ISIS is the world’s most successful pulsed spallation

neutron source that provides beams of neutrons and muons

that enable scientists to study the properties of the matter at

the atomic level. Restrictions are imposed on the muon target

regarding thickness as this will affect the proton transmission

to the second neutron target. However, it could

be possible to improve the muon production by optimizing

the target geometry. Currently the muon target is a 7 mm

thick graphite plate oriented at 45 degrees with respect to

the proton beam. A set of slices placed at variable distance

is proposed instead of the 7 mm thick graphite target. The

performance of the set of slices is examined in this pape

Bungau, Adriana

Bungau, Cristian

Cywinski, R.

King, Phlip

Lord, James

English

University of Huddersfield Repository

University of Huddersfield RepositoryBungau, Adriana, Cywinski, R., Bungau, Christian, King, Phlip and Lord, JamesGeometry Optimization of the ISIS Muon Target Original CitationBungau, Adriana, Cywinski, R., Bungau, Christian, King, Phlip and Lord, James (2010) Geometry Optimization of the ISIS Muon Target. In: IPAC'10 - Proceedings of the 1st International Particle Accelerator Conference. IPAC 10 OC/ACFA, Kyoto, Japan, pp. 250-252. ISBN 9789290833529This version is available at http://eprints.hud.ac.uk/12228/The University Repository is a digital collection of the research output of theUniversity, available on Open Access. Copyright and Moral Rights for the itemson this site are retained by the individual author and/or other copyright owners.Users may access full items free of charge; copies of full text items generallycan be reproduced, displayed or performed and given to third parties in anyformat or medium for personal research or study, educational or not-for-profitpurposes without prior permission or charge, provided:• The authors, title and full bibliographic details is credited in any copy;• A hyperlink and/or URL is included for the original metadata page; and• The content is not changed in any way.For more information, including our policy and submission procedure, pleasecontact the Repository Team at: E.mailbox@hud.ac.uk.http://eprints.hud.ac.uk/GEOMETRY OPTIMIZATION OF THE ISIS MUON TARGETAdriana Bungau, Robert Cywinski, University of Huddersfield, Huddersfield, UKCristian Bungau, Manchester University, Manchester, UKPhilip King, James Lord, STFC/RAL, Chilton, Didcot, Oxon, UKAbstractISIS is the world’s most successful pulsed spallationneutron source that provides beams of neutrons and muonsthat enable scientists to study the properties of the matter atthe atomic level. Restrictions are imposed on the muon tar-get regarding thickness as this will affect the proton trans-mission to the second neutron target. However, it couldbe possible to improve the muon production by optimizingthe target geometry. Currently the muon target is a 7 mmthick graphite plate oriented at 45 degrees with respect tothe proton beam. A set of slices placed at variable distanceis proposed instead of the 7 mm thick graphite target. Theperformance of the set of slices is examined in this paper.THE ISIS MUON FACILITYThe requirement for MuSR science is to make availableintense beams of polarised muons. The principal continu-ous muon sources are at TRIUMF and PSI while the pulsedfacilities are at KEK and ISIS. The ISIS accelerator pro-duces intense pulses of high energy protons for neutronproduction by the spallation process and muon productionthrough pion creation. The ion source produces negativehydrogen ions which are sent to the Radiation FrequencyQuadrupole accelerator. Intense RF electric fields are usedto focus, bunch and accelerate particles before they arepassed to the linac where they are accelerated to 70 MeV.Final acceleration of the beam occurs in the synchrotronwhere the bunches are accelerated to 800 MeV. The accel-eration process is repeated 50 times per second, so a meancurrent of 200 μA is delivered to the targets. The muontarget is an edge water cooled plate made of graphite withdimensions (50x50x7) mm, oriented at 45 degrees to theproton beam (rotated about a vertical axis) giving an ef-fective length of 10 mm along the beam. The pions andmuons are extracted into two beamlines each at 90 degreeswith respect to the proton beam. The muons are collectedusing a thin aluminium beam window situated at 15 cmfrom the target centre and having a diameter of 8 cm. Themuons emerge from the target within a vertical acceptanceof ±5 mm and a horizontal acceptance of ±30 mm, with adivergence of 35 mrad in the horizontal direction and 180mrad in the vertical direction and a momentum in the range25-27 MeV/c per unit charge. The muon beam is fully po-larised and this polarisation is maintained as the beam istransported to the muon spectrometers. As the muon fa-cility runs in parallel with the neutron facility, the protontransmission through the muon target must be kept at therequired level (usually above 96%). The target is followedby a set of two collimators. The collimators are angledcones of 40 cm length and are made of Cu . The first colli-mator has an inner radius of 37.5 mm and an outer radius of54.15 mm and the second collimator has an inner radius of51 mm and an outer radius of 61.4 mm. The interaction ofthe protons in the graphite target produces intense pulses ofpions with a wide energy spectrum. The low energy pionscome to rest and decay in the muon target. These muonshave low energy and the only ones with sufficient range toescape the target are those that stop near or at the surfaceof the target. These are surface muons and are 100% po-larised in any chosen direction of emission from the target.In order to improve the surface muon production, substan-tial gain in intensity can be achieved through optimisationof the target geometry as there is very little that can be doneto the energy of the proton driver to improve muon beam in-tensities. A target geometry optimisation is presented nextin this paper.GEANT4 SIMULATIONSThe Monte Carlo code GEANT4 [1] was used for thesesimulations. GEANT4 is a toolkit used to simulate particleinteractions in matter and it supports a flexible frameworkof hadronic models. The Bertini Cascade physics modelwas used in the current study because it performs well forincident protons and is validated up to 10 GeV incidentenergy. In this model, the target nucleus is treated as anaverage nuclear medium to which excitons (particle-holestates) are added after each collision. The path lengths ofnucleons in the nucleus are sampled according to the lo-cal density and free nucleon-nucleon cross sections. At theend of the cascade the excited nucleus is represented as asum of particle-hole states which is then decayed by pre-equilibrium, fission and evaporation methods [1].Slice Target GeometryIn the computer simulations the ISIS 7 mm graphite tar-get is split along the proton beamline into two and threeslices, such that the total thickness will still be 7 mm. Thedistance between the slices is then varied gradually. Thesurface muons are detected by the ISIS beam window andin parallel by a spherical shell of inner radius of 14 cmand an outer radius of 16 mm surrounding the target. Thespherical shell is made of vacuum to avoid particle scatter-ing. In order to have a good statistics for surface muonsdetected by the ISIS beam window, 109 protons were sentMOPEA076 Proceedings of IPAC’10, Kyoto, Japan25008 Applications of Accelerators, Technology Transfer and Industrial RelationsU05 Applications, Otherto the graphite slices. When the spherical shell was usedfor detection, only 108 protons were simulated. The valid-ity of the results obtained with the spherical shell configu-ration relies on the fact that the surface muons productionis isotropic [2].The distribution of suface muon vertices was simulatedfor the present design of the ISIS muon target, and alsofor the alternative slices target geometry, for different dis-tances between the slices. Fig. 1 shows the distribution ofthe vertices of the surface muons detected by the ISIS beamwindow and by the spherical shell surounding the target.z0 (mm)-100 -80 -60 -40 -20 0 20 40 60 80 100x0 (mm)-100-80-60-40-20020406080100Slabs_0mm_isis_histEntries  186Slabs_0mm_sphere_histEntries  730Surface muons vertex distributionsFigure 1: Surface muons vertex distributions for the presentISIS muon target design detected by the spherical shell andalso by the ISIS beam window.Taking into acount the factor of 10 difference in the num-ber of protons simulated in these two cases, one can con-clude that the ISIS beam window is detecting ∼2% of thetotal number of surface muons produced in the target. Thisresult is to be expected since the solid angle to the beamwindow is Ω = 0.073π and the surface muon productionis isotropic. The distribution of the vertices for the sur-face muons detected by the spherical shell also indicate thewhole contour of the ISIS target. The ISIS target was splitnext into two slices, each 3.5 mm thick and rotated at 45degrees with respect to the proton beam axis. The muonvertex distributions are shown in Fig. 2, 3 and 4, for 20, 40and 80 mm, respectively, between the slices.z0 (mm)-100 -80 -60 -40 -20 0 20 40 60 80 100x0 (mm)-100-80-60-40-20020406080100Slabs_20mm_isis_histEntries  221Slabs_20mm_sphere_histEntries  758Surface muons vertex distributionsFigure 2: Surface muons vertex distributions for a 2 slicesmuon target design and 20 mm between the slices.Having a two slices target design results in a higher sur-face muon yield, which can be increased by up to 20% withz0 (mm)-100 -80 -60 -40 -20 0 20 40 60 80 100x0 (mm)-100-80-60-40-20020406080100Slabs_40mm_isis_histEntries  223Slabs_40mm_sphere_histEntries  849Surface muons vertex distributionsFigure 3: Surface muons vertex distributions for a 2 slicesmuon target design and 40 mm between the slices.z0 (mm)-100 -80 -60 -40 -20 0 20 40 60 80 100x0 (mm)-100-80-60-40-20020406080100Slabs_80mm_isis_histEntries  174Slabs_80mm_sphere_histEntries  881Surface muons vertex distributionsFigure 4: Surface muons vertex distributions for a 2 slicesmuon target design and 80 mm between the slices.respect to the present target design configuration, for theoptimum distance of 20-30 mm between the slices.Similar simulations have been performed for a 3 slicestarget design, each slice being 2.33 mm thick and rotatedat 45 degrees with respect to the proton beam. The muonvertex distributions are shown in Fig. 5, 6 and 7, for 20,40 and 80 mm, respectively, between the slices. Having athree slices target design results in a further incease in thesurface muon yield, increased by up to 37% with respectto the present target design configuration, for the optimumdistance of 20 mm between the slices. Also the fraction ofsurface muons which are detected by the ISIS beam win-dow for this new configuration has increased to 3.3%.z0 (mm)-100 -80 -60 -40 -20 0 20 40 60 80 100x0 (mm)-100-80-60-40-20020406080100Slabs_20mm_isis_histEntries  254Slabs_20mm_sphere_histEntries  762Surface muons vertex distributionsFigure 5: Surface muons vertex distributions for a 3 slicesmuon target design and 20 mm between the slices.Proceedings of IPAC’10, Kyoto, Japan MOPEA07608 Applications of Accelerators, Technology Transfer and Industrial RelationsU05 Applications, Other 251z0 (mm)-100 -80 -60 -40 -20 0 20 40 60 80 100x0 (mm)-100-80-60-40-20020406080100Slabs_40mm_isis_histEntries  247Slabs_40mm_sphere_histEntries  901Surface muons vertex distributionsFigure 6: Surface muons vertex distributions for a 3 slicesmuon target design and 40 mm between the slices.z0 (mm)-100 -80 -60 -40 -20 0 20 40 60 80 100x0 (mm)-100-80-60-40-20020406080100Slabs_80mm_isis_histEntries  202Slabs_80mm_sphere_histEntries  1021Surface muons vertex distributionsFigure 7: Surface muons vertex distributions for a 3 slicesmuon target design and 80 mm between the slices.Variation with DistanceThe muon production rate variation with the distance be-tween slices was studied next. The muons were detected inboth detectors, the spherical shell and the ISIS beam win-dow. When the slices are too close, a fraction of the sur-face muons does not reach the detectors because of chan-neling between the slices. As we increase the distance be-tween slices, the muon production rate is increasing. Forthe spherical shell, the muon production rate is increasingand flatens after some distance as all the muons producedin the target reach the detector (Fig. 8).Regarding the surface muons reaching the ISIS beamwindow, the muons entering under large angles must be ex-cluded because they will not even enter the first quadrupoleand cannot be transmitted. In order to get the usable yield,one must apply the angular cut appropriate to the muonbeam line, which accepts muons with a divergence of 35mrad in the horizontal direction and 180 mrad in the verti-cal direction. As a consequence, a surface muon producedinside a target slice which is more than ∼50 mm from thecentral position, will not pass the angular selection cuts,and cannot be used in the experiment.For a two slice geometry there is an increase in the muonproduction rate but after ∼40 mm, which seems to be theoptimum distance, the rate starts to decrease (Figure 9). Ford(mm)0 20 40 60 80 100Surface muons6007008009001000110012002 slabs3 slabsMuon Production rate for a slab geometry targetFigure 8: Variation of the number of surface muons de-tected with the distance between the slices for the sphericalshell.a three slices geometry the rate increases initially with thedistance but when the distance between the slices is greaterthan ∼50 mm, the only surface muons collected by thebeam window are those produced in the central slice, andthe muon rate becomes constant.0 20 40 60 80 100 12005101520253035402 slabs3 slabsMuon Production rate for a slab geometry targetd (mm)Surface muonsFigure 9: Variation of the number of surface muons de-tected with the distance between the slices for the ISISbeam window.CONCLUSIONA target geometry optimisation for an increased surfacemuon yield was presented in this paper. The performancesof the double and triple slice geometries were compared.It was found that for the three slices target the muon yieldis higher than for the two slices geometry. The variationof the muon production rate with the distance between theslices shows the optimum value for the surface muon de-tection in the ISIS beam window being at about 40 mm fortwo slices and 20 mm for three slices target geometry.REFERENCES[1] Geant4 - a toolkit for simulation of the pas-sage of particles through matter, version 4.9.3.p01,http://geant4.cern.ch.[2] A.Bungau et al., “Material Studies for the ISIS Muon Target”,MOPEA077, IPAC 2010, Japan.MOPEA076 Proceedings of IPAC’10, Kyoto, Japan25208 Applications of Accelerators, Technology Transfer and Industrial RelationsU05 Applications, Other

a toolkit for simulation of the passage of particles through matter, version 4.9.3.p01,

Material Studies for the ISIS Muon Target”,

Geometry Optimization of the ISIS Muon Target

Geometry Optimization of the ISIS Muon Target

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