Nuclear Physics Group
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Nuclear Physics Group |
Nuclear Structure of the heaviest nuclei
Rodi Herzberg, Peter Butler, Robert Page
What is the heaviest element we can make?
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 super heavy 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 elusive open questions in modern physics.
A new and very promising approach to understanding
the physics of SHE has been pioneered by a collaboration led by the Liverpool
group: 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. Furthermore
production cross sections are large enough to allow detailed spectroscopic
studies giving access to rotational properties as well as single particle states.
Thus the study of transfermium nuclei in this region gives the most pertinent
experimental clues as to the limits of existence of super heavy nuclei. For a
recent review see e.g. [1].
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 1h9/2, 1i13/2, 2f5/2 and
2f7/2 proton orbitals, respectively. The 2f7/2-2f5/2
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 1i11/2 and 1j15/2 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 2f5/2 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.
Theoretical effort to predict the properties of SHE
has been varied. On the one hand the deformed lighter nuclei around nobelium
are reasonably well described by Wood-Saxon approaches with phenomenological
potentials and a flat density distribution. In contrast, when challenged by the
new data provided mainly through our work, state-of-the-art self consistent
approaches fail to reproduce the detailed single particle structure, especially
for high-j orbitals. In addition, they predict large density fluctuations which
rule out reliable extrapolations to the island of stability from Wood-Saxon
descriptions alone. This work has reinvigorated theoretical efforts around the
world and we are in close collaboration with several theory groups (Bruxelles, Bordeaux
A key experimental challenge is given by the very
large internal conversion coefficients in very heavy systems. This process
removes gamma ray intensity from the transitions of interest and leads to the
emission of electrons instead, giving us the opportunity to perform conversion
electron spectroscopy which allows experimental assignments of
multipolaritities and parities to transitions even if they are weakly
populated.
The new SAGE spectrometer is currently being
constructed in Jyväskylä and underpins a range of physics themes. It will
finally allow the much needed simultaneous
conversion electron and gamma ray spectroscopy of heavy and super heavy
elements. In the near term SAGE will be
finished (grant ends 10/09) and a wide program of exploitation, ranging from
SHE to heavy neutron deficient nuclei around Pb, is envisaged. An upgrade of
the spectrometer to instrument both the Si and the Ge parts fully digitally
will further increase its sensitivity and versatility.
More information can be found on
my personal
pages.
1 R.-D. Herzberg
Spectroscopy of Superheavy Nuclei
Invited Topical Review
Journal
of Physics G 30, (2004), R123.
2 R.-D. Herzberg et al.,
Nuclear
isomers in superheavy elements as stepping stones towards the island of
stability