Muon Science Laboratory Activity Report 2011

2011 at MSL

◤ Temporary recovery of the MUSE facility from the Great East-Japan Earthquake

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Fig1:(Top) Damaged pipings that connect helium gas compressor to the cryogenic system for superconducting solenoid magnet on the D-line. (Bottom) After restoration, which was completed in late October 2011. [Top:enlarged view (86KB) / Bottom:enlarged view (201KB)

 Although the damage caused to the MUSE (MUon Science Establishment) facility was relatively minor as compared to that observed in the remaining parts of J-PARC, MUSE was found to have been severely damaged by the gigantic earthquake that occurred on March 11, 2011. Owing to the ceaseless efforts of the staff as well as contractors, an extremely large amount of reconstruction and repair work (consisting largely of temporary fixes) was completed by the end of November 2011 (see Fig. 1) in order to ensure that MUSE would be ready for accepting a test beam during the December–January period. In January 2012, after a long shutdown of nearly ten months, operations at MUSE were finally resumed in order to deliver a muon beam to the D1/D2 areas.
 During a time of great uncertainty in the early stages of the recovery schedule for J-PARC, the RIKEN Nishina Center kindly offered a share in the beam time at the RIKEN-RAL Muon Facility in the U.K. and hosted many of the experimental proposals approved for the 2011A term. In addition, KEK provided support for travel expenses and manpower through the Inter-University Research Program. Further, the Paul Scherrer Institute (Switzerland) implemented the priority handling of muon experiment proposals from researchers residing in Japan.
 Meanwhile, the call for J-PARC proposals for the 2011B term and subsequent processes were executed in the normal manner with the expectation that the operations at J-PARC, including MUSE, would be resumed in the February–March, 2012 period (although this announcement was highly provisional when it was made in July 2011), and in fact, they were conducted as originally planned.


◤ Facility upgrading of MUSE

1. D-line

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Fig2:(Top) A bird view of the MLF west wing hall, where D-line (under green shields) and U-line (under pink shields) are seen. (Bottom) Superconducting curved solenoid magnet. [Top:enlarged view (176KB) / Bottom:enlarged view (74KB)

 The commissioning of the beam kicker system on the D-line was resumed upon the recovery of J-PARC. The kicker system, consisting of a pair of pulse magnets and steady-field magnets for beam stirring, became fully operational to allow single-muon pulse experiments at the D1 and D2 areas (mostly for surface muons). The installation of the kicker system necessitated the re-tuning of all D-line components to optimize the beam profile at the ends of the respective beam areas. The electric noise originating from the kicker has been suppressed to a level tolerable for muon spin rotation (μSR) experiments. In the meantime, the beam-slicer previously installed in the D1 area was removed in order to create space for the U-line (see Fig.2).


2. U-line

 Construction of the ultra-slow muon beamline made steady progress in parallel with the restoration of the damaged facility. Fabrication of a superconducting curved solenoid system was completed towards the end of 2011 with a few months of delay owing to the difficulty in securing the necessary materials and manpower in the aftermath of the giant earthquake (Fig. 2). The system has cleared the factory tests for cooling and magnetic field excitation, and hence, it will be ready for installation on the U-line during the summer shutdown in 2012. An axial-focusing superconducting coil system (which will be installed downstream from the curved solenoid) was also commissioned to Toshiba Co., and its fabrication process is approaching the final stage.
 While KEK-MSL holds responsibility for the beamline components necessary for delivering the ultrahigh-intensity surface muons (beam energy = 4 MeV) at the exit of U-line, a group headed by E. Torikai of Yamanashi University secured major funding from MEXT (Grant-in-Aid for Innovative Research) for the generation of ultra-slow muons (USM, beam energy ≈ 0.2 eV) and their application to cutting-edge research over a variety of fields including nanomaterials science. This grant will extend over the period FY 2011–2015 to cover the cost of the experimental equipment including a hot-tungsten muon moderator, a high-power laser system, an electrostatic USM transport beamline, and a μSR spectrometer. Design work for the moderator and muon transport beamline is now in progress at KEK, while an all-solid-state high-power laser is being prepared by a RIKEN group. Delivery of the first USM beam is expected toward the end of FY 2012.

◤ Scientific activities

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Fig3:(Top) Bulk magnetic susceptibility observed in Sr2IrO4 (inset: crystal structure consisting of IrO6 octahedrons and Sr ions. (Bottom) μSR time spectra observed in Sr2IrO4 under a zero external field, where the sinusoidal oscillation of signals (asymmetry) due to the spontaneous muon spin precession induced by an internal magnetic field associated with the magnetic order are clearly seen. [Top:enlarged view (229KB) / Bottom:enlarged view (156KB)

 One of the scientific topics that gained prominence in 2011 through the Inter-University Research Program at MUSE was the electronic properties of Sr2IrO4, which is a newly developed transition metal oxide with a crystal structure that is highly similar to that of the cuprate superconductors (i.e., the "perovskite" structure, see Fig. 3). In contrast to the cuprates, whose properties are determined by the electrons in the 3d orbits, the new compound is characterized by iridium 5d electrons that are subject to strong spin-orbit interaction and a wide energy band in their metallic state, suggesting the possibility that a new avenue will be added to the physics of strongly correlated electron systems. While using the neutron diffraction technique for this purpose is ineffective owing to the large neutron absorption cross section of Ir nuclei, muon spin rotation experiments have demonstrated that the pristine compound (= insulator) becomes antiferromagnetic below 240 K which reveals bulk properties that are remarkably similar to cuprates. This topic was selected as the "Editors' Suggestion" of Phys. Rev. B (by H. Okabe et al., NIMS-Aoyama Gakuin U.-KEK collaboration).


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Fig4:(Top) MuSAC members and MUSE staff in a group photo. (Bottom) A snapshot of the committee meeting. [Top:enlarged view (283KB) / Bottom:enlarged view (238KB)

 The Muon Science Advisory Committee (MuSAC) convened during February 17–18, 2012, at the Tokai campus (Fig. 4). The committee was appointed by KEK–IMSS with the task of reviewing the activities of the Muon Science Laboratory (MSL) for the past year and evaluating the Inter-University Research Program conducted at J-PARC, MUSE; the committee was also appointed by the J-PARC Center (under the name Muon Advisory Committee or MAC) to review the operations at MUSE and the related technical developments in the past year. The committee members were H. Amitsuka (Hokkaido University), R. Cywinski (University of Huddersfield), E. Morenzoni (PSI, Chairman), J.-M. Poutissou (TRIUMF), A. Shinohara (Osaka University), J. E. Sonier (Simon Fraser University), and E. Torikai (Yamanashi University). Considering the long shutdown period of J-PARC since the last MuSAC review conducted shortly before the earthquake (February 2011), the charges for the committee remained mostly the same as those for the previous meeting.
 The committee expressed congratulations to all the people involved at J-PARC, MUSE, and MSL for the reconstruction and repair work performed in 2011 after the gigantic earthquake and tsunami that struck Japan in March 2011.
 In the beginning of the executive summary, MuSAC summed up the vision and global plan for MUSE as the following three missions: 1) to develop and build state-of-the-art muon beams and instruments to exploit these beams and advance science, 2) to support a large user community, and 3) to engage in front-line research in material science and muon physics.
 For the first two missions, MuSAC recommended the optimization of the experimental instruments at D1 and D2 with high priority as these instruments will be used for all the competitive applied muon science for the next few years. The committee also recommended designing a new, fully operational spectrometer optimized for D2 (and later for a part of the S-line), furnished with various sample environments that would greatly increase the number of applications from the physics community.
 The scientific results from the Inter-University Research program won high acclaim in view of the extremely difficult circumstances imposed by the earthquake and the concomitant heavy demand on KEK-MSL.
 MuSAC reinforced their recommendation for the ultra-slow muon beam line as the top priority. The committee expressed pleasure upon hearing that considerable overall progress was made in 2011, despite the many difficulties, and endorsed the current roadmap towards the completion of the beamline before the end of 2012. Progress in preparing for the installation of the S-line quadrupoles and the front end of the H-line was also acknowledged by the committee with a recommendation for careful improvisation using limited manpower. It was recommended that attempts should be made to secure sufficient manpower to complete the scientific and technical projects, referring to the possibility in the short term to appoint temporary contract staff while adequately balancing the operational and staffing cost.
Finally, for expanding the user community and establishing collaborations, MuSAC recommended further advertising of MUSE, which is at J-PARC, via workshops and seminars (possibly in collaboration with the local neutron and synchrotron facilities) as necessary steps for involving the broader condensed matter physics and materials science communities. In this respect, designating a person within MUSE for outreach programs (which include the summer challenge KEK program and other training courses, for young scientists, in μSR techniques) was suggested for the coordination of these efforts.


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