MAC-2017 Meeting
Held on February 28th and March 1st 2018
at J-PARC Center, Tokai

Table of Contents
I. Facility Overview

		• Introduction and J-PARC Overview

		• MUSE Facility Overview

				• MLF Facility Overview• Inter-University Research Program

II. MUSE Facility Activity		• Muon Production Target		• D1/S1 Sample Environment

		
		
		
		
		
		
		

• Muon Source Facility: M1/M2 Area
• D1/S1 mSR Spectrometer/DAQ Upgrade• Negative Muons at D2• U-line/Ultra Slow Muons: Beam Line• U-line/Ultra Slow Muons: Laser System• H-line• MLF Second Target		III. General Comments and Recommendations–3–

I. Facility Overview
Introduction and J-PARC Overview:
Prof. K. Yamada, Director of KEK-IMSS, welcomed the committee and introduced the facility andthen J-PARC Director Professor N. Saito gave an overview of J-PARC status and activities, high-lighting the improving operational efficiency of > 90 % in combination with the establishment of
500 kW operation and the continuing progress towards 1 MW operation. Within MUSE the S1 user

program is now ramping up, U-line commissioning is in progress 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 gave an overview of operations in FY2017. The key development has been the installation of the newly designed #8 neutron target allowing 400 kW operation from January 2018 and 500 kW operation from April. A test of 1 MW operation of the #8 target
is scheduled for June. Future plans involve progressing to #10 target, which has a constraint-freedesign. The benefits of the switch to more stable operation in FY2017, even at limited operation
power, are revealed in a return to the trend of increasing numbers of MLF users after the previous
fall due to the serious target trouble.

Overall MLF shows a good publication record with the fully operational muon instruments show-

ing a particularly strong rate of publication per experiment.

MUSE Facility Overview:
An overview of the current status of the MUSE facility was presented by Prof. R. Kadono. Workhas progressed on each of the D, U, S, and H-lines as well as on the muon production target. Hissummary demonstrated the steady progress of each case. Detailed comments in each area will begiven in subsequent sections of this report.

Inter-University Research Program:
After a healthy number of proposals for beam time in 2017, early indications show a significant
reduction in 2018. It is too early to tell whether this is a statistical fluctuation or represents a wor-

–4–

rying trend. However, if it indicates the latter, then causes and remedies should be addressed vigor-

ously as soon as possible. One suggested possibility is a loss of interest by users, due for example,

to accumulated frustration with the difficulty of getting experiments approved and getting beamtime scheduled. It should be considered carefully if the current proposal system presents a barrier
to inexperienced users, or whether it makes sense to change the policy of the proposal review com-mittee, for example by dedicating some time for new users (similar to P type proposals). Outreachto the user base is also crucial, because many of the factors that have frustrated new users in the re-cent past have been addressed, and, looking forward, users can now expect a higher availability ofbeams. Initiating a mentoring program for new users and including training on proposal writing in
future summer schools are other avenues worth considering.

Although the overall publication record is good, the number of publications produced in 2017 wasrelatively low. This is likely to be a lagging response to the low availability of beam over the previous years. Despite this, the highlights of recent progress from the IURP were encouraging for theirbreadth and novelty, including new applications of muonic X-rays to industrial problems like Li
batteries, and the role of cosmic ray muons in electronics failures.

It was very encouraging to see that a number of hires were made at MUSE in the past year andmore are planned. This is important in view of retirements expected in the near future, as well as
the ending of fixed term positions. Even after these hires, there remains a substantial ongoing staff
shortage at MUSE, the total staff number having decreased by 3 between 2017 and 2018, which isaround 10 % of the total. It might be helpful to have a clear statement of a realistic target number of
personnel per beamline that is in line with other facilities like neutrons and synchrotron X-rays, tomake the case for more staff positions as clear as possible.

II. MUSE Facility Activity
Muon Production Target:
The current rotating muon target has operated for 40 months without trouble, making 10 M revo-

lutions out of its full design life of 50 M. Work continues on improving the target diagnostics anddeveloping next generation muon targets using materials such as SiC coated graphite and SiC composites. Participation in the RaDIATE international collaboration covering radiation damage in accelerator target environments is a new and welcome development.

–5–

Muon Source Facility: M1/M2 Area;
Stable operation and maintenance of the primary beam transport section from the muon target to theneutron target and the front-end elements of the four muon beam lines is the responsibility of themuon section. The airflow at the M2 tunnel is closed and slowly and continuously exhausted by aturbo molecular pump (TMP) through the MLF stack. The primary proton beam produces tritium inthe graphite production target, but there is no real time monitoring system for the tritium concentration in the exhaust air due to the difficulty of the detection of its low energy beta ray. A 2 kBq/cm2

tritium contamination was detected around the TMP and this contamination is believed to be due tothe adsorption of water containing tritium formed by isotope exchange with the tritium produced in
the remaining air in the tunnel.

The risk of an abrupt tritium emission from the graphite muon production target is recognized,

and a task force has been created to work on this issue. Diffusion of tritium in graphite is not sup-posed to be fast at the operation temperature of the target, but there is currently no reliable data forthe graphite material used for the muon target. To get some data on this issue, a test sample will beirradiated with a proton beam at RCNP of Osaka University, and the tritium produced in the sample
will be examined by a radiochemical method.

For the M2 tunnel area an upgrade to a new exhaust monitoring system with two buffer tanks is

proposed to guard against abrupt emission of tritium.

MAC recognizes the effort being made to identify and assess potential risks associated with thetritium production in the beam tunnel.

D1/S1 Sample Environment:µSR sample conditions have been upgraded steadily at the D1 and S1 ports. Experiments at tem-

peratures from 20 mK to 500 K can now be conducted. Temperatures lower than 1.5 K are only

achievable with the dilution refrigerator at the D1 port. High demand for measurements below 1.5 K
leads to congestion at the D1 port where various experiments other than µSR are conducted. A 3He

refrigerator is in preparation and will soon be available at the S1 port providing temperatures down
to 0.3 K that will help to alleviate the tight beam demand at the D1 line from low temperature experiments.

New micro transverse field coils installable inside the µSR detector system were introduced that

can apply fields up to 400 G. A new airtight sample holder has been made and a holder for verysmall samples has been made, which is very effective for µSR measurements of such samples withthe aid of a fly-past setup. A new cryo-furnace enabling seamless wide range temperature control
from 1.5 K to 500 K has been installed and is expected to reach 800 K.

–6–

The unified user-interface for temperature and magnetic field control independent of the cryostat

shows good usability features and provides for easy maintenance by the support staff.

MAC evaluates highly the continuing efforts to supply stable and user-friendly measurement

control systems and to extend the range of available sample conditions in terms of temperature andmagnetic field.

D1/S1 µSR Spectrometer/DAQ Upgrade:
Kojima-san reported the development of the new solid-state positron detector system, Kalliope,for which he was awarded the Koshiba-prize in 2018. Although the initial version yielded a

distorted spectrum at early times, these problems have now been solved. Now, Kalliope has
become a very reliable system, which also has enough capacity for the 1 MW operation in thenear future. A potential future issue of noise events related to high proton beam power has been
noted and will require careful testing.

By using the auto-run software, IROHA2, µSR experiments are now more comfortable andreliable than in the past. Furthermore, users can now check the current experimental situationand download their measured data files through the internet. Such progress has made MUSE an
excellent world-class facility in terms of user interface to the experiments.

One great area of progress is in using the combination of small TF coils and a rotating sample rod

controlled by IROHA2 to provide the detailed angle dependence of the muonic hyperfine constantfor a single crystal sample within an auto-run mode.

A 5T muon spectrometer, CYCLOPS, will soon be tested in the S1 area. For the future operationof CYCLOPS that may be installed in the S2 or S3 area, stray fields should be reduced down to anegligible level at the other S areas.

In order to further develop the D1/S1 µSR Spectrometer and DAQ system, particularly for thenear future 1 MW operation the committee recommends to increase the number of staff in this area
to continue the current progress and activities.

Negative Muons at D2:
Creative advances in beam diagnostics enabled a substantial improvement in beam transport ofnegative muons at D2. This enabled implementation of a new collimator system and measurement
chamber and resulted in demonstration experiments with these new capabilities.

This progress, complementary to standard µSR, is excellent, and it should soon move beyond

demonstration experiments and begin to have impact in other fields. We look forward to high profile–7–

uses of muonic X-ray elemental analysis, positive and negative muon-induced single event upsets
diagnosis of operating electronics, and negative muon spin rotation and relaxation (µ-SR) in the
coming year.

U-line/Ultra Slow Muons: Beam Line;
The MAC is pleased to see the good progress made in beam tuning and beam transport

simulations, which now provide a good understanding of the electro-magnetic transport systemof the ultra-slow muon (USM) beam line. As a result of these efforts, the first USM muon spin
precession signals were presented in applied fields of 20 and 200 G for muons with 30 keV energy
stopping in Ag. MAC expresses its congratulations to MUSE on this important milestone.

Compared to the rate reported to MAC last year, the USM rate was doubled, which can be

attributed to both increased proton beam intensity and improved focusing of the surface muon beam
after insertion of an additional axial focusing superconducting solenoid in the U-line.

Problems in the USM extraction after photo-ionization due to both the bent hot tungsten foil

(which distorts the accelerating field) and the stray magnetic field of the additional axial focusing
solenoid have been investigated. The bending of the tungsten foil has been reduced by a different

method of installation of the cold foil, and the vertical beam offset due to the stray magnetic fieldhas been compensated by asymmetric operation of the beam focusing electric quadrupoles, assuggested by beam transport simulations. Further countermeasures against the stray fields using
new magnetic shields are in progress.

Open issues at the moment are:i) the detection efficiency of the MCP detectors used for measuring the USM beam spot andintensity at various positions of the USM beam lineii) the detection efficiency of the positron detectors of the spectrometer and the low positron event rateiii) the low observed decay asymmetry of 0.06.

A proper detector efficiency calibration is essential in order to calculate the rate of ionized Mu

atoms at the tungsten source, using the known transmission from the beam transport simulations. Thisrate 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 – whichis in addition crucial for the planned muon g-2/EDM experiment and the Transmission Muon
Microscope at the H-line.

The low positron event rate reflects a small solid angle of < 20 % covered by the positron–8–

detectors. MAC suggests increasing the solid angle of the positron detectors in order to significantly
reduce the time needed to record a µSR spectrum with the current low USM rate. The low measured

asymmetry of 0.06 can be only partly explained by the 50 % loss of polarization due to Mu
formation at the tungsten foil. The origin of this low asymmetry should be investigated and action
should be taken, if possible, to increase the observable decay asymmetry.

U-line/Ultra Slow Muons: Laser System;
The development and operation of a long-term stable laser system to generate Lyman-alpha lightfor the photo-ionization of thermal muonium is pivotal for the progress of the U-line research
programme. At the same time, it is crucial for the production of a high quality bright positive muon
beam at the H-line for use in the muon g-2/EDM experiment at the J-PARC H-line.

MAC acknowledges, in particular, the progress that has been made in the past year concerning theoperation and the further development of the laser system for muonium ionization at the U-line.

The laser system at the U-line has benefited from well-prepared upgrades concerning the noble

gas optical mixing cell, as well as refinements of the optical beam path, and a 20-year-old pumplaser will be replaced by a more modern system. The laser system appears to be working morereliably now, yet still there are remaining issues concerning the time stability of the produced laser
light, on the scale of hours.

MAC is confident that the measures taken will provide a remedy, and the continuous programme

for the upgrade of components promises further steady improvements. MAC also looks forward

to hearing about results from the new way of producing Nd:YGAG with, in particular, improved
optical quality, and the developments around a new material Nd:YSAG.

MAC appreciates the structured activities that have been started in order to develop a backup

solution concerning the important ceramic frequency mixing material. We appreciate in particular

the professional approach in an area where pioneering technological solutions are being developedby the J-PARC team and their partners. The past hiring of dedicated staff and the attention of
J-PARC management appears to be paying off. New pump laser and new crystal making procedures

promise higher stability and the short-term drop in intensity due to gas impurities should disappear.

The expected overall improvement in Lyman-alpha-light intensity by a factor of 2 to 3 due tovarious system improvements appears to be well justified.

MAC realizes that besides further development of the system, stable operation is also being givenhigh priority, as many experiments depend on the reliable operation of the system. In particular, theremaining issues concerning the obtaining of crystals or optical ceramics of sufficient optical quality

needs further attention and continued full support. The measures taken in this respect reflect a well-–9–designed development programme. We are convinced that this is the right approach, and it will

provide the desired stability for the operation of ultra-cold muon production using muonium laser

ionization at the heart of the new facilities. Being at the core of several activities, strengthening of
the laser expertise by additional staff is strongly advised.

H-line:The H-line will be the spinal cord of a broad new research programme at J-PARC. Its speedy com-pletion is strongly needed and MAC recommends to J-PARC/ KEK management that all possiblemeans should be taken to prioritize and realize the H-line and its promising scientific programme.

We are pleased to learn that the necessary steps were already taken to provide an additional 4 MWof electric power to the experimental hall. We assume that this development, that has now started,
will be completed within the next couple of years. MAC recommends that the necessary funds be
secured for a speedy completion of a fully functional H-line.

The timely completion of the H-line will be important for the scientific success and impact of ex-

periments such as the muonium hyperfine structure, muon g-2/EDM, DeeMe and the muon micro-

scope. All of these projects are timely now and have tremendous scientific discovery potential. The

exciting science programme planned for the H-line can only be enabled with a functioning H-line,
which we recommend to be completed with high priority.

MLF Second Target :Working groups of J-PARC MLF division and of the Japanese Science Societies for neutron andmuon research were organized to discuss the 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. The extraction of the muons from the entrance windowof the spallation target will be achieved by a large acceptance (1 sr) solenoid, and a surface muon

beam intensity increase by a factor of 50 compared to the existing muon beams at target station 1
are expected.

This is a project for the time horizon beyond 2030 and MAC welcomes and encourages the fur-ther development of these initial ideas for future large-scale muon projects at J-PARC.

– 10 –

III. General Comments and Recommendations
MAC appreciates the recent hiring of new staff. This both compensates for the retirement of experienced

personnel in the near future and provides new opportunities. In particular, we appreciate the opening of a
new full professorship for muon science and the potential for future research areas that it offers.

As part of the move from construction to operational mode, the user support requirement fromstaff will naturally increase. We strongly recommend to consider hiring additional staff with part oftheir duties assigned to the ‘instrument scientist’ role that includes assisting new users in preparing
proposals and learning to run their experiments. Developing a strong and stable muon user community relies on such support from the facility.

MAC acknowledges the achievement of stable running of the facility over the last year and we arelooking forward to the scheduled intensity upgrades, working towards 1 MW beam power. These

activities can build reliably on the experience gained from operation in recent years. We appreciate
in particular that safe operation of the facility is visibly given highest attention.

MAC applauds the initiative to set up a new 4 MW power station for supporting in particular the

installation of the H-line in the experimental hall. We recommend a top level of priority for theprovision of sufficient resources for speedy completion of this infrastructure task in order to takeadvantage of this unique scientific window of opportunity.

MAC appreciates the visible progress in further building up the facilities and the high level ofengagement of the mostly young scientists actively working on the instruments in the experimental
hall. With the highest priorities given to the U- and H-lines, we emphasize that the completion of
the remaining S-lines will be very important for the long-term operation of the overall muon facili-ties at J-PARC with a broad science portfolio. Establishing and operating a second S-line spectrometer will soon be needed in order to make efficient use of the available beam intensity as the accel-

erator returns from single pulse to double pulse operation following the planned increases in beam
power.

– 11 –

Charges Given to MAC:
• Evaluate the appropriateness of the facility operation and its upgrades with respect to thefollowing 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 uniqueness of the
facility, attracting not only domestic users, but also international users• Any suggestions for improvements are appreciated. Our particular concerns include, but are notlimited 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 theremaining S beam lines– 12 –