Superheavy elements

Abstract

The outstanding aim of experimental investigations of heavy nuclei is the exploration of spherical “SuperHeavy Elements” (SHEs). On the basis of the nuclear shell model, the next double magic shell-closure beyond 208Pb is predicted at proton numbers between Z=114 and 126 and at neutron number N=184. All experimental efforts aiming at identifying SHEs (Z⩾114) were negative so far. A highly sensitive search experiment was performed in November-December 1995 at SHIP. The isotope 290116 produced by “radiative capture” was searched for in the course of a 33 days irradiation of a 208Pb target with 82Se projectiles, however, only cross-section limits were measured. Positive results were obtained in experiments searching for elements from 110 to 112 using cold fusion and the 1n evaporation channel. The produced isotopes were unambiguously identified by means of α-α correlations. Not fission, but alpha emission is the dominant decay mode. The measurement of excitation functions revealed that the cross-section maxima arise at decresing excitation energy with increasing element number. Microscopic effects and, especially, the transfer of nucleons seem to be important processes for the initiation of fusion. The trend of the cross-section data suggests that elements beyond Z=112 may still be possible for being synthesized.