Mathematical Physics and Astrophysics

Masaaki Kimura

Exploring the Form of Atomic Nuclei

Masaaki Kimura , Associate Professor

Faculty of Science, Graduate School of Science(Department of Physics, School of Science)

High school : Okayama Prefectural Okayama Joto High School

Academic background : Graduate School of Science, Kyoto University

Research areas
Nuclear Physics
Research keywords
Nuclear Structure, Nuclear Reaction, Nuclear Force, First Principle Calculation, Unstable Nucleus

What is the Goal of Your Research?

I am doing theoretical research on the internal structure of atomic nucleus (nuclear structure) and the nuclear reaction generated by the collisions of nuclei. The goal of my research is to continue these fundamental studies in order to answer the following questions: “What characteristics do atomic nuclei have?” and “What kinds of reactions do nuclei cause, and as a result what nuclei are generated?” Elements existing in the natural world are generated by the nuclear reactions inside stars. We also artificially produce and utilize the elements that are useful in industry and medical care through nuclear reactions. As a result, the fundamental study of nuclei is to explore “When, where, and how the elements that constitute nature are generated?” and to pursue “the possibility to utilize various nuclei for human beings.”


What Type of Form do Nuclei Take?

Fig. 1 Density Distribution of Carbon 16 Nucleus. Usually, it takes the form of (a), however, it changes to the form in (b) with the addition of a small amount of energy.

Atoms that constitute material objects can be divided into nuclei and electrons, and furthermore the nuclei are composed of protons and neutrons. The radii of the nuclei are extremely small, around 10-12 cm, in which many protons and neutrons move around at an incredible speed reaching roughly 30% of the speed of light. Nevertheless, the protons and neutrons can form nuclei by staying in proximity of each other, because of the exertion of great attractive force called nuclear force between them.
The nuclear force of which the origin was revealed by Dr. Hideki Yukawa differs so much from gravity or the electrical force that we feel in our daily life. It is a far stronger force, but it loses its strength if there is separation even at a slight distance. Thanks to these very unique characteristics of nuclear force, the nuclei have extremely varied structures and cause a variety of reactions.
As an example, let’s look at the structure of carbon 16 nuclei (Fig. 1) which was theoretically calculated. Usually it has a spherical shape like (a), however, the addition of a slight amount of energy changes the form tremendously to (b). The field of nuclear physics reveals the world of nuclei, rich in variety and diversity, with support in both theory and experiment.


How Do You Do Research?

In our laboratory, we analyze and predict the structures and reactions of nuclei by making theoretical models explaining nuclei and carrying out numerical calculations using computers. The best part of the research lies on the verification of our newly predicted nuclear structures or reactions based our own actual experiments. Recently, with the help of the progress in computer technology, the first principle calculation which requires no assumption or approximation has developed greatly. Fig. 2 shows a cross section of photodegradation reactions of helium 4 nuclei obtained by Asst. Prof. Horiuchi et al. using the first principle calculation. It coincides beautifully with the experimental results and can explain quite clearly how actual nuclear reactions develop. Also, extremely accurate theoretical analysis has been made possible for nuclear reactions that are difficult when applied to experiments.

Fig. 2 Cross Section of Photodegradation Reactions of Helium 4 nuclei. The solid line shows the result from the first principle calculation. The other symbols show the experimental data

Fig. 3 Superconducting Ring Cyclotron (SRC) of RIKEN’s RI Beam Factor

On the other hand, other experimental research can be conducted using accelerators. In Japan, there are many experimental facilities to boast of, one of which is the RIKEN’s RI Beam Factory shown in Fig. 3 (Reference 1). This facility can generate a great amount of unstable nuclei which don’t exist in the natural world and allows the research of those characteristics. As a result, more and more phenomena which challenge conventional understanding about nuclei have been discovered (The carbon 16 nucleus introduced in Fig. 1 is one of them.), which forces people to drastically reconsider traditional nuclear theory. We expect more success from young researchers who are not constrained from conventional thinking.


What Will be Your Next Goal?

As mentioned above, nuclear theory has acquired very high accuracy these days. Thus, the possibility of various applications opens up beyond the frame of fundamental studies. We are looking forward to making our way into new areas such as high-accuracy analysis of nuclear reactions, which occurs in cancer treatment using heavy particle beams.