The main focus is the study of the structure of unstable nuclei using the technique of in-beam γ-ray spectroscopy with direct reactions induced by fast-moving radioactive beams. Various different reaction mechanisms may be used for this purpose, for example, Coulomb excitation, proton-inelastic scattering, nucleon removal reactions and nucleon transfer reactions.

Evolution of nuclear shell structure

One of the current major interests in nuclear physics is the evolution of shell structure far from stability; such changes may be traced through the study of the low-lying states. In particular the properties of the first J^π = 2⁺ states in even-even nuclei such as E_x(2⁺) and B(E2) values, which are obtained by the Coulomb excitation, proton inelastic scattering, or fragmentation reaction, are used as a measure of the nuclear collectivity. We also study odd-mass nuclei by transfer reaction induced by alpha particle or fragmentation reaction to obtain energies of single particle states. Our experiments made at RIBF facility thus far have clarified the magicity loss at the traditional magic number N = 8 and 20. In the future experiment, we will extend this study to heavier region in N = 50, N = 82 and Z = 50.

Proton and neutron motion

Neutrons and protons behave in a very similar manner in stable nuclei as a result of the strong interaction between these two types of fermions. In very unstable nuclei, though, the extreme unbalance between neutron and proton numbers could cause decoupling of neutron and proton distribution and/or motion. One of the well-known cases is the neutron halo, in which neutron distribution has a very long tail outside the core. We are investigating such a phenomenon in more collective property by using several probes, which have different sensitivity to neutrons and protons.

In the neutron-rich carbon isotopes, the quadrupole collectivities are found to be very different for neutrons and protons. Another example is the comparision between the quadrupole collectivities of neutrons and protons in the nuclei close to the N = Z line. When the neutron or proton number is close to magic, the collectivity of this type of nucleon is reduced, which results in the different collectivity for neutrons and protons. On the other hand, when both the proton and neutron numbers are far from the magic number, the neutrons and protons contribute almost equally.

Development of collectivity

The collectivity is one of the unique properties of nucleus as a finite many body system. The collectivity could appear in many ways with different strength reflecting the shell structure. For exapmple, the ³²Mg nucleus is known to have very large collectivity inspite of its neutron magic number N = 20. We have recently studied the character of this collectivity and have shown that the deformation is not sufficiently statical in this nucleus and that the ground state band may correspond to the transisional one between the rotation of a rigid object and the vibration of soft nuclear surface. On the other hand, the more neutron rich ³⁴Mg nucleus could be regarded as a well-deformed nucleus because of the location of (the candidate) of the 4⁺ state, which we have identified for the first time. We have also confirmed that the similar deformation region exist around the N = 40 nucleus ⁶⁴Cr.

Detector/Facility development

We are continually developing and improving the experimental devices used at the RIBF, such as the NaI(Tl) γ-ray detector array (DALI2), the liquid hydrogen and helium targets (CRYPTA), and the Ge detector array (GRAPE). The development of a next-generation γ-ray detector array utilizing LaBr₃(Ce) scintillators (SHOGUN) is also in progress.

The new-generation, high-intensity RI beam separator BigRIPS gives us the potential to expand our research to more exotic and heavier mass regions on the chart of nuclides. The first secondary-reaction experiment at BigRIPS, which used a ⁴⁸Ca primary beam, was accomplished in December 2008, and a series of experiments using ⁴⁸Ca and ²³⁸U beams was performed in late 2009. Further experiments are planned.