Experimental studies on shell structure of superheavy nuclei

Existence of superheavy elements essentially depends on shell structure of superheavy nuclei. It is theoretically predicted that stable doubly-closed-shell spherical superheavy nuclei should exist around the region of the proton number 114 to 126 and the neutron number 172 to 184, although their exact locations and degree of stability have not been established because of large variety of theoretical predictions. On the other hand, we have already succeeded in producing superheavy nuclei with the proton number up to 118 and the neutron number up to 177, indicating that we have already reached the doubly-closed-shell superheavy region. In addition, some pioneering experiments have been performed to directly establish the level structure of superheavy nuclei through spectroscopic methods. This paper introduces the current status of experimental studies on nuclear shell structure of superheavy nuclei. (author)