Spectroscopy of the Heaviest Nuclei
R. M. Clark 1*
1 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
The existence of superheavy elements implies that there are substantial shell effects, beyond the macroscopic liquid drop energy, which stabilize the nucleus against fission. The specific “magic” proton and neutron numbers, representing major spherical shell gaps, are a matter of considerable debate. Shell gaps can also occur when the nucleus distorts to non-spherical shapes leading to enhanced stability at particular deformations. It is well established that nuclei near Z=100, N=152 (252Fm) have well-deformed prolate shapes. Orbitals that originate from above the predicted shell gaps can intrude close to the Fermi surface of these deformed nuclei. There are also many high-K orbitals, which lie close to both the proton and neutron Fermi surfaces. This favors the occurrence of high-K multi-quasiparticle isomeric states at low excitation energy. By identifying such high-K states, and studying their decay to states with lower-K, we can learn about the shapes, single-particle structure, pairing correlations, and excitation modes of the heaviest nuclei.
Many experiments have been performed over the last two years using the Berkeley Gas-filled Separator (BGS) at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. Isomeric states have been identified, and their decays studied, in many trans-fermium nuclei including 253,254,255No (Z=102), 255Lr (Z=103), 256,257Rf (Z=104), and 261Sg (Z=106). Detailed gamma-ray and electron decay spectroscopy has been performed, and a new understanding of the structure and properties of these heavy nuclei is emerging. I will discuss the latest results, and their physics implications. I will also describe the latest technical advances at LBNL including a new BGS focal-plane detector upgrade giving an order-of-magnitude improvement in resolving power for decay spectroscopy, with the first results expected in the spring of 2011.