FEATURE ARTICLE

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

RNC’s initiatives in the ImPACT program

nuclear reaction control methods,

The ImPACT Program
Since 2007, Radioactive Isotope Beam Factory(RIBF) 1) has been one of the world-leading heavy-ion accelerator facilities for low-energy nuclear physics. RIBF
has contributed to the expansion of the nuclear chart by
producing more than 150 new isotopes and has provided
a multitude of opportunities to discover new phenomena
originating from high isospin asymmetry. In parallel tosuch pure nuclear physics activities, a new and national
program of nuclear engineering was launched for social
beneﬁts. In the period of 2014–2018, the program entitled “Reduction and Resource Recycling of High-level
Radioactive Wastes through Nuclear Transmutation” 2)
was carried out as one of the ImPACT R&D programs
conducted by the Cabinet of Japan.
Since Hideki Yukawa left the Japan Atomic Energy
Commission in 1957, the nuclear physics and nuclear
engineering communities have been separated in Japan.
The incident of the Fukushima Dai-ichi Unit in 2011,
however, impelled Japanese nuclear physicists to seriously consider possible contributions to nuclear engineering. In 2014, a demonstration experiment was performed with RIBF. In this experiment, the inverse kinematics with energetic radioactive-isotope (RI) beams of137
Cs and 90 Sr was employed to study spallation reaction to ﬁnd an eﬃcient reaction pathway for reductionof the radioactivity.3) The experiment provided an opportunity to the physicists and engineers to work together. The potentials of RI beams at RIBF were also
demonstrated, and the fundamental reaction data obtained with RI beams were found highly useful for designing an accelerator-based transmutation system.
High-level radioactive waste has two main components: long-lived ﬁssion products (LLFPs) and minor
actinides (MAs). The transmutation of MAs has beenstudied in detail for the purpose of reprocessing spent nuclear fuel under the concept of using fast-breeder reactors
or accelerator-driven systems. The LLFP nuclides aresupposed to create risks in the geological disposal strategy because their half-lives are much longer than thoseof MAs. However, the transmutation of LLFPs has notbeen studied extensively. Considering the present situation of R&D activities in the ﬁeld of the radioactive
waste, the ImPACT program was jointly proposed by the
nuclear physicists and engineers, to focus on acceleratorbased transmutation of LLFPs.
The ImPACT program emphasized on the “resource
recycle” of radioactive waste. Thus, 107 Pd and 93 Zr werehighly prioritized as objectives. The program consisted
of ﬁve projects as follows:
Project-1: Development of separation and recovery
technologies,
Project-2: Production of nuclear reaction data and new

Project-3: Reaction theory modeling and simulation,
Project-4: Evaluation of the nuclear transmutation system and development of elemental technologies, andProject-5: Process design concept by nuclear engineers.
All the projects were managed coherently, and Project-2
and Project-4 were conducted by RIKEN Nishina Center.
An overview of the activities in Project-2 and -4 ispresented in this article.
Transmutation Reactions for Accelerator System
To design an accelerator system for the transmutation, the possible reactions for the transmutation wereintensively discussed at an early stage of the ImPACT
program.
First, we adopted nuclear reactions, not electromagnetic reactions. In the case of nuclear reactions induced
by “hadron” particles, which interact strongly with LLFP
nuclei, several reactions, such as spallation, knockout reactions, are applied with reaction cross sections of the order of 1 barn. Conversely, in the case of electromagnetic
reactions, the reaction candidate is only the photoabsorption process (γ, n), and its cross section is approximately 0.2 barns for LLFPs. To induce the (γ, n) reaction, secondary high-energy photons, of which energy ismore than 10 MeV, should be produced by high-energy
electron beams through the bremsstrahlung process in aconverter material or through the inverse Compton process with intense laser lights. However, the high-energy
photons are wasted to create the electron-positron pairs,
simply because the cross section of the pair creation is
10 barns, much higher than that of the photoabsorption. Thus, most of the electron-beam energy is wasted
in creating the electron-positron pairs, not in transmuting LLFP nuclides signiﬁcantly.
Next, several types of nuclear reactions were considered for transmutation and were categorized with respect
to transmutation throughput and reaction controllability. The candidate reactions are spallation reaction, neutron knockout reaction, such as (n, 2n), and muon capture reaction. The spallation reaction induced by highenergy proton- or deuteron-beams is very promising interms of throughput, but it is not excellent in terms ofcontrollability. When the spallation reaction is applied,the beams are irradiated directly to LLFP materials,
and the target thickness is determined from the stopping range of the beams. Higher energy beams lead tothicker targets, and hence, the throughput is increased
as a function of beam energy. However, according to theresults obtained in Project-2, higher beam energy produces more nuclides as spallation products, because a- S1 -

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RIKEN Accel. Prog. Rep. 55 (2022)

higher energy beam deposits higher energy in the target
nuclei, and the higher deposited energy in the nuclei isused to emit more protons and more neutrons from thenuclei. Thus, in the case of the spallation reaction, theoptimal beam-energy should be carefully considered interms of not only the throughput but also the costs necessary to treat radioactive nuclides produced in LLFP
materials after long-term irradiation.
The neutron knockout reaction is induced by highenergy neutrons with energy more than 8 MeV. In thecase of even-odd separated LLFP, especially for the Pd
nuclides, the neutron knockout reaction has very highreaction controllability. After the even-odd separation,
105
Pd and 107 Pd are left as nuclear waste (105 Pd is a stable nuclide, and 107 Pd is a long-lived one). The (n, 2n)reaction produces 104 Pd and 106 Pd which are both stable, and (n, 3n) reaction also could be applied to produce103
Pd and 105 Pd. The 103 Pd nuclide is not stable, but its
half-life is as short as approximately 17 days. The neutron energy range appropriate for (n, 2n) is 8–18 MeV,
and it gives the cross section of approximately 2 barns.
Higher energy neutron beams also induce (n, 3n). The
high energy neutron beams could be created by high energy deuteron beams via breakup reaction. The broad
energy range of neutrons is very useful because this leads
to the generation of high ﬂux neutron for transmutation.
The target thickness for the neutron knockout reaction
is determined from the mean-free path of the reaction,
which is of the order of 100 g/cm2 in Pd metal.
The other reaction candidate is the negative muon capture reaction. The negative muon capture process also
has high reaction controllability. A negative muon is aunique particle, which interacts electromagnetically andcan form a muonic atom where, like an electron, the negative muon is trapped by a nucleus. Then, the muon is
captured by a proton inside the nucleus by weak interaction, and the proton is converted to a neutron. The
capture probability per muon is almost 100% for Z >40 nuclides, when the negative muons are implanted tothe material with Z > 40. Because s muon is approximately 100 times heavier than an electron, the energy
transfer in the process is rather high, and a few neutrons
are emitted from the nucleus. Hence, the muon capture has reaction controllability for the proton number
and decrements the number by one. This feature of themuon capture is very useful for chemical separation. In
the case of Pd with atomic number of 46, the products
are only Rh with atomic number of 45. Although thenegative muon is very promising in terms of controllability, the production of a muon is rather energy consuming; the production of one muon needs 5 GeV. A negative
muon is created through decay of a negative pion, andthe pion is created by a high-energy nuclear reaction, forexample, proton induced nucleus collision, in which theproton energy should be higher than 300 MeV. The pion
production cross section highly depends on the energyof beams and isospin of beams. Accelerator facilities formuons, such as TRIUMF, PSI, RAL, and J-PARC, de-liver high-energy and intense proton beams with energyof more than 500 MeV, and the target thickness is limited
to avoid pion capture in the target. These facilities are
not optimized for negative muon productions. Instead ofproton beams, deuteron beams can produce four times
more negative muons than proton beams because of theisospin eﬀects.
Negative muons are also useful to produce 14 MeV
neutrons via muon catalyzed fusion of d + t. In the process, one muon can create approximately 100 14 MeV
neutrons. Thus, the energy cost for a 14 MeV neutron isapproximately 50 MeV. This value is smaller than thatin case of an accelerated deuteron beam inducing d + t
fusion, which is approximately 2 GeV.
Regarding the neutron capture process, it is noted thatneutron capture cross section for 107 Pd is higher than 1barn at a neutron energy below 100 keV and 10 barns at
10 eV to 1 keV. Thus, neutrons produced via spallation
reaction are also very useful for transmutation. In thecase of the even-odd separation scheme for the Pd nuclides, the neutron capture changes 105 Pd and 107 Pd to106
Pd and 108 Pd, respectively, which are both stable nuclides. Thus, a combination of the knockout reaction andneutron capture could lead to high throughput for transmutation. Considering all the above, we have decided todesign an accelerator system for deuteron beams. The
deuteron beams are utilized for a variety of applications:spallation reaction in direct irradiation as well as spallation neutron production, high energy neutron production
via the deuteron breakup reaction, and eﬃcient production of negative muons.
Nuclear Reaction Data with RIBF
Project-2 aimed to produce reaction data of the LLFPs
that could be evaluated and utilized as input for a simulation code as part of Project-3. As introduced in theprevious section, all the possible reactions for transmutation were discussed: proton-, deuteron-, and neutroninduced reactions and muon capture reactions.
In the case of direct irradiation of charged-particles
into an LLFP material, an energy-dependence study of
the reaction is quite important because of the stopping
power in the material. This argument is also applied
for the high-energy neutron-induced spallation reaction.
A challenge for the energy-dependence study is to build
a bridge over the Fermi energy; at a lower energy, thecollective natures of the reaction are dominant and at ahigher energy, the nucleon-based natures become dominant.
According to the experiences in the previous experiment of 137 Cs and 90 Sr,3) the reaction study for proton-,deuteron-, and neutron-induced reactions was conducted
in inverse kinematics at RIBF under a large domestic collaboration comprising the University of Tokyo, Tokyo Institute of Technology, Kyushu University, Miyazaki University, Niigata University, and RIKEN. Compared withthe traditional activation method via normal kinematics- S2 -

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FEATURE ARTICLE

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(;D D D.$ @.@?;87,)9?@;.D7AD!;83)7D18;>.D6.?18-D .-9DD D.%@DFig. 1. Reaction study for long-lived ﬁssion products at RIBF in the ImPACT program. Major spectrometers such as, ZeroDegree, SAMURAI, and SHARAQ, were utilized.4)

with stable beams and RI targets, the inverse kinematics
technique gives clear particle identiﬁcation for reaction
products and easy control of RI beam energies for the
study of energy-dependence. In addition, we do not haveto prepare RI targets but stable-isotope targets, such as
protons and deuterons. All these advantages of inverse
kinematics result in reaction data of excellent quality at
RIBF.
As shown in Fig. 1, intense beams of LLFPs, such as
107
Pd and 93 Zr were produced at the in-ﬂight separator “BigRIPS” via ﬁssion reactions with a uranium beam
accelerated at Superconducting Ring Cyclotron (SRC),
and then were delivered to three spectrometers (ZeroDegree, SAMURAI, and SHARAQ), which were utilized
to identify and analyze the reaction products.5–9) ZeroDegree is suitable for inclusive measurements with relatively heavy fragments. SAMURAI has a wide acceptance in both momentum and scattering angle for reaction products. Hence, semi-exclusive measurements were
performed to detect reaction products as well as neutrons
in the projectile frame. At the SHARAQ spectrometer, CNS, the University of Tokyo and RIKEN worked
together to develop an eﬃcient deceleration scheme for
RI beams. A new device “OEDO” 10) was installed in
the SHARAQ beamline to control emittance growth bytransverse RF electric ﬁelds. An operation mode of
BigRIPS was developed to realize energy-decelerated
beams.11,12) The combination of OEDO and energydecelerated beams successfully led to a reaction study
at an energy of 20 MeV/nucleon.
Enhancement of element production by incomplete fusion reaction with weakly bound deuteron was observed
at 50 MeV/nucleon,9) as shown in Fig. 2, and the incident energy was much higher than the energy scale ofcomplete fusion reaction. The reaction mechanism could
be applied to produce a higher Z element by increasing
the atomic number by 2 via heavy loosely bound nuclei
such as 9 Be and 6 He. Another highlight is a deductionof neutron capture reaction via the Trojan horse methodwith a low energy RI beam at 20 MeV/nucleon.13) In
future, the new method can be widely applied not only
for nuclear engineering but also for nuclear astrophysics.
All these data stimulated theoretical works, and the
JAEA nuclear data group developed a new data library“JENDLE/ImPACT–2018” for proton- and neutroninduced reactions up to 200 MeV/nucleon.14) The newlibrary has been built in as a package of PHITS simulation code.15)
A test of 107 Pd transmutation with macroscopic quantities was conducted with normal kinematics by utilizing- S3 -

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RIKEN Accel. Prog. Rep. 55 (2022)

a low energy deuteron beam at the AVF cyclotron at
RIBF.16) Almost 100% enriched 107 Pd was prepared asa target and irradiated for several days. After the irradiation, the reaction products were identiﬁed from gammaray and ICP-MS measurements. The production yields
were found consistent with PHITS predictions.15)
Accelerator System
Project-4 proposes an accelerator-based transmutation system for LLFPs, and focuses on three objectives:(1) to design an accelerator system with a possible transmutation scenario, (2) to develop essential technologies
for a high-power accelerator system, and (3) to encourage ideas for new high-power and energy-saving accelerators. Four activities by the RIKEN Nishina Center are
selected and brieﬂy introduced.

Linear accelerator system ImPACT2017
Delivering high-energy and intense deuteron beams isessentially important for the nuclear transmutation as
described in the second section. The next question is theintensity of deuteron beams. In the existing facilities inoperation, the maximum intensity of proton beams is of
the order of 1 mA. As discussed in the ImPACT project,
the beam intensity necessary for the transmutation isat least 1 A, which is three orders of magnitude higher
than that of the present facilities. It should be noted that
1 A corresponds to approximately 1 mol/day. If all the
beams contribute to the transmutation, the throughput
would be approximately 100 g/day for the mass numberof 100, which corresponds to 36 kg/year.
To achieve a 1-A deuteron beam, we have made aconceptual design for a linear accelerator system called“ImPACT2017,” 17) as shown in Fig. 3. There are a few
key aspects of ImPACT2017. First, the system does notstart with a radio-frequency quadrupole (RFQ). Second,the RF cavity of the linear accelerator is made as a single
cell. The RFQ device is convenient and widely employedbecause a single RFQ device has three functions: beam
bunching, acceleration, and focusing. However, a small
beam aperture of RFQ (∼1 cm) limits the beam current
due to the space charge eﬀect. Thus, we must develop
other types of acceleration scheme, which is capable fora large size beam. To accelerate a high-intensity beam,
we must supply high RF power to the cavities. A standard conﬁguration has several cells in one cavity, and the
RF power is supplied to the cavity through RF couplers.
Each RF coupler has a limitation for transmitted power;
hence, many RF couplers should be attached to the cavity. In this case, a multi-cell conﬁguration may cause
problems in operation. Decrease the number of RF couplers is signiﬁcant for reasonable operation of the linear
accelerator, and we have decided to employ a single cell
conﬁguration for the cavity. Further details of the newidea are found in Ref. 17).

Fig. 2. Proton-induced and deuteron-induced cross sections
for the 103, 104, 105 Ag isotopes produced by 107 Pd as a function of total incident energy, taken from Ref. 9). Cross
sections on proton and deuteron are displayed by circles
and squares, respectively. Enhancement of the 103, 104 Ag
cross sections with deuterons is clearly observed at theincident energy of 100 MeV, i.e., 50 MeV/nucleon for
deuterons.

R&D of key technologies for high-power accelerator systemAs essential components for high power accelerator
systems, a superconducting RF cavity for low-velocity
charged-particles, material-free window-system for vacuum sealing, and liquid-target system for production
targets have been developed by the RIKEN acceleratorgroup.
R&D of a quarter-wave superconducting RF-cavity for
low-velocity charged-particles 18) is essential to achieve
low power consumption and space-saving. A prototypeof the cavity was designed, constructed, and tested. The
cavity was successfully cooled down to 4 K with liquid
He, and acceptably high values of Q (∼109 ) and acceleration electric ﬁeld (up to 9 MV/m) were achieved. Basedon the experiences in this R&D process, the linear accelerator RILAC has been recently upgraded by installinga similar type of superconducting RF-cavities.19)
A plasma-window with a diameter of up to 20 mm wasdeveloped as the material-free window system for vacuum sealing. The achieved pressure and the V -I characteristics as a function of diameter were investigated.

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free society to avoid global warming. However, risk management in reactor operation has become a serious concern since the Fukushima incident, and the cost of nuclear energy has increased. Regardless of the promotionof or opposition to nuclear energy, the problem of highlevel radioactive waste needs to be solved because thewaste already exists. In Japan, approximately 18,000tons of spent fuel exists, and the location for the geographical disposal has not been decided. Mitigating therisk of the waste is an important issue because this negative legacy should be left to posterity. Pure nuclear
physics is now playing an important role in realizing aradioactive-waste free society.
Fig. 3. Schematic of the 1-A Linac, taken from the cover pageof Proc. Jpn. Acad. Ser. B 95(7).17)

Detailed information can be found in Ref. 20).
The new idea of a target accepting the high intensity
beam was proposed and published as patent.21)
Summary and Future Perspectives
The ImPACT program ran from 2014 to 2018 as theﬁrst step in the nuclear transmutation of LLFPs and successfully stimulated the interest of the nuclear chemists,
nuclear physicists, accelerator physicists, and nuclear engineers in Japan. The program created new data as well
as new ideas. In the second step, it is necessary to identify R&D items and proceed. For example, the transmutation scenario with high-energy neutrons still needs
nuclear reaction data with 99 Tc and other nuclides. In
addition, 1-A Linac would be highly desirable. R&D fora target to accept the very intense deuteron beam must
be conducted in the future. It should be noted that theseR&D can be linked to our future of pure nuclear physics,
where high intensity beams may become necessary. The
methodology of inverse kinematics can also be extended
to a reaction study for MAs.
In the program, as an alternative scenario, a transmutation with 14 MeV neutrons produced via muoncatalyzed fusion was also discussed; for which, an efﬁcient production of negative pions has been proposedwith an FFAG accelerator.22) To promote further themuon scenario, production cross sections of negative pions with a deuteron beam would be highly desirable. A
negative muon was also recognized as a charming particle that causes muon capture reaction and changes theatomic number of nuclei capturing the muon. Based on
the muon capture reaction, a new application of muons
was proposed to produce the isotope 99 Mo for medicaluse, where 99 Tc in the waste is utilized as the substancefor 99 Mo production via muon capture.23) Experimentsof muon capture were organized at the facility of RCNP
Osaka University, RAL muon facility, and J-PARC. The
results are being published.24,25)
The future role of nuclear reactors is being intensively
discussed in conjunction with the realization of a carbon-

Acknowledgment
The works introduced here were funded by the ImPACT Program of Council for Science, Technology, andInnovation (Cabinet Oﬃce, Government of Japan).
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