Research Interests

What is atomic nucleus, and how it is important?

Atomic nucleus is the core of atom, and is the many-body system of nucleons, including protons and neutrons. Number of protons, indicated as Z, defines the total charge of the nucleus, and hence its chemical identity. Number of neutrons, indicated as N, also contributes to the total mass of the nucleus, whereas neutron itself is electrically neutral. Combinations of (Z, N) can explain the phenomenon of isotopes or nuclides, that is, the same atomic number with different atomic mass of A = Z + N. The atomic nuclei exhaust about 99.93% of total mass of existing matter. The study of atomic nuclei has been important for the characteristics of matter, tests of fundamental interactions, and even for practical applications.

Since the first observation of atomic nuclei by Ernest Rutherford (UK) in 1911, a variety of interests and efforts has been paid to investigate them. As observed in the nuclear experiments, they show the unique properties and behaviours, for example, nuclear energy, nuclear reactions, and radioactivity. However, in the nuclear physics, there still remains a tremendous amount of problems, which need the high-expertise and inter-disciplinary investigations to be resolved. For practical applications, the physical properties of atomic nuclei have been connected with various branches and have attracted their interests.

Astrophysics - there has been a close collaboration with the nuclear and astrophysics, that is, "nuclear-astrophysics". Several essential problems remain in this branch. For example, the current knowledge on the origin of elements is up to the light ones, including hydrogen and helium generated in the Big Bang, while the other elements are considered as produced in the cosmic objects, such as the nucleosynthesis process in stars and their explosions. Structure of neutron stars, explosion mechanism in supernovae, gamma ray burst, as well as the abundance of chemical elements are also known as typical topics in this field.

Quantum computing - to utilize quantum systems for computing is one of the most fascinating topics in the recent science. There, several quantum-mechanical properties, for instance, the superposition and entanglement, can be utilized to boost the efficiency of computing. (i) The employment of quantum computers should be an important task also in the near-future nuclear physics. Since atomic nuclei are quantum systems, for the simulation of them, the quantum computers can be a better option than the classical computers. For this purpose, the development of suitable algorithm of nuclear-physical calculations to quantum computers is necessary. (ii) Another, possible collaboration is the nucleon-based quantum computers. In quantum computing, so-called “qubits” are responsible to represent the quantities. As one idea to realize the quantum computers, the nuclear magnetic resonance (NMR) has attracted the interests. In this way, the qubits are realized with the spins of atomic nuclei.

Nuclear industry and engineering - there has been a long-standing interest on how to extract and utilize the nuclear energy with a solid security. Since the typical order of energy resource in atomic nuclei is one-million larger than the chemical energy, it has been expected to provide the solution for the modern energy crisis. Discovery of the novel nuclides may profit us to obtain the new materials. Transmutation technique of elements, e.g. to produce rare metals, is also based on the nuclear-physical knowledge. In parallel, the huge risk of utilizing the nuclear energy has been considered, where typical problems include (i) how to safely control the nuclear-power plant or devise, (ii) how to deal with the radioactive waste, and (iii) how to prevent the abuse of nuclear energy. Notice that, for these applications, the correct knowledge on atomic nuclei is indispensable. My research also aims to make a significant contribution in these applications.

Role of nuclear physics

Considering the role of nuclear physics, one natural question appears: Do we already know all the physical properties of all the atomic nuclei? Unfortunately but of course, the present answer is NO. For example, in the data base of nuclides, one can find that many nuclides lack the consensus on their binding energies, radioactive modes, lifetimes, etc. The maximum number of nuclides in this universe is estimated as 3000-4000 at least, whereas the complete information has been available only for 200-300 nuclides, which play a fundamental role in the application science or industry with an accumulation of experimental data. Namely, about 90% of the existing nuclides still need to be investigated. Because of the technical difficulties, the experimental approach to the full set of nuclides is still demanding at present. In this situation, the theoretical approach can be alternative. For this purpose, so-called “universal theory” for atomic nuclei has been on a serious demand.

Universal theory of atomic nuclei

Universal theory of atomic nuclei has been one dream since the origin of nuclear physics. This theory, when it would be completely established, is able to reproduce all the physical properties for arbitrary nuclei: those include the binding energy, excitation features, active mode(s) of radioactivity, lifetime, etc. However, even lots of efforts have been devoted, the complete version of the universal theory is not established. The main difficulty originates in (i) the quantum many-body problem and (ii) the variety of physical properties observed in atomic nuclei. Toward the goal of the universal theory of atomic nuclei, there are several, promising archetypes in the nuclear theory. Those include the large-scale shell model, ab-initio calculation, chiral effective field theory, energy-density functional (EDF) theory, and also combinations of these theories.

Relativistic energy-density functional (REDF) theory for atomic nuclei

Oct. 2018 - Present

Energy-density functional (EDF) theory has been utilized to establish the universal framework of atomic nuclei, which can widely explain the rich properties of various nuclides. The self-consistent mean-field (MF) calculation has been developed on the non-relativistic or relativistic EDF as basement. Now I am continuously studying the collective excitations, but replacing its basement to the relativistic EDF. With my colleagues, Prof. Nils Paar and Mr. Goran Kruzic, I am investigating the magnetic dipole (M1) mode, which can be useful to determine some model parameters in the relativistic EDF. To investigate this M1 excitation, I developed the relativistic QRPA scheme. The pairing-model dependence of this M1 excitation is another topic in progress now.

[2020OKP] = T. Oishi, G. Kruzic, and N. Paar, Journal of Phys. G, Vol. 47, 115106 (2020).

[2019OP] = T. Oishi and N. Paar, Phys. Rev. C 100, 024308 (2019).

Time-dependent approach to proton-emitting processes

April 2014 - Sep. 2018

Quantum time-dependent system or meta-stability is a basic concept to understand several dynamical processes of atomic nuclei. Two-proton (2p) emission is one of those processes with quantum tunneling with many degrees of freedom. We have developed a time-dependent (TD) three-body model for 2p-emitting nuclei. This method can provide an intuitive way to discuss the broad-resonance system, for which the multi-particle dynamics should be carefully taken into account. By applying the TD model to 2p emission, we have found that, (i) 2p emission is controlled by the proton-proton pairing interaction, which is indispensable to explain the experimental data of energy and width; (ii) diproton correlation with the spin-singlet configuration is suggested [2010OHS, 2016OKP]. In paper [2018Oishi], the effective Lambda-proton interaction in the one-proton emission is investigated. Future plan: TD picture is essential to understand the nuclear dynamics. I am planning to develop the TD-relativistic EDF framework for several research targets.

[2010OHS] = Phys. Rev. C 82, 024315 (2010).

[2016OKP] = Phys. Rev. C 96, 044327 (2017).

[2018Oishi] = Phys. Rev. C 97, 024314 (2018).

Skyrme energy-density functional approach to collective excitations

April 2014 - Sep. 2016

For an optimization of EDF parameters, the collective excitations give useful references. The quasiparticle random-phase approximation (QRPA), within a framework of the EDF theory, has been a standard tool to access the nuclear collective excitations. However, the full description of various collective modes has not been completed. The traditional matrix QRPA has been numerically demanding especially for heavy and neutron-rich nuclei. Recently, finite amplitude method (FAM) for the efficient QRPA calculation was implemented into the self-consistent Hartree-Fock-Bogoliubov (HFB) code. That calculation was based on the non-relativistic nuclear EDF of the Skyrme type. In our recent work [2016OKH], FAM-QRPA scheme showed a remarkable efficiency, which enables us to perform the systematic analysis of the giant-dipole resonance for heavy rare-earth nuclei. The experimental energy and width can be well reproduced with our FAM-QRPA. A role of the isovector effective mass, which is an important pseudo-observable quantity of the infinite nuclear matter, is discussed.

[2016OKH] = T. Oishi, M. Kortelainen, and N. Hinohara, Phys. Rev. C 93, 034329 (2016).