Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

Historical overview of the RIKEN-RAL Muon Facility
from catalyzed fusion to materials/life science
K. Nagamine ∗1,∗2a) fusion phenomena within a small muonic atom/
molecule and its successive reactions, b) no need forhigh temperatures, c) no radioactive waste production,d) no fuel problem, e) no critical phenomena for freerunning, and f) already realized in a laboratory scaleand only a few eﬀorts are required for a break-evenachievement.3)
μCF consists of the following processes: 1) the formation of a small muonic molecule and subsequent
intra-molecular fusion reaction, and 2) the mediationof a chain of fusion reactions by a single μ− (Fig. 2),
up to more than 100 cycles. The numbers such as rates
are quoted from a standard textbook.
After μ− injection and stopping in a D-T mixture,
either a (dμ) or a (tμ) atom is formed. Because of
the diﬀerence in the binding energies, the μ− initiallyin the (dμ) undergoes a transfer reaction to the (tμ),
which reacts with D2 or DT to form a muonic molecule(dtμ) at a rate of λdtμ , where the formation of a speciﬁc state of (dtμd2e− ) through a resonant molecularformation (RMF) takes place. The (dtμ) molecule, after a rapid cascade transition, causes a fusion reaction
since the distance between d and t is suﬃciently close
to allow fusion to occur, producing a 14.1-MeV n anda 3.5-MeV α.
After the fusion inside the (dtμ) molecule, most of
the μ− is liberated to participate in the second μCF.
Some small fraction is captured by the emitted α++(called the initial sticking probability ωS0 ). Once the(αμ)+ is formed, because of kinetic energy, the μ− canbe stripped from the (αμ)+ ion and liberated again

Introduction
Construction of the RIKEN-RAL Muon Facility was
proposed by RIKEN in 1989. It had been realized as
the ﬁrst large-scale scientiﬁc contribution from Japanto UK. Major research outputs achieved thus far andfuture prospects are brieﬂy summarized here.
Birth of RIKEN-RAL Muon Facility
At the time of the cold fusion (proposed nuclear fusion phenomena of D2 conﬁned in special materials
such as Pd) in 1989, muon-catalyzed fusion was recognized as a real nuclear fusion process without theneed for high temperature. Immediately after the coldfusion report, RIKEN proposed the construction of amuon facility at the world’s strongest pulsed proton
accelerator facility, the Rutherford Appleton Laboratory in UK, and the proposal was approved by theJapanese Government in 1990.1) A facility composedof a superconducting pion-decay section with 4 muon
extraction lines was designed, constructed, and completed in 1994, as shown in Fig. 1, and various experimental programs were started.2)
Muon Catalyzed Fusion (μCF)
Among the various muon sciences, μCF, which is
nuclear fusion catalyzed by the muon, is the most important as a new possible energy source. Fascinating
features of the μCF can be summarized as follows:

Fig. 1. Layout of the RIKEN-RAL Muon Facility in 2010.
∗1∗2

Chief Scientist of Muon Science Laboratory (1984–2002)
Muon Science Laboratory, IMSS, KEK

Fig. 2. Conceptual diagram of the D-T μCF cycle.

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Research Activities 1991–2016(regeneration fraction R). Thus, μ− of either a nonstuck μ− or a regenerated one can participate in thesecond μCF cycle, while the thermalized (αμ)+ is left
out from the μCF cycle with an eﬀective sticking ωS =(1 − R)ωS0 . More than 100 fusion processes are known
to take place with a single μ− .
The phenomena of sticking and regeneration weredeeply investigated at RIKEN-RAL by measuring both
the X-ray from stuck (αμ)+ representing initial sticking and the yield of fusion neutrons representing resultant phenomena including regeneration. Here, thestrong pulsed muon was essentially helpful to remove the background related to T β-decay. The
temperature-dependent measurement was conducted
for solid/liquid D-T.5) Although the X-ray yield remains constant, the loss rate decreases at higher temperatures; the regeneration seems to increase in thehigh-temperature solid (Fig. 3).
At RIKEN-RAL, the muon molecular formation ratein D2 μCF (λddμ ) was investigated by controlling
ortho- and para-states to study the energy matching inRMF. Contrary to theoretical expectation, the opposite tendency was discovered in solid and liquid D2 .6,7)
Although the result is preliminary, a similar opposite
tendency was discovered at RIKEN-RAL for the μCF
in liquid D-T by using ortho- and para-D2 with T2 in a
non-equilibrium liquid mixture.8) The observed orthopara dependence of the fusion neutron yield seems tobe opposite to the theoretical expectation; a greater
fusion neutron yield was observed for the ortho case inexperiment. All of these observations clearly demonstrate the importance of the molecular correlation effect in RMF in D2 and D-T μCF. The molecular correlation may change the energy-level diagram in D2 andD-T in a scale of meV at an energy spacing of the orderof eV.
Based on these results, several remarks can bemade on possible further increase in the energyproduction capability. By using ortho-D2 rich andhigh-temperature (30 K) solid D-T μCF towards economical break-even, the following can be achieved:a) ortho-para optimized D2 can be used to increase

λdtμ to 800 μs−1 b) and in order to decrease ωS (increase R), use of high-temperature solid D-T at 30 K
may cause a decrease of ωS to 0.1% (increase of Rto 0.9%). Then, the fusion neutron occurrence is
1200/μ− , which corresponds to an energy production
capability of 21 GeV/μ− , well exceeding the cost ofmuon production of 5.0 GeV/μ− .
Energy production in solid D-T (30 K) is not realistic. Another idea is has been proposed: strongly correlated μCF. Let us consider the spatial structure of
successive D-T μCF chain, where α++ ions are emitted in a space of within (120 μm)3 from a conﬁned
spot (within (0.2 μm)3 towards a larger (αμ)+ ion
space (within (480 μm)3 ). Therefore, if we have strong
pulsed muons with stopping region and μCF region
overlapping each other, because of collisions between
the (αμ)+ ions and α++ ions from nearby μCF reactions, exotic regeneration of (αμ)+ stripping throughhigh-temperature plasma caused by α++ may occur
in the high-density μCF of D-T mixtures at moderately high temperature. There, ωS = 0.1% can be expected, leading to the same energy production capability, which is beyond the break-even energy balance.

Fig. 3. Observed X-ray spectrum 4) (left) and temperaturedependence of muon loss probability (ωS ), and the ratioof X-ray neutron yield for D-T μCF 5) (right).

Fig. 4. Observed phase diagram of LSCO by a change ofrelaxation rate of muon (open circle) compared withelectron resistivity (closed symbols).

Condensed Matter Physics
By using the capability of long-time-range measurement of the μ+ SR time spectrum, various condensed
matter studies have been proposed and conducted,
mainly by external users amounting to more than 200
scientists. Topics covered a) magnetic properties ofhigh-TC superconductors, b) dynamic character of exotic magnetic materials, c) diﬀusion of μ+ in variousmaterials d) reactions of μ+ and muonium in chemicalmaterials, and e) light-element diﬀusion in battery materials. A distinguished example is presented below.

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RIKEN Accel. Prog. Rep. 50 (2017)

Careful measurements of zero- and longitudinal-ﬁeld
muon-spin relaxation have been performed in order toinvestigate the dynamics of the electronic and magnetic states in the normal region of La2−x Srx CuO4
over a wide range of hole concentrations from 0.024
to 0.15.9) It has been found that the dynamic depolarization rate of muon spins in zero ﬁeld starts toincrease monotonically from approximately 0.02 μs−1
with decreasing temperature at a high temperature ofapproximately 100 K. This eﬀect is enhanced near ahole concentration of 1/8 per Cu, suggesting that thedynamics of ﬂuctuating internal ﬁelds at the muon site
in the normal state of La2−x Srx CuO4 changes at a hightemperature of approximately 100 K and is correlatedwith the mobility of holes (Fig. 4). The result should
be related to the origin of high TC superconductivity.
For the μ+ SR studies of magnetism at surfaces andinterfaces, the production of ultra-slow μ+ is in highdemand. By coupling pulsed lasers with pulsed muons,a production method of ultra-slow μ+ by the laser
resonant ionization of thermal muonium was developed at KEK. Advancements have been achieved at
RIKEN-RAL by placing the system at the experimental port of a surface muon beam line 10) (Fig. 5). Energy was proved to be tunable from 0.1 keV to 20 keV.
Space-time structure advancement was conﬁrmed. The
project is now under further development at J-PARC.

Most elementary molecules have been isolated, andtheir complicated structures have been determined one
by one through crystallographic analysis. The remaining problem is the elucidation of the electron transfer
mechanism.
Cytochrome c is a small hemoprotein and one of themembers of the respiratory chain in mitochondria. Ittransfers an electron from protein complex III to IV,coupled with redox reactions of Fe located at the centerof a porphyrin ring.
At RIKEN-RAL, μSR spectra of cytochrome c andmyoglobin were measured.11) μSR data were analyzed
by the Risch-Kehr (R-K) theory, which was developedin order to analyze the μSR data of conducting polymers. The principle of the electron transfer in biological macromolecules studies by μSR is as follows(Fig. 6). During the slowing-down process, the injected
μ+ has a chance to pick up electron(s) to form a neutral
muonium. Then, the muonium becomes thermalized,stops at an electronegative site, and forms a chemicalbond to a molecule. Then, depending on the natureof the molecule, the electron brought in by the μ+ becomes localized to form a muonic radical state and/or
diﬀuses rapidly along either intra-molecular chains orinter-molecular paths. These behaviors of the electron
caused by the μ+ can sensitively be detected by measuring the spin-relaxation process of the μ+ by using
the μSR method. The discrimination of intra- andinter-molecular electron transfer can be discussed on
the basis of the magnetic-ﬁeld dependence of relaxation rates. Thus, the spin relaxation of the μ+ due tomagnetic interaction with the moving electrons nearmuons was derived as a characteristic function (R-Kfunction), G(t) = exp(Γt)erfc(Γt)1/2 , for a rapid electron spin ﬂip rate with the relaxation rate Γ, which

Life Sciences
By utilizing the unique and strongest pulsed muons,long-time range (more than 10 times the muon lifetime) became easily measured; consequently, the delicate muon spin response in biological environments iseasily detectable.
The electron-transfer process plays highly important roles in biological activities. Two representative examples are the photosynthetic electron transfer chain and the respiratory chain in mitochondria.

Fig. 6. Conceptual diagram showing the measurementof electron transfer in macromolecules by the μ+ SR
method (left), and the measured temperature dependence of longitudinal (right-up) and transverse (rightdown) diﬀusion rate.

Fig. 5. Conceptual diagram showing the generation of an
ultra-slow muon from an MeV muon by the laser ionization of thermal muonium (left). Layout of the dedicatedexperimental facility installed at port 3 10) (right).

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Research Activities 1991–2016

has a characteristic dependence on the applied longitudinal ﬁeld (B). In the case of one-dimensional intramolecular electron motion, Γ is proportional to 1/B.
This experiment and several succeeding experiments
on proteins and DNAs mainly performed at RIKENRAL have revealed the following facts: a) electron
diﬀusion along the chain is temperature independent,
while inter-chain diﬀusion has T-dependence reﬂecting
glass transition at 100 K (Fig. 6); b) the hydrationeﬀect causes enhanced diﬀusion; c) the stopping position of the μ+ in proteins seems to be away from Fecontaining porphyrin but close to the part of the main
poly-peptide chain.
Conclusion and future of RIKEN-RAL
As an extension of the pulsed muon facility, a facility to produce stronger pulsed muons has been constructed in Japan, the J-PARC MUSE, by KEK andJAEA. However, it is still important to keep RIKENRAL, since various experimental instrumentations are
more easily equipped at RIKEN-RAL, promoting theincrease of proposals and users. The author is grateful for the long-time collaboration of the following
persons who contributed to the development of theRIKEN-RAL Muon Facility: Drs. M. Iwasaki, T. Matsuzaki, K. Ishida, I. Watanabe, R. Kadono, Y. Matsuda, S.N. Nakamura, P. Bakule, and F. Pratt.
References
1) D. Swinbanks, Nature 354, 98 (1991).
2) T. Matsuzaki et al., Nucl. Instrum. Methods Phys.
Res. A 465, 365 (2001).
3) K. Nagamine, in Introductory Muon Science (Cambridge Univ. Press, Cambridge, 2003), p. 694) S. N. Nakamura et al., Phys. Lett. B 473, 226 (2000).
5) N. Kawamura et al., Phys. Rev. Lett. 90, 043401(2003).
6) A. Toyoda et al., Phys. Rev. Lett. 90, 243401 (2003).
7) H. Imao et al., Phys. Lett. B 632, 192 (2006).
8) K. Ishida et al., Proc. of μCF-07 (Dubna, 2007) p. 99.
9) I. Watanabe et al., J. Phys. Soc. Jpn. 77, 124716(2008).
10) P. Bakule et al., Nucl. Instrum. Methods Phys. Res. B266, 335 (2008).
11) K. Nagamine et al., Physica B 289–290, 631 (2000).

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