## Research

Atomic nuclei are mesoscopic systems, dominated by an interplay of microscopic and macroscopic behaviour. Accordingly, their properties are described within the framework of very different models, e.g. by the shell model, emphasising the microscopic structure, or the liquid drop model, explaining macroscopic behaviour.

We are investigating details of nuclear structure by bringing nuclei into extreme states using nuclear reactions with stable and radioactive heavy-ion beams. In such reactions nuclei can be excited into states with very high excitation energy, angular momentum (spin), deformation or isospin.

At very high angular momentum the interplay between the pairing and Coriolis interactions can be studied. Nucleons are excited into various orbitals and, for deformed nuclei, rotational bands are built on different excited states. Deformation may change as a function of angular momentum, which in term influences the rotational properties [see Publications].

In recent years we have discovered a new and unexpected mode of excitation at high spin: the occurrence of rotational-like bands in near-spherical nuclei. For rotational bands to occur, the symmetry of the quantum mechanical system must be broken. The long known rotational bands in deformed nuclei are good examples. We have found cases where the symmetry of spherical nuclei is broken by anisotropic currents - and the associated magnetic moments - of a few high-spin nucleons, see Fig.1. This type of excitation which leads to bands of strongly enhanced magnetic dipole transitions, see Fig.2, has been named 'Magnetic Rotation' [see H. Hübel, Prog.Part.Nucl.Phys. 54, 1 (2005)].

In different ranges of excitation energy and spin the occupation of certain nuclear orbitals can drive the nuclear shape to extreme values. We have recently studied triaxiality, super- and hyperdeformation.

Triaxiality had been predicted theoretically for a long time, but
it has been difficult to prove experimentally. This is a shape
where all three major nuclear axes are different. We have recently
found unique evidence for triaxiality by the discovery of the
'Wobbling Mode' in several Lu isotopes. The fingerprint for
wobbling are families of rotational bands which differ only by the
tilt angle of the collective angular momentum with respect to the
nuclear shape and which are connected by unusually strong
inter-band transitions [see P. Bringel *et al.*,
Eur. Phys. J. A 24, 167 (2005)].

Very elongated shapes, where the nuclear
axis ratios c:a approach 2:1 and 3:1, have been called super- and
hyperdeformed, respectively. We have been studied a number of nuclei
which show superdeformation in various mass regions and have
determined their quadrupole moments. As an example for a
superdeformed rotational band, see Fig.3 [see A.K. Singh *et al.*,
Nucl. Phys. A 707, 3 (2002)].

The theoretically predicted hyperdeformation at very high spins is difficult to find. So far, our experiments to search for discrete-line rotational bands of such extremely elongated nuclei produced no convincing results. Weak hints for hyperdeformation have been found in continuum gamma-ray correlation spectra [see Publications].

The investigations mentioned above have been performed with large
multi-detector spectrometers, in recent years mainly with the
EUROBALL and GAMMASPHERE arrays, see Fig.4 and
Fig.5, respectively. These instruments allow to
study very weakly populated structures in nuclei, down to about
10^{-5} of a reaction channel. A big step forward is
expected in the future when spectrometers with even higher
efficiency, AGATA in Europe and GRETA in the USA, become
operational.

Nuclei far off the valley of stability
have become more accessible recently via reactions with
radioactive beams. For such nuclei with large isospin shell model
calculations predict a modification of shell and subshell
closures. This is an important question to be investigated
experimentally since shell effects greatly influence nuclear
properties. We have measured reduced transition probabilities to
the first excited 2+ states in radioactive ^{56}Cr and
^{58}Cr by relativistic Coulomb excitation using the
FRS-RISING setup at GSI, see Fig.6.

The results show that the 2+ state in
^{56}Cr with neutron number N=32 is less collective than its
neighbours. This is evidence for a new subshell closure [see A. Bürger *et
al.*, Phys. Lett. B, submitted (2005)].

## Electron Spectroscopy

From January 23 to 24 2003 there was a Mini-Workshop on Future In-Beam Conversion-Electron Spectroscopy here at the HISKP.

The talks held for this workshop are available for download (PDF format, files up to 4 MB): Piotr Bednarczyk, Alexander Bürger, Andreas Görgen, Paul Greenless, Yves LeCoz, Witold Meczynski, Edgar Mergel, Macin Palacz, Costel Petrache, Peter Thirolf. The list of participants and the program are also available in PDF format.