Muon Science Laboratory Activity Report 2012

2012 at MSL

◤ Overview of KEK–MSL Activities

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Fig1: (top) People gathering around the brand-new Superconducting Curved Solenoid Magnet being installed at the exit of the U-line. (bot-tom) Axial-focusing Superconducting Solenoid placed in tandem with the above solenoid. [Top:enlarged view (676KB) / Bottom:enlarged view (70KB)

 J-PARC MUSE had a busy year since the resumption of user operations in February 2012 following the recovery from the Great East-Japan Earthquake of 2011. It hosted 39 user experiments in FY2012 at the muon D-line under the Inter-University Research Program, operating for 60 days at 220 kW (2012A) and 63 days at ~300 kW (2012B) of proton beam power. In addition, 20 days were allocated to one project, and 36 days were used for commissioning by the instrument group throughout the year.
 Meanwhile, infrastructure for the D-line made steady progress so that muon spin rotation (μSR) measurements can now be performed at temperatures as low as 20 mK using a 3He−4He dilution refrigerator. The DΩ1 μSR spectrometer at the D1 area, which was a second-hand instrument serving only as a stop-gap measure, was finally furnished with an all-semiconductor-based high-density μ-e decay positron/electron detector system (named "Kalliope" for the KEK Advanced Linear and Logic board Integrated Optical detector for Position and Electrons, developed under collaboration with the Electronics System Group in KEK-IPNS). This upgrade of the D1 instrument was a major milestone for the planed replacement of DΩ1 with a full-fiedged next-generation μSR spectrometer in FY2013.
 Concerning the facility upgrade, MUSE staff had an enormously busy summer installing the front-end beam-line components for the S- and H-lines near the muon production target during a relatively short shutdown period. The construction of the U-line, comprising a superconducting curved solenoid, axial-focusing solenoid systems (see Fig. 1), and an ultra-slow muon beam-line at the U-line exit (supported by Kakenhi) came close to the final stage, nearly on schedule for the completion and commissioning of the first beam in FY2013. Installation of ultraslow beam optics components, a high-power laser system, and the μSR spectrometer are in progress. In the meantime, the government announced a major supplemental budget near the end of FY2012 in which the construction of a muon S-line in part (covering up to the S1 branch) and installation of a rotating muon production target were partially funded under the strict condition of completion by the end of FY2013.


◤ Facility development

1. "Kalliope", a big step forward

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Fig2: (top) New Kalliope detector module. (bottom) Six of them were installed on the DΩ1 spectrometer. [Top:enlarged view (111KB) / Bottom:enlarged view (135KB)

 The Kalliope detector is a module comprising 32 scintillation counters and avalanche photodiodes assembled together with electronic circuits for analog and digital signal processing on a single palm-top-sized board (see Fig. 2), for measuring μ-e decay positrons to get timing information relative to the external trigger (timing for the proton beam pulse) using a time-to-digital converter (TDC). The obtained positron timing information is converted into a bunch of TCP/IP packets by a newly developed one-chip interface (SiTCP) on the board, and transferred to host computers for every each proton pulse through Ethernet with at a speed of 1 G bit/s. The adoption of Ethernet and the associated TCP/IP protocol as a detector-computer interface makes the system quite fiexible, and readily scaled up without relying on specific interface hardware for interface that quickly tends to become quickly obsolete.
 There are 12 Kalliope detector modules that have been installed in the DΩ1 spectrometer (six each forward and backward from the sample position) since November 2012. They served to double the solid angle for positron detection from 8% to 17% (where the old detector system based on phototubes was kept operational as a benchmark). While it has become clear throughout commissioning that there still remained some room for improvement of the on-board electronic circuits (in particular for analog signal processing), the usefulness of the Kalliope detectors has been demonstrated by a doubled data rate compared with previous measurements.


2. Breaking the world record for pulsed muon intensity

 The commissioning of the U-line commenced with the resumption of beamtime in October 2012 after completing all the installation work including that for the superconducting curved solenoid, axial-focusing superconducting solenoids, Wien filters, beam blocker, and preparation of associated utilities such as electricity, cooling water, vacuum system, interlock and remote-control devices, and so forth within the short period of the summer shutdown. The highlight of the commissioning was on November 7, 2012, to confirm the world's highest intensity of 2.5 × 106 "surface muons" per pulse, where the figure was estimated by counting the μ-e decay positrons from a metal specimen exposed to the beam at the exit of axial-focusing solenoid. The observed intensity broke our own record achieved at the D-line (1.8 × 105) by more than an order of magnitude, promising for the final goal of generating a high-intensity ultraslow muon beam in FY2013.

◤ Scientific activities


Fig3:Lithium diffusion coefficient in a series of cathode materials LiTPO4, T = Fe, Ni, Co derived from muon measurements. [After J. Sugiyama et al., Phys, Rev. B 85, 054111 (2012).] [enlarged view (164KB)

 One of the scientific topics that appeared in 2012 from the Inter-University Research Program at MUSE was the lithium diffusion property of battery materials inferred from μSR. Lithium-based batteries are currently regarded as being among the most promising candidates for future secondary (rechargeable) battery systems. Transition metal oxides are drawing particular attention as key materials for electrodes because they have a relatively flat open-circuit voltage over a large lithium solid solution with desirable characters for practical application such as being inexpensive, non-toxic, and easy to handle. A group headed by J. Sugiyama of the Toyota Central Research Lab has been committed for years to extensive research on transition metal oxides, which are promising for electrodes in lithium bat-teries. Recently, they were successful in estimating the diffusion coefficient of lithium ions in cathode materials, lithium transition-metal phospho-olivines (LiTPO4, T = Fe, Ni, Co), where diffusive behavior was clearly observed via the relaxation due to nuclear dipolar fields above ~150 K. From the temperature dependence of the nuclear field fluctuation rate, self-diffusion coeffficients of Li+ ions at 300 K and their activation energy were estimated to be ~10−10 cm2/s and 0.10 − 0.17 eV, respectively (see Fig. 3). These figures are now under careful scrutiny in comparison with results obtained by other methods and with theoretical predictions to clarify the microscopic mechanism of lithium diffusion.



Fig4: Renewed MuSAC members and MUSE staff in a group photo. [enlarged view (123KB)

 The 11th Muon Science Advisory Committee (MuSAC) was convened on February 22 − 23, 2013, at the Tokai campus (Fig. 4). The committee was newly appointed by KEK−IMSS for the next 3 years with the task of reviewing the activities of the Muon Science Laboratory (MSL) for the past year and evaluat-ing the Inter-University Research Program conducted at J-PARC MUSE. The committee was jointly appointed by the J-PARC Center (under the name Muon Advisory Committee or MAC) to review in particular the operation of MUSE and related technical developments in the past year.
 Following the overview session introducing the new MuSAC/MAC members to the current status of J-PARC as a whole, the Materials and Life Science Facility (MLF), and KEK-IMSS from both directorates (which have also been renewed since FY2012), the first day was spent mostly on the review of hardware developments including the muon generation target, beamlines, and other experimental equipment, whereas the second day was devoted to the post-evaluation of the Inter-University Research Program and scientific activities of the KEK-MSL staff in general.
 At the beginning of the executive summary, MuSAC expressed congratulations to all people involved at J-PARC, KEK/IMSS, and MUSE for their great contribution to the work performed in 2012. The committee acknowledged that several beamline components (e.g., for the U-line) have been delivered and installed according to the scheduled plan, and great progress has been achieved in the development and upgrading of spectrometers and on the project to replace the production target with a rotating target. Considering the shortage of manpower with respect to the multitude of tasks, the committee granted that it was a remarkable and outstanding achievement.
 The committee was charged with providing advice on the following four issues, 1) user operation, particularly concerning D1/D2 instruments, 2) construction of the ultraslow muon beamline in view of its roadmap and scientific goal, 3) future plans and instrumentation, particularly for the S- and H-lines, and 4) Expansion of science and the community. The final report will be delivered to both the KEK-IMSS and J-PARC Center directorates sometime in May 2013.


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