MAC-2018 Meeting
Held on March 1st and March 2nd 2019
at J-PARC Center, Tokai

Table of Contents
I. Facility Overview
		•Introduction and J-PARC Overview

		•

MLF Facility Overview

		•MUSE Facility Overview

		•Inter-University Research Program

II. MUSE Facility Activity		•Muon Source Facility: M1/M2 Area		•

Muon Production Target		•µSR Spectrometer and Sample Environment		•Negative Muons at D2

		•U-line/Ultra Slow Muons Beam line		•

Laser system		 •H-line		

MLF Second Target•

III. Research Projects (S-Type Proposals)
		•Precision Measurement of Mu Hyperfine Structure and Muon Magnetic Moment•

Frontiers of Research on Condensed matter, Life Science, and Particle Physics

			

explored by Ultra-Slow Muon Microscope		•Search for Muon-Electron Conversion utilizing Pulsed Proton Beam from RCS		•Development of General-Purpose µSR Spectrometer with Semi-conductor-based			

Optical Detectors and Measurement of New Element Strategy Samples with			

New Functions

–1–

		

•

Application of Muon Radiography for the “Photon and Quantum Basic Research			Coordinated Development Program”

		Development of D1 Spectrometer Instruments and µSR experiment in Strongly•			

Correlated Electron Systems

		•Transmission Muon Microscopy		•Precision Measurement of Anomalous Muon Magnetic Moment/EDM

IV. General Comments and Recommendations

Appendix
I. Members of MAC
II. Charges given to MAC–2–

I. Facility Overview
Introduction and J-PARC Overview
Professor N. Kosugi, Director of KEK-IMSS, welcomed the panel and introduced the IMSS activities at J-PARC and the KEK Tsukuba campus. IMSS, responsible at KEK for carrying out multiprobe research for life and materials science with photons, positrons, muons and neutrons, is asking
for suggestions to efficiently operate the KEK beamlines at the two facility sites with the current
limitations on budget and manpower. Then J-PARC Director Professor N. Saito gave an overview
of J-PARC status and activities, highlighting the scientific output of MLF and the particle physics
program. At MLF, there is an increasing importance of the combination of industrial applications
and academic research. The J-PARC accelerator and MLF continued the 500 kW operation in JFY
2018, again providing stable beam with > 90% availability. Operation at 1 MW has been demonstrated for one hour in July, and preparations for routine operation at beam power > 500 kW arecontinuing. Within MUSE the S1 user program has been established, U-line commissioning is inprogress and the construction of the H-line power substation is under way. Finally, a set of charges
was given to MAC to evaluate the facility operation and make suggestions for improvements.

MLF Facility Overview
Prof. T. Kanaya, the Director of the MLF, showed an overview of facility status and beam operations in FY2018. A stable operation at 500 kW beam power has been maintained, with a successful
one-hour beam test on targets at 1 MW. Plans with improved neutron targets with constrain-freestructure with a gradual increase of the beam power are revealed. The overall output in scientificpublications from the MLF shows a continuous upward trend accompanied with the increase of thenumber of users.

MUSE Facility Overview
Prof. R. Kadono presented an overview of MUSE facility. MUSE has been accepting 500 kW proton beam since April 2018. The target system was tested with 1MW beam for one hour, and theresults show that it is ready for a long-term test. One concern was the damage of a coaxial joint of
the rotating muon target found in the shutdown period. Necessary measures such as an improved
monitoring and interlocking system were taken to minimize the possible problems during proton
beam operation. Detailed comments on the progress in each of the experimental areas will be given
in subsequent sections of the report.
–3–Inter-University Research Program
Although there was concern about a significant reduction of the number of proposals in 2018A, theresults of the following two terms show a solid recovery: 49 proposals in 2018B and 54 in 2019A.
This recovery is closely tied to the achievement of stable operation of the S1 beamline that has
increased beam time availability into the community. MAC also appreciates that the MUSE staff
efforts to expand the user base has paid off as an increase in the number of international users andP-type proposals. MAC recommends continuing a careful survey of the degree of satisfaction ofusers and various levels of outreach activities.
Scientific results from the Inter-University Research Program maintain a high standard of productivity, achieving the second highest number of annual publications in the past ten years. MAC was
impressed to see that a variety of significant studies have recently been completed by the membersof the program. A few highlights are the observation of ferromagnetic fluctuations in heavily doped
high-Tc Bi2201 cuprates, identification of a novel Griffiths phase in a layered iridate, and a muonneutron multiprobe study on quantum fluctuations in a spin-1/2 uniform triangular lattice. It is also
highly appreciated that a joint research initiative between IMSS and Toyota CRDL, Inc. has steadily
been developing to establish the technique to non-destructively determine the distribution of Li
metal in battery materials using negative muons.
MAC is also very pleased to hear that the development of science and technology regarding negative muons at MUSE provided crucial contributions to the approval of the project of “Toward newfrontiers: Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum
beams” for the Grant-in-Aid for Scientific Research on Innovative Areas in 2018. This will accelerate the further progress in generation of the focused negative-muon beam followed by the establishment of the muon-microscope technique.
Although MAC recognized that several personnel actions (promotions, hires and changes) havebeen made carefully but steadily in the past year, the importance of strategic human resource management will become increasingly important and urgent in view of retirements expected in very
near future, together with the ongoing expansion of the MUSE facility. MAC has a strong concern
about the shortage of manpower, particularly regarding the maintenance of M1/M2 tunnels involving the muon target with a risk of breakdown, and the active management of the ultra-slow-muon
project.

–4–II. MUSE Facility Activity
Muon Source Facility: M1/M2 Area
It is crucial to guarantee the radiation safety of the facility, at both the current 500 kW, and the future 1 MW, both for workers and for the neighbouring community. MUSE personnel continues their
responsible stewardship of the proton beamline M1/M2 tunnel, taking active measures to mitigate
the radiation hazard, primarily from tritium.
While the track record in this area is excellent, in comparison with other facilities throughout theworld, it is unusual that facility scientists would be given such a task. Moreover, with such limited
manpower, this represents a substantial diversion of effort away from their principal role in facility
development and operation.

Muon Production Target
The muon production target is key infrastructure for the entire MUSE facility, and thus crucial tomaintain reliable safe operation with minimal downtime. During the 1 MW test, the target was
found to operate according to prediction, but a longer term test is essential to be sure it can bear the
extra heat load safely. This test should be a very high priority to determine if any modification will
be necessary in future with regular 1 MW operation.
Previously, it was thought that the target lifetime would be limited by the bearings of the rotating
wheel, so it was surprising to hear that the coupler on the rotating feedthrough failed. The replacement plans for the second coupler, involving significant radiological hazards, are in place and seemsound. A backup plan to replace the entire target is also prepared. The origin of the failure has been
identified to be an inappropriate manner of key groove machining. New couplers, properly fabricated, will surely make another breakage unlikely.
Development of target monitoring capabilities continue, motivated by the need to ensure safe operation including all foreseeable failures. In this effort, it would be useful to connect the MUSE
target effort with other groups with expertise in these issues, both in Japan (e.g. Haba at Riken)
and abroad (PSI). One can anticipate in the near future that the demands of developing and refining
the target will subside, and a rather lower demand regime will follow. It would be appropriate to–5–remove direct responsibility for the target from MUSE and place it on a larger J-PARC wide entity
tasked with operation and maintenance of targets.

µSR Spectrometer and sample environment
With sufficient and predictable muon beam time, the next key criterion to expand the user base andincrease the scientific productivity of MUSE is to ensure the spectrometer capabilities are available
for the users. This includes functioning data acquisition and sample environment (primarily field
and temperature). In this respect, the facility continues to progress and mature with the S1 and D1
areas providing duplicate capabilities in many cases. In one respect, D1 remains a bottleneck forthe demand on the low temperature dilution fridge. This is particularly problematic as the demand
for negative muons at D2 will continue and these two are largely incompatible. The new 3He fridge
may mitigate this to some extent, but it may also be worth considering some means to duplicate thedilution fridge capability at S1. The users may be willing to try to raise funding for this.
Another key criterion for a flourishing user facility is a reliable data acquisition system. While theDAQ works quite well, it still suffers from occasional problems, and surprisingly this accounts forsome significant downtime. An occasional problem that can be addressed in less than an hour would
be acceptable (tolerable), but if there are problems that account for loss of days of beam time (asignificant fraction of the time for a single user), then effort is still required to reach an acceptablelevel of reliability. It is clear that while the system operates very well, refinements are still in order,
for example, the DAQ should not be so dependent on the continuous availability of a good network
connection to Tsukuba. Since this is a key capability for facility operation on an ongoing basis,
there should be a clear plan in place to maintain and improve the DAQ system on an ongoing basis
into the future. The 1MW operation of the DAQ looked very promising with reasonable results at
the maximum count rates. In the run up to regular 1MW operation, it is important to anticipate issues such as detector deadtime related distortions to be prepared to address them as required.

Negative Muons at D2
A continuous commissioning effort has successfully enabled the simultaneous beam usage at theD1 and D2 lines with the electric beam kicker to direct each of the two bunches of the muon beamof a momentum up to 30 MeV/c to different beam lines. An upgrade of the radiation shield for theneutrons emitted after negative muon capture by nuclei was implemented. Experiments with nega-–6–

tive muon beams of 40 MeV/c can safely be conducted.  Electric discharge of the Wien filter at theoperation voltage higher than 150 kV was reported. The issue will be resolved in the next summer
maintenance period and the filter will be operable at 250 kV.
The world’s first measurement of the µ-SR of the Kubo-Toyabe function at the D1 port was also reported as a remarkable achievement in FY2018.

U-line/Ultra Slow Muons: Beam line
Good progress has been made in beam tuning and beam transport simulations, which allowed ultra
slow µSR experiments on Ag in magnetic fields up to 0.14 T and for the first time at implantationenergies down to 500 eV. The measured ultra-slow muon rate at the intermediate focus F3A could
be increased from 70/sec to 120/sec, measured with a MCP detector. The detected rate in the positron spectrometer at the sample position reached about 22/sec. Taking into account the solid angleof 17% of the spectrometer, this translates into at least 130 ultra-slow µ+/s at the sample position.
The measured beam spot size at sample position is about 10 mm FWHM (about twice larger than
expected from simulation), and the measured time-of- flight distribution is 10 ns FWHM. The observed decay asymmetry of 0.1 indicates a USM beam polarization of about 40%, which might bepartially coming from the cloud-muon contamination of the U-line surface muon beam, reducing
the U-line beam polarization to less than 90%. Further progress has been made in the improvementof the positron spectrometer performance and the magnetic shielding of the stray magnetic field of
the last axial focusing superconducting solenoid of the U-line. The effect of stray field on the muon
beam transport has been minimized in the simulation by asymmetric operation of the electrostatic
quadrupoles.
Open issues to be addressed are i) disagreements between experiment and simulation of beam spot
sizes and transmissions from F3A to sample position, and ii) the estimation of the rate of ionized
Mu atoms at the tungsten source, using the known transmission from the beam transport simulations. This rate is of fundamental importance, since it allows estimating the overall USM generation
efficiency. This is key information for understanding and optimization of the laser ionization process – which is in addition crucial for the planned g-2 experiment and the Transmission Muon Microscope at the H-line. MAC suggests increasing the solid angle of the positron detectors to reduce
the time needed to record a µSR spectrum with the currently low USM rate.

–7–

Laser System
Achievements of the previous year include an upgrade of the laser system to improve the stabilityof operation (modifications of the optical table), and a significant improvement in the timing jitterand peak power of the ω2 generation, which is important for stable generation of the Lyman-α light.
The problem of the high-quality large size Nd:YGAG laser ceramic is still not solved. Two routesof improvement are currently envisaged: a bonded large size Nd:YGAG has been obtained and isunder evaluation, and an alternate material (transparent Nd:YSAG ceramic) was obtained, where
the emission spectrum was as expected (but not yet tested for Mu ionization). There is still a factor
25 missing in the laser pulse energy (4 µJ compared to the design 100 µJ).

H-line
MAC is pleased to see the construction of the new power station is ongoing and completion is expected during FY2019. Since the beam-line has the highest priority, we strongly support the effort
of J-PARC center to secure any additional budget to deliver the first beam in 2020. MAC applauds
the willingness of the DeeMe team to contribute to the building and commissioning of the beamline to ensure the timely start of the physics run with the high intensity beam.

MLF Second Target
Working groups of J-PARC MLF division and of the Japanese Science Societies for Neutron andMuon Research continued their evaluation plans for a future target station 2 in MLF. This plan includes an upgrade of the proton beam power to 1.5 MW, a splitting of the proton beam in front of
the existing MLF hall by a fast magnetic kicker, and the building of a new experimental hall with a
combined muon/neutron spallation target. Improvements of a factor 20 of neutron intensities, and afactor 50 – 100 of muon intensities are expected, allowing for new kind of applications of muon micro beams under extreme conditions (high pressure, high external magnetic field), bio science (small
protein crystals), and industrial applications. A CDR is in preparation, and a letter of intent will be
submitted in March 2019 to the Science Council of Japan (SCJ) for the Japanese Master Plan ofLarge Research Projects. The MLF second target is a project for the time horizon beyond 2030 and
MAC furthermore supports this important development of initial ideas for future large-scale muon
projects at J-PARC.

–8–

III. Research Projects (S-type Proposals)
Precision Measurement of Mu Hyperfine Structure and Muon Magnetic Moment
Progress on the muonium hyperfine structure and muon magnetic moment experiment MuSEUM
has been reported. The zero-field resonance measurement of νhfs reached now a world-record statistical precision of about 1 kHz, a factor 20 improvement in the past 2 years, and about 30% better
compared to the LAMPF experiment of 1999. The present expected systematic error is about 8 Hz
(2ppb), and 20 ppb on the magnetic moment for the final experiment in high magnetic field. In thenext five years, a factor of 200 improvement in statistics compared to the LAMPF experiment is
expected, requiring the availability of the H- line from FY2020. This will bring the experimental
accuracy of νhfs to a few Hz (a few ppb), which is in the range of the most precise theoretical calculations, and thus allowing for the most precise test of bound state QED.

Frontiers of Research on Condensed matter, Life Science, and Particle Physics
explored by Ultra-Slow Muon Microscope
An update of the USM microscope has been presented. The prospects of the project are still unique,offering real space 3D µSR mapping and probing of electronic states and spin states and their dynamics across interfaces with ~10 µm spatial resolution. However, the project suffered from significant delays due to long J-PARC shutdown times since 2011, and from the limited USM rate of theorder of 100/s due to the limitations in the generation of Lyman- α light. The further steps are notclear from the presentation, and MAC recommends the preparation of a plan on the further strategy,
especially in view of the problems in the generation of a sufficient rate of USM. MAC has a concern about the project progress after the end of the external funding.

Search for Muon-Electron Conversion utilizing Pulsed Proton Beam from RCS
The DeeMe project received stage-2 approval from the muon-PAC at KEK/IMSS, and the detector
system is now ready to use. Completion of the H-line should have highest priority in order to run
the experiment in a timely manner, to provide a result before COMET phase-I, and in order to maximize the potential of a major discovery at J-PARC. MAC applauds the willingness of the DeeMe
team to contribute to the building and commissioning of the beam line to ensure the timely start ofthe physics run with the high intensity beam.

–9–

Development of General-Purpose µSR Spectrometer with Semi-conductor- based
Optical Detectors and Measurement of New Element Strategy Samples with New
Functions
Good progress has been reported in the application of µSR to the characterization of hydrogenstates in the technologically relevant semiconductors ZnO and InGaZnO4 (IGZO). Publications
on the results are in preparation, while other studies of the MUSE team on interstitial hydrogen in
FeS2 and on electronic correlation ins Y2C were published in PRB in 2018, with the FeS2 paper
receiving special attention in the Physics Buzz Blog run by APS. MAC is pleased to see the production of interesting results within the MEXT “Elements Strategy Initiative to Form Core ResearchCenter for Electron Materials”, and MAC is looking forward to see new results in upcoming experiments on new functional materials.

Application of Muon Radiography for the “Photon and Quantum Basic Research
Coordinated Development Program”
MAC acknowledges the good progress being made in the negative muon radiography program fornon-destructive elemental analysis using muonic x-rays. The application to depth selective lithium
concentration determination in lithium ion batteries with a few tens of micrometres resolution is remarkable.

Development of D1 Spectrometer Instruments and µSR experiment in Strongly
Correlated Electron Systems
Several improvements of the D1 spectrometer have been implemented to reduce the back- groundcontribution in the µSR spectra. These include a large bore spectrometer magnet, development of a
new beam monitor, beam tuning and the development of a fly-past chamber. This allows now measuring small samples (5x5mm2) with a high muon decay asymmetry, down to mK temperatures byusing a dilution refrigerator. These improvements are beneficial for the investigation of crystals ofstrongly correlated electron systems, which are often only available in small amounts/sizes. MAC islooking forward to seeing new results of topical questions in strongly correlated electron systems.

– 10 –

Transmission Muon Microscopy
The Transmission Muon Microscope TµM at the H2-line is in a R&D phase where several prototype components are being prepared for tests. If successful, this facility will offer unique capabilities to visualize nanoscale structures and function of >~10-µm-thick specimen. As in the case of the
USM microscope, the feasibility of the TµM will crucially depend on the USM generation, which iscurrently limited by the generation of intense Lyman-α pulses. MAC suggests the preparation of aplan of the further strategy and a time frame within the realization of the project is planned.

Precision Measurement of Anomalous Muon Magnetic Moment/EDM
The collaboration of the g-2/EDM experiment has now grown to 102 members from 8 countries.
Preparations including construction of experimental components and developments are advancing
well to measure the anomalous magnetic moment and the electric dipole moment of the muon in acompletely different way compared to the BNL/FNAL method. It represents a key experiment for
testing the standard model, and it received now stage-2 status from the IPNS director, and a stage-2
recommendation by the IMSS-PAC. Next milestones are the construction of the H-line, the laser
ionization of muonium at the H-line to generate USM, to accelerate the thermal muons, and toreceive the full funding for the entire experiment. MAC strongly supports this experiment, but expresses concerns about the efficient generation of Lyman-α light for producing USM, which is critical also for the TµM project and for the U-line experiments. MAC is pleased to hear that alternative
solutions for the USM production are under consideration as a backup plan.

IV. General Comments and Recommendations
MAC is pleased to see the outstanding work of the MUSE team in all fields of its responsibilities,
and acknowledges the achievement of another year of stable operation at 500 kW beam power. Thistranslated into a significant increase of the number of annual publications in the past ten years.  A
one-hour test at 1 MW last July was successfully carried out, thanks to the experiences gained fromoperation in previous years. We are looking forward to stable long-term operation at 1 MW beam
power. MAC is very pleased to see the growing interest from broad scientific communities in muonscience at J-PARC.
MAC recognizes that safe operation of MUSE continues to be of highest priority. An appropriate– 11 –

plan has been set up to avoid tritium release from the muon production target. The muon target experienced an unexpected failure of a coaxial coupler. A good repair/exchange plan is available, andcontinuous improvements on monitoring of the status of the muon target, including improved interlocking system, will ensure a safe operation of the facility.
The research reports presented to MAC demonstrate the rich field of muon applications, from fundamental particle physics to condensed matter physics, material science, elemental analysis, andideas to use the muons as novel microscopy/tomography probe. This broad field of topical applications is a unique strength of the MUSE facility.
MAC recommends shifting away the responsibility for the safe delivery of proton beam from the
MUSE staff to a single dedicated J-PARC-wide group with the expertise and capabilities to deal
with these issues in a uniform site-wide manner, ensuring a uniform safety concept. MAC strongly
recommends transferring the present MUSE people responsible for M1/M2 proton beam delivery
and muon production target to support MUSE users in order to develop and establish a strong andstable muon user community with the highest international standards. In this way a uniform operation combining KEK and JAEA resources is achievable, which is key to the further success of MLF.
It will also be helpful to increase efforts to make tighter collaborations with Universities.
The laser for USM generation is of pivotal importance for the experimental program at MUSE.
MAC recommends setting up an external advisory team of laser experts to evaluate the present situation, and as a mid-term measure, to establish a “laser development and maintenance team” under
the responsibility of J-PARC to ensure long-term stable laser operation for the various experiments
requesting USM.
MAC strongly supports the effort of J-PARC to secure the additional budget for the constructionof the H-line to deliver first beam in 2020, in order to start as soon as possible the exciting science
program with the experiments muonium hyperfine structure, muon g-2/EDM, DeeMe and the muontransmission microscope.
MAC recommends providing project and risk management plans for the further U-line/USM commissioning including the issue of efficient generation of USM, and for the completion of the S2-S4
areas.

APPENDIX I– 12 –

Members of MAC:
Hiroshi Amitsuka (Hokkaido University)
Andrew MacFarlane (University of British Columbia)
Kenya Kubo (International Christian University)
Martin Mansson (Royal Institute of Technology)
Takashi Nakano (Research Center for Nuclear Physics)
Thomas Prokscha (Paul Scherrer Institute) Chair
Tadayuki Takahashi (Institute for the Physics and Mathematics of the Universe)
Unable to attend: Klaus Kirch (ETH Zurich and Paul Scherrer Institute)

APPENDIX II
Charges given to MAC:
• Evaluate the appropriateness of the facility operation and its upgrades with respect to the following points :−safe, stable and efficient operation towards the production of science in a timely manner−

timely  construction  of  beam  lines  and  their  preparation  to  maintain  the uniquenessof the facility, attracting not only domestic users, but also international users• Any suggestions for improvements are appreciated. Our particular concerns include, but are
not limited to, the following :−Limited manpower to operate and maintain the beam lines−

Urgency of commissioning of the U-line, preparation for H-line construction and the remaining S beam lines– 13 –