There are about 300 elements in nature. How are these elements (in another word, stable nuclei) produced in nature? This is one of most important question in current physics.
The universe began from Big-Bang (BB). At first, in BB, proton and neutron were produced. Next, in hot-plasma, proton and neutron induced nuclear reactions and heavier nuclei were produced. In BB, up to He nucleus must be produced. It is considered that nuclei heavier than He were produced with long time in stars. Sun is one of a good example. To understand nucleosynthesis, information of nuclear reactions and nuclear structure are indispensable.

Nevertheless, how is uranium (Z = 92), that is the heaviest element in nature (Fig. 1), produced? It is already known that nucleosysnthesis in the star can not reach to uranium even using long time. Lighter stable nuclei next to uranium is bismuth (Z = 83), however, all nuclei between bismuth and uranium are unstable nuclei with several days or less of life-time.
However, uranium exists even in the earth. It is considered that uranium were produced by explosive neutron capture during very short time. Supernovae explosion and/or neutron-star merger are considered to be its major source. Neutrons with high-energy produced by explosive phenomena were captured by nucleus continuously. By adding neutrons, neutron-rich unstable nuclei were produced. On the other hand, unstable nuclei induced beta-decay (neutron converted to proton by emitting electron and anti-neutrino) and approached to the stable nuclei. By inducing beta-decay, proton number increased. Thus, by passing the region to balance neutron capture and beta-decay, it is considered that uranium is produced in the universe.

Figure 1. Nuclear chart for the rare-RI ring. Black squares show stable nuclei and gray squares show nuclei with known masses. Red squares represent rare-RIs where the beam intensity is more than 1 event/day/pnA in the rare-RI ring. The r-process path estimated theoretically is shown by the green line, and the traditional magic number is shown in blue with numbers. [A. Ozawa et al., PTEP 2012, 03C009.]

Where does this process occur in nuclear chart? In theory, it is predicted that this process passes in very neutron-rich region (r-process in Fig.1), where neutron number is about twice to proton number. However, these neutron rich nuclei are not experimentally confirmed yet. Before we clarify the process to create uranium, we need to show existence of these unstable nuclei. In the next, we need to measure their neutron-capture probabilities (cross-sections) and life-times. It is not easy to measure the neutron-capture probabilities experimentally. Since, in the present technique, we can not prepare neutron as the target, it is impossible to measure neutron-capture probabilities directly. We need to determine them by in-direct methods. Neutron capture reaction is relatively simple reaction. If neutron separation energies to relevant nuclei are experimentally known, the probabilities can be estimated. Furthermore, by using the mass differences, rough estimation for life-times are possible with the help of beta-decay theory. Thus, in order to clarify the process to create uranium, mass measurements for unstable nuclei are indispensable. In another word, if we know the mass, we can estimate the process roughly. By the above motivation, we study developments and improvements of device to measure the mass of unstable nuclei. In the mass analysis field, Japan is one of top countries in the world. (Dr. Kouichi Tanaka got Nobel prize on 2002. His subject was innovation of mass-spectrometer for protein. ) Further, Japan has the top technique for RI-beams. However, systematical mass measurements for the unstable nuclei have not been done in Japan, so far. We will perform systematical mass measurements for the unstable nuclei by using ‘new’ experimental device ‘Rare-RI Ring’ in RIKEN RI beam factory (RIBF).