01/29/08

Let's talk shop!

My research interest are mainly focussed on the spectroscopy of superheavy elements (SHE). Go directly to the isomer.

The study of nuclei at the limits of stability is one of the main challenges faced by nuclear structure physics. Only in the exploration of extremes are the finer details of structure isolated and identified. One such extreme is that of high mass and charge – the regime of superheavy elements (SHE). The question whether an island of stability exists for nuclei with more than 100 protons and where the borders of such an island may lie has been at the centre of nuclear physics for nearly half a century and is still one of the most fascinating and elusive open questions in modern physics.

(Picture taken from S. Cwiok et al, Nuclear Physics A 611 (1996) p 211)

One major challenge to experimental investigations is the increasing difficulty with which heavier SHE can be produced. Fusion evaporation reactions give low cross sections as the product of fusion cross section and survival probability becomes increasingly smaller. For the most part, structure information has mainly been derived from the study of alpha decay fine structure. Here the selective population of states with a similar single-particle structure as the mother state has allowed studies aimed at the systematic mapping of single-particle orbitals close to the ground state in a large number of nuclei. The main difficulty in this approach lies in the assignment of spins and parities from just the alpha decay observables such as energies and hindrance factors alone. The heaviest nucleus in which a direct measurement of the ground state spin has been made via optical spectroscopy measuring the hyperfine splitting is ²⁵³Es. More commonly one has to rely on an evaluation of decay chains over a wide range of nuclei and comparison with the very theories one wishes to constrain.  Thus this method alone generally does not pin down configurations.

One further, indirect approach is open to the study of SHE. In the deformed nuclei around  ²⁵⁴No (Z=102) the deformed Nilsson orbitals built on spherical single particle levels around Z=120 come down in energy and lie at or close to the Fermi surface. Here the sequence of levels at a deformation β₂ ≈ 0.3 probes the 7/2^-[514]_p, 9/2⁺[624]_p, 1/2^-[521]_p and 5/2^-[512]_p states stemming from the spherical 1h_9/2, 1i_13/2, 2f_5/2 and 2f­_7/2 proton orbitals, respectively. The 2f_7/2-2f_5/2 spin-orbit partners are especially important here, because the strength of the spin-orbit interaction governs the size of a possible Z=114 shell gap. In addition, the strongly downsloping low-Ω levels stemming from the spherical 1i_11/2 and 1j_15/2 orbitals well above Z=126 come close to the Fermi level, and the structure of the well deformed nuclei in the nobelium region will depend strongly on variations in the energies of those spherical orbitals. In our recent work we were able to pinpoint a level in ²⁵⁴No that involves the 2f_5/2 proton orbital which sits directly above the spherical Z=114 gap (RDH et al., nature 442 (2006) 896, see also here and here for additional commentary.

Together with our colleagues in Daresbury and Jyväskylä we build the SAGE spectrometer to allow simultaneous in-beam gamma-ray and conversion electron spectroscopy.

(Coming soon)

(Coming Soon)