Dr. F. G. Kondev
Argonne National Laboratory

Observation and characterization of the limits in mass and charge of the nuclear landscape is one of the fundamental goals of nuclear physics. With increasing proton number, the long-range Coulomb repulsion in heavy nuclei leads to a decrease in stability, and, according to the liquid-drop model, elements with proton number Z>100 should disintegrate immediately via spontaneous-fission. However, the observation of super-heavy nuclei with atomic numbers beyond Z=100 indicates that these nuclei still have a sizable fission barrier. The latter owes its existence to the presence of shell gaps in the single-particle energies, which lead to a barrier against fission despite the vanishing liquid-drop contribution.

Nuclei in the trans-fermium region near Z~96-104 are also known to be deformed and they are frequently found to exhibit high-K isomerism, owing to the presence of high-K orbitals near both the proton and neutron Fermi surfaces. The properties of such isomers provide important information on the single-particle structures in the region, as well as on the role played by the pairing and residual nucleon-nucleon interactions. Their existence can also lead to enhanced stability for the heavier elements.

In this presentation, an overview of an on-going experimental heavy-elements spectroscopy program at Argonne National will be given. Recent new results describing a search for isomeric states in ²⁵⁴Rf using the ²⁰⁶Pb(⁵⁰Ti,2n) reaction and the Argonne Fragment Mass Analyzer will be presented. For the first time, a digital data acquisition system was deployed, which allowed comprehensive pulse-shape analysis of the recoil-decay pile-up events to be performed and identification of implant and decay events separated by decay times as short as hundreds of nanoseconds. Furthermore, this novel approach resulted in a much lower ~50 keV threshold for conversion-electron events, associated with decays of isomeric states within the first 6 s following implantation, independent from the energy threshold set in the digitizer firmware.

*This work was supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357.