Disappearance of magic numbers in neutron-rich nuclei

A nucleus is a quantum many-body system composed of protons and neutrons, which exhibits shell structure as is evident in atoms and metal clusters. It is characteristic feature that protons and neutrons in nuclei have their own magic numbers, and nuclei with these numbers can be more tightly bound compared to neighboring nuclei. Magic numbers in nuclei are closely related to the scenario of the element synthesis in the universe. Recently, it has been shown that magic numbers in nuclei can be changed if one looks into very neutron-rich nuclei. Here, we first introduce a pioneering Coulomb excitation experiment on 32Mg, which motivated similar studies with RI (radioactive isotope) beams. Recent progress in experimental studies is also discussed with an emphasis on the exotic shell structure of neutron-rich nuclei, such as disappearance of magic numbers, and appearance of new magic numbers. (author)