusneutron energy for a filter com-posed by 1 block of 2 cm.(b) Attenuation Coefficient versusneutron energy for a filter com-posed by 1 block of 5 cm.(c) Attenuation Coefficient versusneutron energy for a filter com-posed by 1 block of 10 cm.(d) Attenuation Coefficient versusneutron energy for a filter com-posed by 2 blocks of 10 cm.(e) Attenuation Coefficient versusneutron energy for a filter com-posed by 3 blocks of 10 cm.(f) Attenuation Coefficient versusneutron energy for a filter com-posed by 3 blocks of 15 cm.Figure 6. Attenuation Coefficient versus neutron energy for Al2O3 filters for different geometries.December 2015(a) Attenuation Coefficient vs crystal thickness(mm).(b) Signal to noise ratio vs crystal thickness(mm).Figure 7. Attenuation Coefficient and Signal to noise ratio for different thicknesses (mm) of Si filter.sDecember 2015(a) Attenuation Coefficient vs crystal thickness(mm).(b) Signal to noise ratio vs crystal thickness(mm).Figure 8. Attenuation Coefficient and Signal to noise ratio for different thicknesses (mm) of Al2O3 filters.December 2015References[1] S. Peggs et all., ESS 2013-001, 2013.[2] F. Cantargi et. al Annals of Nuclear Energy 80 (2015) 43-46[3] By D. F. R. Mildner et. al J. AppL Cryst. (1993). 26, 438-447[4] D. D. Julio et al., “Benchmarking Geant4 for spallation neutron Source Calculations ”, submitted toECNS 2015 proceedings.[5] S. Agostinelli et. al., Nucl. Instrum. Methods Phys. Res., Sect. A. 506, 250 (2003)[6] N. Cherkashyna et. al., J. Phys. Conf. Ser. 528 (2014) 012013[7] N. Cherkashyna et. al., arXiv:1501.02364[8] N. Cherkashyna et. al., Phys. Rev. ST Accel. Beams 18 (2015) 083501[9] J. Stenander and D. D. DiJulio, Nucl. Instr. & Meth. Phys. Res. A 798 (2015) 167-171[10] V. Santoro et. al. “MeV Neutron Production from Thermal Neutron Capture in 6Li Simulated withGeant4” , submitted to ECNS 2015 proceedings , arXiv:1510.07937[11] M. Boin et all., J. Appl. Cryst. 45 (2012) 603-607.[12] M. Boin et all., J. Appl. Cryst. 44 (2011) 1040-1046.[13] M. Boin et all., J. Phys:Conf. Ser. 340 (2012) 012022.[14] T Kittelmann et. al J. Phys. Conf. Ser. 513 (2014) 022017.[15] H.W. Bertini, Phys. Rev. 131 (1963) 1801.[16] H.W. Bertini, Phys. Rev. 188 (1969) 1711.[17] T. Kittelmann and M. Boin Comp. Phys. Comm. 189 (2015) 167-171

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