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Advanced Science Research Center Japan Atomic Energy Agency
Makoto Oka, Hiroyuki Koura
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Advanced Science Research Center Japan Atomic Energy Agency

MAKOTO OKA AND HIROYUKI KOURA
ADVANCED SCIENCE RESEARCH CENTER, JAPAN ATOMIC ENERGY AGENCY

ADVANCED SCIENCE RESEARCH CENTER

The Advanced Science Research Center (ASRC) is a research center that is part of the Japan Atomic Energy Agency (JAEA), and focuses on the fundamental sciences related to atomic energy research and development. ASRC was established in April, 1993, in the former Japan Atomic Energy Research Institute (JAERI). When JAERI consolidated with the Japan Nuclear Cycle Development Institute in 2005 to become JAEA, the center continued operating without any constitutional changes. The aims of ASRC are (1) to focus on fundamental research pertaining to the origin of principles and phenomena, and to apply the results to atomic energy research and development, and (2) to conduct research that leads to the development of other fields along with the development of atomic energy. The center has been managed by director generals, who, when they are chosen, are external to the ASRC's present staff members. The foundation of the center was established by the first Director General Prof. Muneyuki DATE (Osaka Univ.), and Profs. Hiroshi YASUOKA (Univ. Tokyo), Yoshihiko HATANO (Tokyo Inst. Tech.), and Sadamichi MAEKAWA (Tohoku Univ.) have served as directors general in the 25 years that have passed since. Makoto OKA is the fifth director general, with Hidehito ASAOKA acting as the associate director since April 2018.

The center has seven research groups in FY2019, which cover heavy-element nuclear science, interfacial reaction field chemistry, hadron-nuclear physics, material science for heavy element systems, spin-energy transformation science, the nanoscale structure and function of advanced materials, and advanced theoretical physics. There are presently about 75 current staff members, including the guest group leaders.

LOCATION AND FACILITIES

 



Fig. 1: The ASRC Building.

The location of the center (Fig. 1) is at the Tokai campus of JAEA, in Tokai village, Ibaraki prefecture of Japan. The campus has many large facilities devoted to nuclear science. Among them, the Japan Proton Accelerator Research Complex (J-PARC) is a high intensity proton accelerator complex operated jointly by JAEA and KEK. J-PARC is a multidisciplinary research facility that consists of a 400 MeV linear accelerator as the injector, a 3 GeV rapid-cycling synchrotron for multi-purpose neutron and muon beam lines, and a 50 GeV (currently 30 GeV) main ring for secondary hadron beams (pion and kaon) as well as a neutrino beam for the long-baseline T2K experiment. The Tokai tandem accelerator, another nuclear physics facility, is a Van-de-Graaff type electro-static accelerator, which accelerates ions to a maximum voltage of 25 MV. The accelerator provides various species of ion beams, from hydrogen to bismuth on various radioactive target materials, including the actinides such as uranium, americium, einsteinium. With the accelerator, we study the properties of chemical elements and nuclei. By combining these accelerators and other facilities, we engage in a high amount of activity in the frontiers of nuclear science.

HUMAN RESOURCES

ASRC considers its human resources to be its most valuable asset, and offers a high-quality research environment so that researchers can conduct spontaneous and original work at the center. Moreover, we believe that exchanges and collaborations with domestic and foreign institutions are critically important in order to gain broader perspectives on research. Currently, we have six guest group leaders that are external to JAEA, and two of these guest group leaders are foreigners. Furthermore, we foster the professional development of many postdoctoral fellows and we also support many graduate students. As a result, ASRC is able to have young researchers accepted to other JAEA departments as well, and thus ASRC serves as a source of excellent research talent. We believe that these steps will encourage the development of ASRC over the next 25 years.

RESEARCH HIGHLIGHTS

Heavy Element Nuclear Science

Studies of the chemical properties of the heaviest elements and determining the limits of nuclear stability are among the most interesting, but also challenging topics in modern nuclear chemistry and physics. By using the JAEA tandem accelerator facility (Fig. 2), we focus on nuclear and atomic structure studies in the heaviest element region and investigate new reaction mechanisms to access these exotic heavy nuclei.

 



Fig. 2: JAEA's tandem accelerator.

The first successful measurement of the first ionization potential (IP1) was carried out for lawrencium (Lr, element 103) at the JAEA tandem accelerator facility (T.K. Sato et al., Nature, 2015 [1], Fig.3). The first ionization potential is a key observable to unveil electronic configurations of the heaviest elements. Here, we developed a new method based on a surface ionization process coupled to an on-line mass separation technique to determine the IP1 of the nuclei, where the method produced only one atom at a time. These results, compared with a relativistic theoretical calculation, clearly demonstrate that the 5f orbital is fully filled at nobelium with the [Rn]5f147s2 configuration and that the next outer orbital starts being filled at Lr surprisingly in the 7p1/2, not 6d1/2.

 

Fig. 3: The first ionization potentials (IP1) for lanthanides and actinides. The IP1 of fermium (in 2018), nobelium (in 2018) and lawrencium (in 2015) were measured at the JAEA tandem accelerator for the first time.

In the field of nuclear physics, we have developed a multi-nucleon transfer reaction. By using the method, we can have access to much heavier nuclei that cannot be treated in conventional ways. We have established a method to study fission for a large set of nuclei in a single experiment (R. L챕guillon et al., Phys. Lett. B, 2015 [2], Fig.4), and we have generated widespread data in the unexplored neutron-rich region, by using various radioactive nuclides (232Th, 235U, 237Np, 248Cm, …) as targets.

 

Fig. 4: Fission fragment mass distributions as a function of mass number obtained in a multi-nucleon transfer reaction for 18O+232Th. Colored lines (red, blue) are the theoretical calculations.

MATERIAL PHYSICS FOR HEAVY ELEMENT SYSTEMS

Actinide condensed matter involves strongly correlated f-electron systems, which provide exotic electronic properties and show another face of superconductivity and magnetism.

 

Fig. 5: Map of the magnetic fluctuation amplitude on URhGe. Around the field-induced quantum critical point, HR~13 Tesla, reentrant superconductivity (RSC) occurs in the compound URhGe.

Around the reentrant superconductivity in a uranium compound, URhGe, the field induced Ising-type ferromagnetic (FM) fluctuations are revealed to be enhanced strongly by nuclear magnetic resonance (NMR) measurements (Y. Tokunaga et al., Phys. Rev. Lett. 2015 [3], Fig.5). The theoretically predicted wing structure around the tri-critical point has been confirmed. Related UXGe (X: Co, Rh, Ir) have been investigated to clarify the relation between the Ising FM fluctuations and p-wave superconductivity.

SPIN-ENERGY TRANSFORMATION SCIENCE

A spin current is a flow of magnetism. It has been used in non-volatile magnetic memory and energy conversion technologies. These technologies will dramatically reduce energy consumption because of the characteristic property of spins, i.e., its rotational motion is limited in one direction. A mechanical rotation is equivalent to a magnetic field in a rotating frame. The conversion from a macroscopic rotation to a magnetic moment of spin, known as the Barnett effect and having previously been measured only in a ferromagnet, was successfully observed in paramagnetic states of a gadolinium (Gd) metal with a rotational frequency of 1.5 kHz (M. Ono et al., Phys. Rev. B, 2015 [4], Fig. 6).

The conversion between spin and rotation is also realized in the flow of liquid metal (R. Takahashi et al., Nature Phys, 2016 [5], Fig. 7). We call it spin hydrodynamic (SHD) generation. In this case, a vorticity defined by a rotation of fluid velocity corresponds to a rotational motion coupled to spins. The spin current is generated along the vorticity gradient and is converted into electric voltage. The voltage signal is proportional to the square of friction velocity and shows good agreement with the theoretical prediction. The observed voltage generation will be used to make an electric generator and a spin generator without using magnets.

 

Fig. 6: (a) Capsule (left) and Gd sample (right). (b) Schematic illustration of the experimental setup. (c) Rotational frequency dependence of magnetization observed for a Gd sample and a blank capsule.

 

Fig. 7: (a) Schematic diagram of the SHD generation effect. (b) Scaling behavior of the voltage signals due to spin hydrodynamic generation.

NANOSCALE STRUCTURE AND FUNCTION OF ADVANCED MATERIALS

A thermodynamic property of a material usually depends on the normal bulk state, while a novel state is manifested by translational-symmetry breaking as realized at surfaces and interfaces. In addition, different states are formed around defects and impurities, and these states influence the bulk property. Hence, the study of local states is quite important in materials research.

The structures and properties of novel two-dimensional atomic sheets fabricated on metal substrates were investigated. By using total-reflection high-energy positron diffraction (TRHEPD), we experimentally confirmed that graphene-substrate spacing is shifted by more than 1 횇 through the hybridization with d-states of substrate materials, cobalt or copper (Y. Fukaya et al., Carbon, 2016 [6], Fig. 8). Monolayer graphene was also studied by using proton permeability analysis. We found that the defect structures in the graphene play a role in proton permeability, and fundamental knowledge could be obtained for the development of novel hydrogen isotope storage and selective hydron membranes. We have also investigated several monolayer systems and surface states for other materials like germanene (Y. Fukaya et al., 2D Matter [7], 2016).

 

Fig. 8: Graphene-substrate spacing measured by the positron diffraction technique with TRHEPD. The spacing is shifted depending on the substrate materials.

INTERFACIAL REACTION-FIELD CHEMISTRY

We explore novel chemical reactions of actinides and fission products (FPs) with various solid phases, at solid-liquid and at liquid-liquid interfaces in order to contribute to waste treatment technology, materials science, environmental and microbiological chemistry, environmental remediation and actinide & FPs chemistry.

Barite (BaSO4) is a mineral with very low solubility that can be easily synthesized by mixing aqueous solutions of soluble salts such as barium chloride and sodium sulfate. Since barite can incorporate a variety of cations in its structure by coprecipitation, removal of divalent cations from aqueous solutions by coprecipitation with barite has been applied at an industrial scale. We succeeded in coprecipitating larger amounts of anions of Se(IV), Se(VI), and I(V) with barite by adjusting various parameters of coprecipitation (K. Tokunaga et al., Environ. Sci. Technol, 2017 [8], Fig. 9). The optimum coprecipitation conditions for those anions were different from each other but their distribution coefficients reached a level greater than 3횞103 mL/g. There were no methods to selectively incorporate those anions in inorganic solids with low solubility. We are planning to immobilize barite coprecipitated with those anions in a solidified body and will investigate the barrier performance of a solidified body.

In addition, we have embraced the challenge of developing novel waste treatment technologies for difficult-to-treat fission products, iodine and heptavalent technetium, to contribute to the acceleration of the Fukushima Daiichi nuclear power plant (FDNPP) decommissioning program.

 

Fig. 9: Change of distribution coefficient Kd. (left) Se(IV) incorporation increased with crystal lattice distortion made by replacement of Ba2+ with Ca2+. (right) Se(IV) and Se(VI) incorporation increased with decreasing initial SO42- concentration.

HADRON NUCLEAR PHYSICS

We explore hadronic systems with strange and charm quarks mainly at J-PARC, and hot and dense quark/hadronic matter at J-PARC.

In the study of Λ hypernuclei, we found an indication of large charge symmetry breaking in a ΛN interaction by measuring the difference between the first excited state and the ground-state for and (T.O. Yamamoto et al., Phys. Rev. Lett., 2017 [9], Fig. 10).

 

Fig. 10: Level schemes of the mirror hypernuclei, and . Λ binding energies (BΛ) of and are taken from past emulsion experiments. and are obtained using the present data and past 款-ray data.

 

Fig. 11: A photograph of the 'MINO' event and its schematic drawing. The overlaid photograph is made by patching focused regions. Tracks #4, #5, #6, #8, and #9 are not fully shown in this photograph because these tracks are too long to be presented.

Furthermore, we found a beryllium double-Λ hypernucleus at J-PARC. This is the second of only two such events with a double-Λ hypernucleus and is a new species different from the previous event, a helium double-Λ hypernucleus, known as the 'NAGARA' event in 2001. We named this new event the 'MINO' event (H. Ekawa et al., PTEP, 2019 [10], Fig. 11).

 

References

[1] T.K. Sato, et al., Nature 520, 209 (2015).
[2] R. L챕guillon et al., Phys. Lett. B 761, 125 (2015).
[3] Y. Tokunaga et al., Phys. Rev. Lett. 114, 216401, (2015).
[4] M. Ono et al., Phys. Rev. B 92, 174424 (2015).
[5] R. Takahashi et al., Nature Phys. 12, 52 (2016).
[6] Y. Fukaya et al., Carbon 103, 1 (2016).
[7] Y. Fukaya et al., 2D Matter 3, 035019 (2016).
[8] K. Tokunaga et al., Environ. Sci. Technol. 51, 9194 (2017).
[9] T.O. Yamamoto et al., Phys. Rev. Lett. 115, 222501 (2017).
[10] H. Ekawa et al., Prog. Theor. Exp. Phys. 2019, 021D02 (2019).

 

Makoto Oka is the director general of the Advanced Science Research Center, Japan Atomic Energy Agency (JAEA). After receiving a D. Sci from the University of Tokyo, he worked as a post-doctoral researcher at the Institute for Nuclear Study at the Univ. of Tokyo; Kobe Univ.; and Massachusetts Institute of Technology, and as an assistant professor at the Univ. of Pennsylvania. He then moved to the Tokyo Institute of Technology (Tokyo Tech) as an associate professor in 1991. He was a professor of physics at Tokyo Tech from 1996 to 2018 and moved to JAEA in April, 2018 as the director general of ASRC. He was a member of the Japan Science Council since 2006 (council member from 2011-2017) and has been a guest researcher of RIKEN since 2009. His research interests are in the fields of hadron physics, quantum chromodynamics, hadron spectroscopy and hadronic interactions.

Hiroyuki Koura is a principal researcher of the Advanced Science Research Center, Japan Atomic Energy Agency. After receiving a D. Sci from Waseda University, he worked for RIKEN as a collaborative scientist from 2001-2004. In 2004, he moved to the Advanced Science Research Center, Japan Atomic Energy Research Institute (JAERI) as a research scientist. He was a visiting scientist at Oak Ridge National Laboratory in 2005. He has been an adjunct researcher at Waseda University since 2012, a visiting scientist at RIKEN since 2013, a visiting professor of Ibaraki University since 2016, and a part-time lecturer of Tsuda University since 2017. His research interest is in nuclear physics, including superheavy elements and nucleosynthesis in stars.

 
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