Bentley, Philip

Bergbäck Knudsen, Erik

Filges, Uwe

Klinkby, Esben Bryndt

Lauritzen, Bent

Nonbøl, Erik

Willendrup, Peter Kjær

Recently, an interface between the Monte Carlo code MCNPX and the neutron ray-tracing code MCNPX was developed[1]. Based on the expected neutronic performance and guide geometries relevant for the ESS, the combined MCNPX-McStas code is used to calculate dose rates along neutron beam guides. The generation and moderation of neutrons is simulated using a full scale MCNPX model of the ESS target monolith. Upon entering the beam extraction region, the individual neutron states are handed to McStas via the MCNPX-McStas interface. McStas transports the neutrons through the beam guide and by using newly developed event logging capability, the neutron state parameters corresponding to un-reflected neutrons are recorded at each scattering. This information is handed back to MCNPX where it serves as neutron source input for a second MCNPX simulation. This simulation enables calculation of dose rates in the vicinity of the guide. In addition the logging mechanism is employed to record the scatterings along the guides which is exploited to simulate the supermirror quality requirements (i.e. m-values) needed at different positions along the beam guide to transport neutrons in the same guide/source setup

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Downloaded from orbit.dtu.dk on: Dec 15, 2017Application of the MCNPX-McStas interface for shielding calculations and guide designat ESSKlinkby, Esben Bryndt; Bergbäck Knudsen, Erik; Willendrup, Peter Kjær; Lauritzen, Bent; Nonbøl, Erik;Bentley, Philip; Filges, UwePublished in:Conference proceedings - International Workshop on Neutron Optics and Detectors 2013Publication date:2013Link back to DTU OrbitCitation (APA):Klinkby, E. B., Bergbäck Knudsen, E., Willendrup, P. K., Lauritzen, B., Nonbøl, E., Bentley, P., & Filges, U.(2013). Application of the MCNPX-McStas interface for shielding calculations and guide design at ESS. InConference proceedings - International Workshop on Neutron Optics and Detectors 2013Application of the MCNPX-McStas interface forshielding calculations and guide design at ESSE B Klinkby1,2, E B Knudsen3, P K Willendrup3,4, B Lauritzen1,E Nonbøl1, P Bentley2, U Fliges51) DTU Nutech, Technical University of Denmark, DTU Risø Campus,Frederiksborgvej 399, DK-4000 Roskilde, Denmark2) European Spallation Source ESS AB, Box 176, S-221 00 Lund, Sweden3) DTU Physics, Technical University of Denmark, DTU Lyngby Campus, Anker EngelundsVej 1, DK-2800 Kgs. Lyngby, Denmark4) ESS design update programme - Denmark5) Paul Scherrer Institute, 5232 Villigen, Switzerland. ESS design update programme -SwitzerlandE-mail: esbe@dtu.dkAbstract. Recently, an interface between the Monte Carlo code MCNPX and the neutronray-tracing code MCNPX was developed[1]. Based on the expected neutronic performance andguide geometries relevant for the ESS, the combined MCNPX-McStas code is used to calculatedose rates along neutron beam guides. The generation and moderation of neutrons is simulatedusing a full scale MCNPX model of the ESS target monolith. Upon entering the beam extractionregion, the individual neutron states are handed to McStas via the MCNPX-McStas interface.McStas transports the neutrons through the beam guide and by using newly developed eventlogging capability, the neutron state parameters corresponding to un-reflected neutrons arerecorded at each scattering. This information is handed back to MCNPX where it serves asneutron source input for a second MCNPX simulation. This simulation enables calculationof dose rates in the vicinity of the guide. In addition the logging mechanism is employed torecord the scatterings along the guides which is exploited to simulate the supermirror qualityrequirements (i.e. m-values) needed at different positions along the beam guide to transportneutrons in the same guide/source setup.1. IntroductionThe European Spallation Source (ESS), presently starting construction in Lund, Sweden, will bethe most intense source of spallation neutrons ever built[2]. Protons from a 5MW, 2.5GeV linearaccelerator will impact a rotating tungsten target in 14, 2.86ms long pulses every second. Thespallation neutrons hereby created are thermalized in water and some of them are further cooledin liquid para-hydrogen before extracted through individual beam-lines serving 22 cold/thermalinstruments. To fully exploit the long pulse characteristics, many instruments benefit frombeing long - up to 200m is foreseen. Each instrument will by situated behind a guide tailoredspecifically to it’s requirements in terms neutron beam parameters. Guide design is challengingand important since guides and in particular their shielding is a major cost driver for the ESSfacility.For these reasons it would be useful to have a tool that:• Can monitor where in a guide neutrons are lost• Allows to optimise reflectivity requirements along a guide• Serves as an input for dose-rate calculations along guide (n,γ)• Works within the work-flow accustomed to instrument designers (McStas)The newly developed tool McStas Scatterlogger is the backbone that facilitates this usage.2. McStas Scatterlogger: Implementation and usageMcStas[3, 4, 5, 6] combines ray-tracing with Monte Carlo methods to accurately and efficientlysimulate neutron scattering experiments of all types. As seen from the user perspective (typicallyinstrument designers and users) the program is based upon a so-called instrument file in whichyou place predefined or user-supplied components according to the real setup of the instrumentin question. The instrument file is written in a simple meta language, which at runtime iscompiled first to ansi-C, which is turn is compiled to an executable.The Scatterlogger is an additional McStas component which can be placed in an instrument filewrapping around any other component - for example a guide component. At each scatteringin the guide, the incoming and outgoing neutron state is then be temporarily stored, and aniterator is supplied so that the user can analyse the scatterings once the neutron has exited thecomponent. To prevent memory built-up the results of the analysis are histogramized, memoryis flushed and McStas proceeds to the next neutron - as in any other McStas simulation.In the intermediate analysis step (event post-processing), the user has access to the full neutronstate before the scattering as well as the reflected and transmitted neutron state:Incoming state: nin = (x,vin, t,win)Transmitted state: ntrans = (x,vin, t,wtrans)Reflected state: nrefl = (x,vout, t,win−,wtrans)(1)Figure 1. Basic guide geometry - from McStas’ mcdisplay. Also shown in the figure is anexample neutron trajectory. At each encounter with the guide wall, both the reflected andun-reflected neutron ray is stored and both trajectories are visualised. The un-reflected ray isallowed to propagate to a monitor component (green) surrounding the guide where it is detected.Figure 1 contains a visualisation of a guide and the logging utility in operation. At eachscattering of the neutron on the guide wall, the un-reflected neutron weight fraction is emittedas a secondary ray, propagated to a monitor component.3. Use-case 1: Lost intensityIn this simple example, a 10m straight guide of m = 2 coating is subject to the scatterloggingmechanism (see figure 1). At each scattering the transmitted weight wtrans and thecorresponding position along the guide, z, is histogramized and the result is shown in figure 2.As expected the neutron loss occurs predominately at the first scattering in the beginning ofFigure 2. Histogram showing the lost neutron intensity along straight guide as described inthe text.the guide - the strength of the scatterlogging procedure is that it provides a method to quantifythis. Obviously the extent at which intensity is lost along a guide depends strongly on the guidegeometry and coating as well as the source (ie. energy, divergence and spatial extent). Thus, thepresent example is by no means an attempt to describe guide losses in general, on the contrarythe scatterlogging must by utilised individually to aid a particular guide design. Moreover, usersshould anticipate an iterative process where the results of one iteration (eg. figure 2) gives inputto next - eg. increasing the mvalue in the part of the guide closest to the source.4. Use-case 2: ReflectivityNeutrons scattering on a mirror are reflected if the energy/incident angle is sufficiently low.Specifically, the following relations hold (see figure 3):cos(2θ) = (~vin · ~vin) / ~vin2 (2)k = ~vin mh/h¯ (3)Figure 3. Reflected vs. absorbed neutron.Given a neutron state and a guide geometry, the minimum mirror reflectivity requirementwhich would reflect the neutron, mmin, can be calculated [6]:mmin = 2 · k · sin(θ)/0.0219 (4)Figure 4 shows mmin for the first, second and third reflection respectively. The fact that themirror in this example has an mvalue of 2, is seen (on the intensity scale on the right-hand sideof each figure) to cause significant loss of neutrons, in particular between the first an secondscatterings.Figure 4. Histograms showing the minimum m-value required for a reflection, as a function ofz along guide in a simulation of the 10m straight guide described in the text. The color scaleindicates the bin intensity. Note that the simulation was carried out using a m = 2 guide, whichis seen to cause significant intensity loss in the beginning of the guide.5. Use-case 3: Background along a guideTo calculate radiation due to un-reflected neutrons and γ’s resulting from neutron absorptionin the guide material (triggering production from the (n, γ) reaction) several steps are carriedout:First, a Monte Carlo simulation of 2.5GeV protons impacting on a tungsten target wheelis performed according to the baseline ESS MCNPX model from which the geometry shown infigure 5 is taken. Some of the neutrons created in the spallation processes in the target wheelare thermalized in water moderators and further cooled, while others escape the target stationat significant energies. The neutron spectrum (figure 6) and divergence as the neutrons exitthe moderator surface serve as input for a second MCNPX simulations which is illustrated infigure 7. In this second MCNPX simulation neutrons are generated on the blue surface shownleft. Upon reaching the vertical dashed line, they are handed to a McStas simulation throughthe MCNPX-McStas Source Surface Read \Write interface [1]. McStas transports the neutronswithin the guide and returns lost neutron weight to MCNPX where it forms the basis of adose-rate calculation.Figure 8 shows the result of the dose-rate calculation for a 50m straight guide (left) andcurved 50m curved guide (right). The radius of curvature is 1500m which means that line ofsight is lost at approximately 21m explaining the corresponding drop in neutron dose-rate. Thephoton dose-rate does not show the same trend which is due to the over-illumination of theguide entrance by energetic neutrons.To validate that in fact γ’s are created from (n, γ) reaction initiated by transmitted neutrons,figure 9 shows the corresponding dose-rate result, only restricting to the wavelength span: 0.5 A˚-1.0 A˚. Clearly in this setup, the trend observed for neutron dose-rate can be recognised for γ’s.6. Conclusions and outlookThe McStas Scatterlogger has proved a useful tool in guide design. In particular, it facilitates tomonitor where along a guide neutron intensity is lost and allows to optimise where to use high-reflectivity mirror sections, and where less expensive mirrors can be installed with insignificantperformance loss. Moreover it can serve as a input for MCNPX calculations of dose-rates, byFigure 5. Target station MCNPX geometry of the baseline ESS design [2].λ [A˚]Log(Intensity)Figure 6. Spectrum of neutrons exiting the cold moderator surface. The jumps are due thatthe input MCNPX source description is binned in energy rather than wavelength.Figure 7. Left sketch: Longitudinal view of the simulation geometry. The detectors are placed5 cm within the steel shielding. Right: end-view of the guide.which shielding requirements along a guide can be established. Note that the results dependstrongly on the simulated guide geometry and source specifications, wherefore simulationsinvolving the McStas scatterlogger need to be performed based on individual guide\instrumentdesigns.Future extensions include the calculation of backgrounds at the sample and detector position,activation calculations as well as calculation backgrounds and dose-rates at one instrument,caused by neutrons escaping a neighbouring instrument.Length along y [m]0 10 20 30 40 50 Sv/h]µDose [10210310410510610710810 NeutronPhotonLength along y [m]0 10 20 30 40 50 Sv/h]µDose [10210310410510610710810 NeutronPhotonFigure 8. Histograms showing the dose rate along the straight (left) and curved (right) guidedefined in the text and shown in figure 7. Energy- and particle dependent conversion factorshave been applied when converting from fluence to effective dose-rate in accordance with Swedishradiation protection authorities[7, 8].Length along y [m]0 10 20 30 40 50 Sv/h]µDose [110210310410510610710 NeutronPhotonLength along y [m]0 10 20 30 40 50 Sv/h]µDose [110210310410510610710 NeutronPhotonFigure 9. Histograms showing the dose rate along the straight (left) and curved (right) exampleguides defined in the text and shown in in figure 7. Only neutrons in the wavelength range:0.5 A˚-1.0 A˚ are accounted for. Energy- and particle dependent conversion factors have beenapplied when converting from fluence to effective dose-rate in accordance with Swedish radiationprotection authorities[7, 8].References[1] Klinkby E et al February 2013 Nuclear Instruments & Methods In Physics Research A 700 106[2] Peggs S (editor) et al April 23, 2013 ESS Technical Design Report ESS-doc-274, ISBN 978-91-980173-2-8 URLhttp://europeanspallationsource.se/documentation/tdr.pdf[3] Lefmann K and Nielsen K 1999 Neutron News 10 20[4] Willendrup P, Farhi E, Knudsen E, Filges U and Lefmann K 2013 Journal of Neutron Research 13 underpublication, DOI 10.3233/JNR-130004[5] Willendrup P, Knudsen E, Farhi E and Lefmann K 2011 User and Programmers Guide to the Neutron Ray-Tracing Package McStas, Version 1.12c, Risø−R−1416(rev.ed.)(EN)[6] Willendrup P, Knudsen E, Lefmann K and Farhi E 2011 Component Manual for the Neutron Ray-TracingPackage McStas, Version 1.12, Risø−R−1538(rev.ed.)(EN)[7] Seltborg P et al 2005 Nuclear Instruments and Methods in Physics, A 501 313[8] 1998 International Commission on Radiation Units and Measurements ICRU report 57

Application of the MCNPX-McStas interface for shielding calculations and guide design at ESS

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Application of the MCNPX-McStas interface for shielding calculations and guide design at ESS

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