Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

The RIKEN-RAL Muon Facility at the ISIS Neutron and Muon
SourceP. King,∗1,∗2 M. Iwasaki,∗3 K. Ishida,∗3 and I. Watanabe ∗3
The RIKEN-RAL Muon Facility is one of RIKEN’ssigniﬁcant overseas outposts and a major collaboration
between RIKEN and the UK. The Facility is part of the
ISIS Neutron and Muon Source, located at the Rutherford Appleton Laboratory (RAL) in the UK. ISIS isthe UK’s national neutron and muon source, but it has
many international partnership arrangements, and oneof the largest and longest standing is the collaboration
with RIKEN. The Facility performs a very wide varietyof pure and applied studies using muons.
The collaboration began in the late 1980’s under
the leadership of Prof Kanetada Nagamine. The ﬁrst
agreement between RIKEN and RAL to build the Facility and collaborate in muon science was signed in1990 for 10 years. First muons were produced in 1994
and the Facility was oﬃcially inaugurated by the then
RIKEN President Prof Arima in 1995. The Facility
has been performing muon science studies since then,with further agreements between RIKEN and RAL in
2000 (10 years) and 2010 (7.5 years).
In 2015 the Facility celebrated 20 years of muon production and 25 years of formal agreements between
RIKEN and RAL. Over its lifetime, the Facility has
been recognized in a number of ways. This includes
regular presentation at meetings of the UK-Japan
Joint Committee on Co-operation in Science and Technology chaired by the UK Chief Scientist, where it isused as a good example of UK-Japan science collaboration. In 2003 Prof Nagamine received the Toray
Science and Engineering Prize from the Toray Science
Foundation of Japan, and in 2002 Dr Paul Williams(CLRC Chief Executive) received the Minister’s Commendation for Meritorious Service in the Field of International Exchange from the Japanese Minister ofEducation, Culture, Sports, Science and Technology.
Muons for use in the Facility are produced by thedecay of pions generated by the interaction of the ISIS
800-MeV proton beam with a 1-cm thick carbon target.
The Facility is able to accept “surface” muons, produced from pion decays within the carbon production
target. These are positive muons with high ﬂux andwith momentum of around 30 MeV/c which are thendirectly transported to the experiment areas. The Facility also incorporates a decay channel, within which
pions from the carbon production target are allowed todecay in-ﬂight. This enables higher momentum (up toaround 110 MeV/c) positive muons to be produced, as∗1∗2∗3Fig. 1. The RIKEN-RAL Muon Facility at the ISIS Neutron and Muon Source, UK. Muons or pions are generated at the production target, and muons are transported to one of four experiment areas. The superconducting solenoid allows decay of pions in-ﬂight, enabling
variable momenta positive or negative muons to be produced.

well as allowing negative muons to be generated. Thismeans that the facility is very ﬂexible, enabling both
positive and negative muons with variable momenta tobe generated for muon science experiments.1)
The ISIS primary proton beam is pulsed at 50 Hz,
meaning that the RIKEN-RAL Muon Facility is apulsed muon source. The science that can be doneat the Facility is, in many ways, complementary tothat which can be done at continuous muon sources
at PSI in Switzerland or TRIUMF in Canada. The
RIKEN-RAL Muon Facility is the only pulsed sourceof decay-channel muons in Europe, and so is a uniqueresource for muon studies.
The science of the facility is very varied, and broadly
falls into two categories: fundamental physics studies
using muons, and condensed matter and molecular science employing muons as probe particles.
A key science driver for the facility when it wasﬁrst established was investigation of the process ofmuon catalyzed fusion (μCF). μCF is a cold fusion
process occurring between deuterium and tritium nuclei. A dtμ molecule is formed within a liquid or solid
deuterium-tritium mixture, which enables the d andt nuclei to approach closely enough to fuse, releas-

Director of RIKEN Facility Oﬃce at RAL, RIKEN Nishina
Centre (2012–present)
STFC ISIS Neutron and Muon Source
RIKEN Nishina CentreS-43

RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

also feature, for example in organic 11) and inorganic(battery) 12) materials.
The Facility has seen development of key techniques for condensed matter and molecular studies using muons. These include laser-stimulation of samples, allowing excited state spectroscopy,13) and studies of materials under pressure which are enabled bythe higher momentum beams available from the decay
channel.
The traditional muon technique for condensed matter studies allows the bulk of the material to be probed,
as muons penetrate a large fraction of a mm into a sample. In order to allow studies at much smaller depths
closer to the surface, and interface studies, work has
been ongoing to develop a source of low energy muons
on Port 3 of the RIKEN-RAL Muon Facility. The
method being used is particularly suitable to a pulsed
muon facility, and involves laser ionization of muonium
atoms (positive muons which have picked up an electron to form a light hydrogen atom with the muon
as nucleus) to produce keV muons as opposed to theMeV muons normally used. The method has been extensively developed at RIKEN-RAL, and the characteristics of the low energy muon beam that is produced
have been detailed.14) This method is now in operation
at the J-PARC muon facility following its development
at RIKEN-RAL. It oﬀers the potential for fundamental muon studies, such as muon g − 2 measurement, as
well as condensed matter investigations.
Recently, new applications of muons have been developed at the facility. This includes studies of singleevent upsets in microelectronic circuits 15) and elemental analysis of materials with negative muons.16)
Novel methods are also being developed for fundamental studies of the proton radius 17) in order to resolve
discrepancies between diﬀerent measurements on thissubject. Overall, the Facility has produced more than
400 publications since its beginning in 1994, across avery broad range of pure and applied subject areas using muons.
A particular strength of the facility has been the development of collaborations and the attraction of researchers from around the world. Over 80 separate
Japanese groups have used the facility over its lifetime,and around 40 groups worldwide have collaborated on
science at the facility. Particularly of note is the development of links between RIKEN and Asian universities, resulting in MoUs with ﬁve Indonesian universities (ﬁrst signed in 2008 and renewed in 2011 and2015), and an MoU with Universiti Sains Malaysia,
ﬁrst signed in 2012 and renewed in 2015. These Asianlinks have resulted in multiple workshops and meetings, a variety of publications, in, in particular, theinvolvement of around 10 masters and PhD students
from Indonesia and Malaysia whose theses have been
based on work performed at RIKEN-RAL.

Fig. 2. Adam Berlie, joint RIKEN-RAL / ISIS appointment, with the CHRONUS spectrometer.

ing energy and characteristic reaction products. The
RIKEN-RAL Muon Facility provided an ideal sourceof negative muons to enable this process to be studied
in detail, including revealing new eﬀects in its temperature dependence.2) Other fundamental physics studies
at the facility have included precise measurement ofthe muon lifetime 3) and muonic atom spectroscopy forstudies of nuclei.4)
A key application of muons is the study of atomicand molecular-level properties of materials in condensed matter physics and chemistry. Muons produced
at RIKEN-RAL are spin polarized, an eﬀect occurring
naturally from the pion decay. Positive muons maintain their polarization when implanted into materials,
and observation of the time-dependence of the polarization of the muon ensemble within a sample provides information on local magnetic ﬁelds. In turnthis informs on the magnetic properties of the material as well as enabling processes such as diﬀusion tobe studied. The polarization of the implanted muons
is observed by detection of the positrons emitted whenthe muons decay after their average lifetime of 2.2 μs.
These positrons are preferentially emitted in the direction of the muon spin at time of decay, enabling
the behavior of the muons with a material to be followed. The Port 2 area of the Facility has been hometo the ARGUS muons spectrometer since the earlydays of the facility, and this was supplemented with
the CHRONUS spectrometer on Port 4 which began aμSR programme in 2014.
Studies of a wide variety of magnetic and superconducting materials have been a feature of the RIKENRAL Muon Facility. This includes, for example,
studies of critical behavior around magnetic transitions,5) model organic systems,6) non-Fermi liquid
spin dynamics,7) the interplay between magnetism andsuperconductivity,8) spin liquid systems 9) and otherfrustrated systems.10) Diﬀusion and charge dynamics

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RIKEN Accel. Prog. Rep. 50 (2017)

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