Batkov, K.

Klinkby, Esben Bryndt

Lauritzen, Bent

Mezei, F.

Pitcher, E.

Schönfeldt, Troels

Takibayev, A.

Willendrup, Peter Kjær

Zanini, L.

The present study takes its origin in the baseline design of European Spallation Source where a cold and a thermal moderator are situated next to each other enabling bispectral extraction. The study aims at mapping the differences in various neutron distributions depending on the angle and position from which the moderator is viewed. This study does not only show changes in both cold and thermal neutron flux, depending on extraction position, but also shows that there are significant differences in the wavelength spectrum and origin of neutrons depending on the angel of view

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 19, 2017Optimization of cold neutron beam extraction at ESSSchönfeldt, Troels; Batkov, K.; Klinkby, Esben Bryndt; Lauritzen, Bent; Mezei, F.; Pitcher, E.; Takibayev,A.; Willendrup, Peter Kjær; Zanini, L.Publication date:2013Link back to DTU OrbitCitation (APA):Schönfeldt, T., Batkov, K., Klinkby, E. B., Lauritzen, B., Mezei, F., Pitcher, E., ... Zanini, L. (2013). Optimizationof cold neutron beam extraction at ESS. Paper presented at 11th International Topical Meeting on NuclearApplications of Accelerators, Bruges, Belgium.Optimization of cold neutron beam extraction at ESST. Scho¨nfeldt, European Spallation Source, Lund, Sweden & Technical University of Denmark,Nutech, Roskilde, DenmarkK. Batkov, European Spallation Source, Lund, SwedenE. B. Klinkby, European Spallation Source, Lund, Sweden & Technical University of Denmark,Nutech, Roskilde, DenmarkB. Lauritzen, DTU Nutech, Roskilde, DenmarkF. Mezei, European Spallation Source, Lund, SwedenE. Pitcher,European Spallation Source, Lund, SwedenA. Takibayev, European Spallation Source, Lund, SwedenP. K. Willendrup, Technical University of Denmark Physics, Lyngby, DenmarkL. Zanini, European Spallation Source, Lund, SwedenAbstractThe present study takes its origin in the baseline designof European Spallation Source where a cold and a ther-mal moderator are situated next to each other enabling bis-pectral extraction. The study aims at mapping the differ-ences in various neutron distributions depending on the an-gle and position from which the moderator is viewed. Thisstudy does not only show changes in both cold and ther-mal neutron flux, depending on extraction position, but alsoshows that there are significant differences in the wave-length spectrum and origin of neutrons depending on theangel of view.IntroductionThe European Spallation Source (ESS) is going to be thehighest intensity neutron source ever build. Neutrons willbe produced by spallation, driven by a 5 MW, 2.5 GeV pro-ton beam impacting on a massive rotating tungsten targetwheel. At ESS the neutrons will be thermalized in ambientwater moderator placed next to the target, and cooled fur-ther in a 20◦K tank of liquid para-hydrogen. The moderatorwill be surrounded by a reflector system, consisting of aninner beryllium reflector, surrounded by an outer reflectormade of steel, as shown in Figure 1 or described in [1].A moderator system will be placed above and below thetarget, each having two 60◦ by 12 cm windows where neu-trons can be extracted for experiments. The experimentswill each be given a 12 cm high and 5◦ wide instrumentwindow, in one of these 60◦ beam extraction windows.Geometry nomenclatureFor the ESS geometry, as seen in Figure 1 and 2, we de-fine vertically up as the y-direction (0, 1, 0) (upwards onthe left plot of Figure 1), the proton beam to point in theFigure 1: Target-Moderator-Reflector system from MC-NPX [2] model. On the left a zy-slice at x = 0, on theright a zx-slice, of the moderator reflector system abovethe target.z-direction (0, 0, 1) in a right handed coordinate system (xpointing upwards and z pointing right in the right side plotFigure 1 and in Figure 2). The origin (0, 0, 0) is situated atthe spallation hotspot inside the target wheel, which is onthe central axis of the cold moderator cylinder.We define θ as the positive angle in the zx-plane with re-spect to the proton beam axis (θ = 0 at (0, 0, 1) as seen inFigure 1). This study takes its origin at the point closest tothe moderator at which the neutron guides can be placed,namely at the cylindrical beam extraction surface, 2m fromthe center y-axis of the para-hydrogen cold moderator. Atthis 2 m radius cylinder the neutrons position is fully de-scribed by y and θ.We define δθ (in θ-direction) as the relative direction offlight of neutrons with respect to the extraction surface nor-mal.Neutrons origin plays a vital role in this study. At a givenposition the neutrons direction of flight δθ can be used toderive if the neutron originated from the cold or thermalmoderator or from the reflectors.The wavelength (λ) will be divided into four categories:Cold neutrons: λ ≥ 2.0 A˚.1                                                                                                               x                            Proton Beam                  θ                                                                                                                 z           Reflector    Thermal       Cold          thermal       modetator   moderator          side   Reflector       cold       side  ‘  Neutron   position                Neutron                origin   Neutron   direction                                                    Beam     extraction     δθ  orientation      Beam       extraction      window c Figure 2: Horizontal geometry definitions. θ is that angleof the neutron position at the 2 m extraction window. δθis the angle between the 2 m extraction surface normal andthe neutrons direction (the horizontal neutron beam diver-gence).Thermal neutrons: .5 A˚ ≤ λ < 2.0 A˚.Hot neutrons: .01 A˚ ≤ λ < .5 A˚.Any wavelength/all neutrons: .01 A˚ < λ.Note that no neutron with .01 A˚ ≥ λ will be included inthis study.Spatial distribution [deg]θ0 50 100 150 200 250 300 350y [cm]-30-20-1001020300100200300400500600700800(a) Any wavelength (λ > 0.01A˚) [deg]θ0 50 100 150 200 250 300 350y [cm]-30-20-100102030050100150200250(b) Cold neutrons (λ ≥ 2.0 A˚) [deg]θ0 50 100 150 200 250 300 350y [cm]-30-20-100102030050100150200250300(c) Thermal neutrons (.5 A˚ ≤λ < 2.0 A˚) [deg]θ0 50 100 150 200 250 300 350y [cm]-30-20-100102030050100150200250300(d) Hot neutrons (.01 A˚ ≤ λ <.5 A˚)Figure 3: Neutron distribution on the beam extraction sur-face at 2 m radius, with different wavelength requirements.The main part of the excess neutrons in the two forwardpointing windows (θ ∈ [30◦; 90◦] and θ ∈ [270◦; 330◦])are thermal and hot, a visualization of this is shown inFigure 4. Most instrument will be given a single slot of5◦x12 cm. Note that all of these distributions are flat in yand symmetric in θ around the beam axis (180◦ or 0◦).The TDR model of ESS [1] consists of a thermal mod-erator and a cold moderator positioned near the center offour 60◦ beam extraction windows. Some of the neutronsemitted from the thermal/cold moderator or from the re-flectors arrive on one of these four 2 m beam extractionwindows, as seen in Figure 3. Each of these 60◦ beam win-dows will be subdivided into twelve 5◦ beam slots, most ofwhich will host an extraction instrument. These slots havebeen indexed in increasing order with θ, first slot being slot0 at θ ∈ [30; 35].Looking at a flat moderator surface, one will discoverthat the neutrons emitted in an angle with respect to the sur-face will be suppressed because they need to traverse morematerial in order to reach the surface. Traversing more ma-terial, causes more absorption and more re-scatter. Thismeans that the maximum intensity from a moderator sur-face is expected to be directly in front of it, then falling offas something cosine-like as the surface is viewed at an an-gle wrt. the surface normal. This is also the case for thethermal neutrons from the thermal moderator at ESS, eventhough this is not how it looks from Figure 3 and 4. Thisis due to the two figures showing all neutrons arriving onthe extraction surface, and as a trivial consequence thermalneutrons from the reflectors and from the cold moderatorare included. The reason why this is asymmetric is thatthere are more neutrons in the reflector on the cold moder-ator side of the moderator-reflector system (due to the for-ward pointing proton beam and the reflector being closer tothe spallation hot spot), this can be seen from Figure 5.Intensity (I) [PDU]012345) [deg]θHorizontal position (40 60 80 100 120 140 160]Å) [λWavelength (0246810Figure 4: Horizontal wavelength spectra for a forward(30◦-90◦) and a backward (95◦-155◦) extraction windowon the beam extraction surface at 2 m radius (slice-wisenormalized in λ to enhance horizontal structure). Cold neu-trons have equal distribution in both forward and backwardwindow, however, there are more cold neutrons centrallyin the extraction window. Thermal and hot neutrons aremainly forward directed.Angular distributionThe horizontal divergence (δθ) for cold and thermal neu-trons observed from different extraction positions (θ), can20100200300400500) [deg]θHorizontal position (20 40 60 80 100 120 140 160) [deg]θδHorizontal divergence (-6-4-20246(a) Cold neutrons (λ ≥ 2.0 A˚)0100200300400500600700800900) [deg]θHorizontal position (20 40 60 80 100 120 140 160) [deg]θδHorizontal divergence (-6-4-20246(b) Thermal neutrons (.5 A˚ ≤ λ < 2.0 A˚)Figure 5: θ vs δθ for cold and thermal neutrons. The linesrepresents edges between characteristic areas: cold side re-flector - red - cold moderator - blue - thermal moderator -black - thermal reflector. The red line have been removedfrom a region, to obtain better view the thermal hotspotnear 80◦ − 90◦ and 145◦ − 155◦ in (b).be seen in Figure 5. The Figure shows quite a few dif-ferences from extraction position to extraction position.These differences might be exploited by instruments de-pending on their demands on wavelength spectrum andbeam footprint, a couple of ideas/examples of this is foundin the next section. The structure of the cold spectrum,shown in Figure 5a, can be explained by the properties ofpara-hydrogen: scattering on the spin-singlet state of para-hydrogen becomes unavailable at 15.2 meV, and in turn themoderator becomes transparent. As a result of this trans-parency neutrons are (to first order) emitted from the pointwhere they scattered below this energy (λ ∼ 2.3 A˚). Asthermal neutrons entering a cold moderator will be downscattered in very few collisions, with a short mean freepath, they are observed near the interfaces where they en-ter the cold para-hydrogen moderator. This effect is notvery visible for central extraction positions, as there is nosuch interface in line of sight of the moderator (see Fig-ure 1). The effect is enhanced for instruments opposite thecold moderator, as they are viewing the interface betweenthe reflector and the cold moderator, where more neutronsenter the moderator compared to the interface between thethermal moderator and the cold moderator.As seen in Figure 5b, the thermal neutrons are fairlyevenly distributed over all of the thermal moderator. How-ever, when observing thermal neutrons from the reflectornext to the cold moderator, one discovers a hotspot near theinterface between the cold moderator and the reflector, butonly when viewed from the opposite side of the beam ex-traction window. The reason is that most neutrons, emittedthrough a surface are forward directed, and as the surfaceis viewed at the lowest angle from the opposite side of theextraction window (∼ 30◦ wrt. the surface normal) mostneutrons are observed here.Figure 6 shows a more detailed view of the angular dis-tribution of neutrons in different wavelength regions, forthree specific extraction locations. Namely the most for-ward pointing slot, on the cold moderator side of the ex-traction window (6a); one of the 2 central slots in the ex-traction window, directly in front of the thermal moderatorsurface (6b) and the slot at the thermal moderator side ofthe extraction window, where the thermal neutron hotspotat the reflector next to the moderator is most intense (6c).Optimization of instruments positionand orientationFor most instruments the first thing to consider from theknowledge obtained in this study, is background. Mov-ing from a forward pointing extraction window to a back-ward pointing window, reduces high energy background,along with hot and thermal neutron flux, it does howevernot change the cold neutron intensity significantly. Thismeans that instruments which only concerns with cold neu-trons, should be position them self in backward pointingslots. For thermal or bispectral instruments, it should becarefully considered if more thermal neutrons are worth theextra background.One example, of a more complex utilization of a morespecific feature of the spatial and angular distribution ofneutrons could be for bispectral extraction. At first glanceit would seem reasonable to pick an extraction slot near thecenter of the extraction window where both cold flux andthermal flux from the thermal moderator is at its maximum.However, moving the bispectral instrument to the thermalmoderator side of the extraction window, aligning it to lookat the interface between the reflector and the cold modera-tor, will allow the instrument to extract both cold neutronsand thermal neutrons, even without the use of a bispectralsupermirror. The bispectral mirror can then be added to en-hance cold neutron extraction, and in case of failure, due toradiation damage or its like, the mirror can be removed andthe instrument will still accept quite some cold neutrons.Also the instruments interested in neutrons with wave-lengths near 2 A˚ should consider the extraction slots nearthe sides of the extraction window, as these positions havefar more neutrons in this intermediate wavelength range(λ ∈ [1.8 A˚; 2.2 A˚]), originating from the interface be-30200400600800100012001400]Å) [λWavelength (1 2 3 4 5 6 7 8) [deg]θδHorizontal divergence (-6-4-20246(a) Slot 0: Cold moderator side0200400600800100012001400]Å) [λWavelength (1 2 3 4 5 6 7 8) [deg]θδHorizontal divergence (-6-4-20246(b) Slot 5: Central slot02004006008001000120014001600]Å) [λWavelength (1 2 3 4 5 6 7 8) [deg]θδHorizontal divergence (-6-4-20246(c) slot 11: Thermal moderator sideFigure 6: δθ vs λ for three different forward pointing ex-traction slots. Note how the optimal extraction angle forthermal neutrons, changes for different slots.tween the cold and thermal moderator or the cold modera-tor and the reflector.ConclusionThe present studies have mapped the differences in vari-ous neutron distributions for different angles of view andposition of extraction. This study showed that there arequite some differences from slot to slot in the origin andspectrum of neutrons. Any experiment should chose theirbeam slot carefully and heavily consider which directionto align their guide. Changing orientation or moving po-sition changes which ranges of neutrons are accepted bya guide. Such optimizations can not only be used to en-hances the intensities of wanted neutrons and help shapethe beam footprint, but it can also be used to suppress un-wanted background.References[1] S. Peggs, editor. ESS Technical Design Report. April2013.[2] Laurie S. Waters, Gregg W. McKinney, Joe W. Dur-kee, Michael L. Fensin, John S. Hendricks, et al. TheMCNPX Monte Carlo radiation transport code. AIPConf.Proc., 896:81–90, 2007.4

Optimization of cold neutron beam extraction at ESS

http://orbit.dtu.dk/ws/files/69170402/Optimization_of_cold_neutron_beam_paper.pdf

Optimization of cold neutron beam extraction at ESS

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology