The nucleus can be divided into two types; one is stable nuclei which can exist in nature, the other is unstable nuclei (radio isotopes, RI) which decay to stable nuclei within a finite life time. Figure 1 is the nuclear chart that the vertical and horizontal axes indicate the number of protons (Z) and the neutrons (N), respectively. Each square in Fig. 1 means one kind of nucleus. According to a theoretical prediction, it is said that there are about 7000 nuclei. The black squares in Fig. 1 represent stable nuclei and there are about 300 of them, while remaining nuclei are unstable nuclei. So far, the number of discovered unstable nuclei is about 3000, but the existence of many of them is unknown. Unstable nuclei located far from stability line in Fig. 1 have more excess neutrons (protons) than stable nuclei, so these nuclei are called neutron (proton)-rich nuclei generally. From recent studies, unstable nuclei have exotic properties which stable nuclei do not have. In order to understand the nuclear physics closely, it is important to know the structure and properties of unstable nuclei.
Figure 1. Nuclear chart. [Referred from the web page of RIKEN Nishina center]
Experiments for unstable nuclei
Since unstable nuclei do not exist in nature, it needs to produce them for experiments by using accelerators. The improvement of the experiments using RI beams allowed us to study unstable nuclei. Figure 2 shows the superconducting ring cyclotron (SRC) and the RI beam separator (Big RIPS) in RIKEN RI Beam Factory. We perform experiments of unstable nuclei at accelerator facilities.
Figure 2. The superconducting ring cyclotoron (SRC) (Left) and the RI beam separator (Big RIPS) (Right) in RIKEN RI Beam Factory. [Referred from the web page of RIKEN Nishina center]
Nuclear size properties such as radii and density distributions provide us with basic and important information for understanding the nuclear structure. The halo and the skin of unstable nuclei are anomalous phenomena that stable nuclei do not have. The former is the expanded density of the one or two weakly-bound valence nucleons outside of the core nucleus, while the latter is the layer of proton or neutron on the nuclear surface. These phenomena are interesting because protons and neutrons in a stable nucleus are uniformly mixed. Figure 3 shows the radii of light nuclei as a function of their mass number. The solid line indicates the empirical equation for the radii of stable nuclei. The extraordinarily large radii of 11Li (Z=3, N=8) and 14Be (Z=4, N=10) are seen in Fig. 3 compared with the empirical trend.
Investigation of these exotic properties is a motivations of studies for unstable nuclei.
Figure 3. Radii of light nuclei as a function of mass number. [I. Tanihata et al., PRL55, 2676 (1985). I. Tanihata et al., PLB 88, 592 (1988).]