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Birth of an intense pulsed muon source, J-PARC MUSE

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Birth of an intense pulsed muon source, J-PARC MUSE
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  BirthofIntensePulsedMuonSource,J-PARCMUSE Miyake Yasuhiro a , b , ∗ Simomura Koichiro a , b Naritoshi Kawamura a , b Patrick Strasser a , b Shunsuke Makimura a , b Akihiro Koda a , b Hiroshi Fujimori a , b Kazutaka Nakahara a , b Ryosuke Kadono a , b Mineo Kato a , b Soshi Takeshita a , b Kusuo Nishiyama a , b Higemoto Wataru c , b Katushiko Ishida e Teiichiro Matsuzaki e Yasuyuki Matsuda d and Kanetada Nagamine a , e a Muon Science Laboratory, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan  b Muon section, Materials and life science division, J-PARC Center, 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan  c Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan  d University of Tokyo, Graduate School of ARTS and Sciences, Meguro, Komaba 3-8-1 153-8902, Japan  e Advanced Meson Science Laboratory, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198,Japan  Abstract The muon science facility (MUSE), along with the neutron, hadron, and neutrino facilities, is one of the experimentalareas of the J-PARC project, which was approved for construction between the period from 2001 to 2008. The MUSEfacility is located in the Materials and Life Science Facility (MLF), which is a building integrated to include bothneutron and muon science programs. Construction of the MLF building was started in the beginning of 2004, andwas recently completed at the end of the 2006 fiscal year. We have been working on the installation of the beamline components, expecting first muon beam in the autumn of 2008. For Phase 1, we are planning to install onesuperconducting decay/surface channel with a modest-acceptance (about 40 msr) pion injector, with an estimatedsurface muon ( µ + ) rate of 3  ×  10 7 /s and a beam size of 25 mm diameter, and a corresponding decay muon ( µ + / µ − ) rate of 10 6 /s for 60 MeV/c (up to 10 7 /s for 120 MeV/c) with a beam size of 50 mm diameter. These intensitiescorrespond to more than ten times what is available at the RIKEN/RAL Muon facility which currently possess themost intense pulsed muon beams in the world. In addition to Phase 1, we are planning to install, a surface muonchannel with a modest-acceptance (about 50 mSr) mainly for experiments relating to material sciences, and a superomega muon channel with a large acceptance of 400 mSr. In the case of the super omega muon channel, the goal is toextract 4  ×  10 8 surface muons/s for the generation of ultra slow muons and 1  ×  10 7 negative cloud muons/s with amomentum of 30-60 MeV/c. One of the important goals for this beam line is to generate intense ultra slow muons atMUSE, utilizing the intense pulsed VUV laser system. Approximately 2 ∼ 5 × 10 5 ultra slow muons/s will be expected,which will allow for the extension of   µ SR into the area of thin film and surface science.In the symposium, the current status of J-PARC MUSE will be reported. Key words:  muon, J-PARC, MUSE, pulsed muon, MLF, remote handling, slow muon ∗ el. +81-29-284-4624, Fax: +81-29-284-4878, email: ya-suhiro.miyake@kek.jp 1. The 3GeV Proton Beam at J-PARC TheJ-PARC(JapanProtonAcceleratorResearchComplex)willbeconstructedinthesouthpartofthe Preprint submitted to Elsevier 18 July 2008   Tokai-JAEA (Japan Atomic Energy Agency) site,and consists of a 181 MeV LINAC (400 MeV in fu-ture), as well as a 3 GeV and a 50 GeV proton syn-chrotron ring. About 90% of the 3 GeV, 333  µ A (1.0MW) beam is sent to the Materials and Life Sciencefacility (MLF) for the production of intense pulsedneutron and muon beams, while the remaining 10%will be sent to the 50 GeV ring for further accelera-tion for the kaon and neutrino physics programs [1]. The parameters of the 3 GeV proton beam are:(i) The number of protons will be 8.3  ×  10 13 /pulse,(ii) Theaveragebeampowerwillbe0.6MW,sincethe LINAC energy will start with 181 MeV inphase 1, and will reach 1 MW in the futurewhen the LINAC energy is increased up to 400MeV.(iii) The repetition rate is 25 Hz, so each pulse isseparated by 40 ms. One proton pulse consistsoftwobunches;eachwithabunchwidth ∼ 100-120 ns, and separated by 600 ns(iv) The transverse emittance ( ϵ ) will be 81 π  mmmrad (beam core) and 324 π  mm mrad (maxi-mum halo). Fig. 1. The top figure shows a schematic drawing of the firstfloor of the MLF building. A bottom figure shows a cutawayview along the primary beamline showing a schematic of theNM tunnel structure of the MLF building. 2. Design of the MLF Building and the NMtunnel The MLF consists of the proton beamline tun-nel (the so called NM tunnel), and two experimen-tal halls (east wing; Experimental hall No.1, westwing; Experimental hall No.2). The tunnel struc-ture is designed to keep radioactive materials insidethe tunnel in order to ensure safe operations duringmaintenance work on the neutron or muon targets.The height of the building is 31 m inside the tun-nel and 21 m in the experimental halls. The widthof the tunnel is 13.5 m, and the east and west wingsare 24.5 m and 32 m wide, respectively. The protonbeam height is 1.6 m from the floor level. The muonscience facility is 30 m long along the proton beam-line, and is located upstream of the neutron facil-ity. Figure 1 (top) shows a schematic drawing of thefirst floor of the MLF building. The proton beam-line in the MLF building consists of the M1 and theM2 line regions. The M1 line is located upstream of the muon target, where no significant beam loss oc-curs. On the other hand, the M2 line is in the vicin-ity of the muon target where severe beam loss oc-curs due to the surrounding beamline components.Fig. 1 (bottom) shows a cutaway view along the pri-mary beamline showing a schematic of the NM tun-nel structure of the MLF building. Since a fractionof the primary 3 GeV proton beam is scattered pref-erentially downstream toward the neutron target,two sets of scrapers are installed to mitigate damageto the beamline components such as the quadrupolemagnets and the beam ducts. Although some of thescattered beam is deposited on the scrapers, the var-ious beamline components such as the quadrupolemagnets, target chamber, scraper chamber, pillow-seal, and vacuum ducts located along both the pri-mary and secondary beamlines will suffer not onlyfromtremendouslyhighradiation,butalsofromcor-rosion induced by  NO x  in irradiated air [2]. There-fore, following what PSI has done in dealing withtheir 1 MW-class proton beam [3], all of the mainte-nance work along the M2 beamline involving powerand water connections are intended to be performedremotely from the top of the maintenance area, lo-cated4mabovefloorlevel.ConstructionoftheMLFbuilding was started in the beginning of 2004, andwas completed in the end of the 2006 fiscal year.2  Fig. 2. A schematic view of the M2 line in the vicinity of the muon target and pictures of the beam line components 3. Installation of the M2 line beam linecomponents In the M2 beamline, all beam line componentsmust be installed via remote handling from themaintenance area above (FL 4m). For that pur-pose, we installed baseplates with a precision of XY  ±  0.5 mm on the floor of 0.5 m FL, and thenwe placed the alignment plates matching to theindividual beam line components with a precisionof XY  ±  0.1 mm, Z  ±  0.1 mm. Iron guide shieldsequipped with a guiding rail structure were alignedby placing knock pins on the alignment plates. Thetarget chamber, the various beamline magnets (M2primary line; six quadrupole magnets QM1, QM2,QN1, QN2, QN3, QN4 and four steering magnets,X22, Y22, X23, and Y23 and secondary line; DQ-1-3 , SQ 1-3 triplet magnets, DB1and SB1 bendingmagnets), two sets of profile monitor assemblies, 20sets of pillowseal assemblies, a gate valve assemblyand seven sets of duct assemblies were also installedon the alignment plates equipped with the knockpins, which allow precise positioning, guided by theguide shields. The power and control cables are thenconnected between the magnets on the M2 line andthe corresponding power supplies.Finally, a successful test was performed on themagnets on the M2 line by running them contin-uously for 12 hours. In addition, after completionof the vacuum connection, the ultimate vacuumachieved was as low as 3  ×  10 − 5 Pa m 3 /s. Fig.2showsaschematicviewandthelatestpicturesoftheMLF M2 tunnel in the vicinity of the muon target.To celebrate its completion, we had a M2 linecompletion party on July 17, 2007, inviting J-PARCcenter director Prof. Nagamiya, vice directors Dr.Ooyama, and Prof. Yamazaki, IMSS director Prof.Shimomura and related J-PARC members. After-ward, between July and September, 2007, the M1and M2 lines were covered with 3500 ton NM tunnelconcrete blocks. Fig.3 shows a picture of the partycelebrating the completion of the M2 line construc-tion on July 17th, 2007.Also, all the beam line components in the M23  Fig. 3. A picture of the party celebrating completion of theM2 line construction on July 17th, 2007 tunnel were successfully demonstrated to functionproperly, delivering the 3 GeV proton beam fromthe muon target to the neutron source as expectedduring the period from first proton beam injectedinto MLF on May 30th, 2008, to the following com-missioning period. 4. The tandem-type graphite muonproduction target The MLF consists of the neutron and the muonscience facilities which utilizes the 3 GeV, 1 MW,25Hz proton beam. In order to reduce the total costof the project through common use of the utilities,getting rid of the severe beam dump constructionassociated with high-level tritium water handling,and by sharing the beam with the neutron facility,we decided to have a tandem-target muon facility,rather than construct a separate building with ourown proton (1 MW) beam dump, as was the casein KEK-MSL. Through discussion with the neutronscience group, we reached an agreement that the to-tal beam loss induced by placing the muon produc-tion targets should be no more than 10 %, which al-lows us to install 10 mm and 20 mm thick graphitetargetsonthebeamlineupstreamoftheneutrontar-get, corresponding to a beam loss of 3.5 % and 6.5%, respectively. Detailed calculations on heat, radi-ation and duct-streaming in the vicinity of the muontarget were performed by NMTC/JAM and MCNPMonte-Carlocodes[4].Inthecaseofthe20mmthickgraphite target, as much as 3.5 kW is deposited intoa 25 mm diameter region of the target through ir-radiation of the 3 GeV, 1 MW proton beam. Onepossible candidate for the muon production target isa rotating carbon target, which has been developedat PSI and has been working well for more than tenyears[3].Intheend,weadoptedanedge-coolednon-rotating graphite target, because of its ease of han-dling during maintenance. In this target, graphiteis indirectly cooled by the copper frame, which sur-rounds the graphite. For the graphite material,we chose the isotropic graphite, IG-43, which hasa thermal conductivity of 139 W/mK, 1.82  g/cm 2 density at 300 K, thermal expansion of 4.8 ppm/K,Young’s modulus of 10.8 GPa, and Poisson’s ratioof 0.28. In the copper frame, three turns of cool-ing pipe, a SUS tube with O.D. 12.7 mm and I.D.10.7 mm, are embedded through HIP (Hot IsostaticPress). In order to reduce stress, a titanium bufferlayer of 2 mm is placed between the graphite diskand the copper frame. The copper frame, the 20mm thick graphite, and the titanium buffer layer arebonded by silver brazing in vacuum. Calculations of the heat and stress induced by the heat deposit of the 3.5 kW beam through ANSYS demonstrate thatthe edge-cooled graphite target can be used safely asthe 1MW muon production target. Detailed calcu-lations of the neutron irradiation effect on the ther-mal conductivity and the thermal stress induced bythe proton beam will be reported elsewhere [5].The designs of the two sets of scrapers and themuon production target are such that they are bothplaced in one large vacuum chamber in order to Fig. 4. Left: a picture of the target assembly transporteddown to the storage basement; Right: a picture of the main-tenance commissioning of the target slide table in the hotcell 4  make remote handling simple during maintenancework. All the water and cable connections will bedone at the top of the maintenance area. Particularcare was given to the means of mounting the target,moving and inserting the target with the requiredprecision, as well as cooling, monitoring, and chang-ing the target in the hot-cell. The design was com-plete by September 2004, and the installation wasfinished in May 2006. In 2007 we made a dedicatedmuon cask, together with a stand to position it ontop of the target chamber with good precision, forthe maintenance of the muon target assembly, thescraper assembly, and the profile monitor assembly.The damaged target or profile monitor assembly canbe lifted into the cask, and transferred to a storagepit located in the M2 tunnel at a level of 10 m. A newtarget assembly will then be installed in the cham-ber as soon as possible. During long shutdown peri-ods, damaged targets will be lifted into the cask, andtransferred to the hot cell. In the hot cell, the tar-get assembly consisting of the graphite target, waterpipes,andatargetslidetableareexchangedthroughremote handling. Since the beginning of April, 2008,we have been performing maintenance tests of theremote handling system by utilizing the power ma-nipulator or special devices in the hot cell. Fig. 4shows a picture of the target assembly transporteddown to the storage basement and a picture of themaintenance test of the target slide table in the hotcell. 5. Secondary muon channels For Phase 1, the installation of the decay surfacemuon secondary channel in the MLF No. 2 experi-mental hall has started. To begin with, on Decem-ber 25th, 2007, we defined a restricted area nearthe decay surface muon line to be a radiation con-trolled area. These are areas where we may bringin radioactive components such as superconductingcoils, which have been used on the KEK beamlinefor more than 30 years. These superconducting coilswerethenassembledintothesolenoidvacuumcham-ber on January, 2008. On February, 2008, installa-tion of the on-line refrigeration system started withfull operation expected by August, 2008. In parallel,the secondary beam line components such as tripletquadrupole magnets, bending magnets, slits, block-ers, gate valves, separator, beam ducts, and shield-ingblockswerefabricated,andtheirinstallationwillbe completed by the summer of 2008. Fig. 5 shows Fig. 5. A picture of the decay surface muon channel underconstruction. a picture of the decay surface beamline under con-struction.In addition to Phase 1, we are planning to installone surface muon channel with a modest-acceptance(about 50 mSr), and one super omega muon chan-nel with a large acceptance of 400 mSr. In the caseof the surface muon channel, we can expect a to-tal of 1.6 × 10 7 /s surface muons with a 1MW pro-ton beam. By installing a kicker and beam slicer,we are planning to place four experimental ports inthe experimental hall No.1. In the case of the su-per omega beam channel, we are going to install alargeacceptancesolenoidmadeofmineralinsulationcables (MIC) and a superconducting curved trans-port solenoid. We can collect either surface or cloudmuons up to 60 MeV/c very efficiently. Finally, weare expecting 4 × 10 8 /s surface muons and 10 7 neg-ative cloud muons in the experimental hall No.2 [8].Although many of these studies can be performedusing either surface or decay muons, at the superomega channel we are aiming to create a new typeof muon source; the intense ultra-slow muon source.Ultra slow muons are generated through resonantionization of muonium (Mu). Mu is formed by stop-ping intense surface muon on the rear surface of ahot W foil. At the RIKEN/RAL muon facility, 20slow  µ + /s are obtained out of 1.2  ×  10 6 /s surfacemuons [9]. Taking into account the repetition rate of the pulsed laser system and the proton beam, as wellas the surface muon yield between RIKEN-RAL andJ-PARC MUSE, we can expect 1.3 × 10 4 /s of slow µ + /s without any additional laser development. Arate of 1.3 × 10 6 /s slow µ + can be achieved with suf-ficient laser development such as the tripling of 366nm photons with pico second pulse width to match5
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