Nuclear Physics

The activities of the Nuclear Physics Department during the year 2000 concentrated on the following topics:

• The design of the Bremsstrahlung facility at the ELBE accelerator with the aim to provide us an optimized source for intense polarized gamma radiation in an envisaged energy range up to 20 MeV was completed. Particular attention was paid to achieve a low background level of scattered photons and neutrons in the experimental cave. The electron beamline in the accelerator hall including the radiator target chamber and the electron beam dump as well as the aluminum collimator with an optional photon beam hardener is now under construction.
• In preparing future photon scattering studies at ELBE we performed an experiment at the Bremsstrahlung beam of the Dynamitron at Stuttgart in collaboration with the groups at the Universities of Stuttgart and Köln. In this experiment the semi-magic nucleus ⁸⁷Rb was investigated for the first time to find indications whether the N=50 neutron core excitations can contribute to the dipole states observed in photon scattering.
• The extensive data taken in an EUROBALL experiment of our group in Legnaro have been analysed and evaluated. Interesting new features concerning high spin structures of the N » Z unstable nuclei produced by the ⁴⁰Ca+⁴⁰Ca reaction were found. For instance, rotational bands identified in ⁷²Br up to high spin values manifest a striking case of signature inversion. According to the applied Nilsson-Strutinsky model this observation provides evidence for a triaxial shape of this nucleus which continuously changes with spin. In the neighboring isotope ⁷³Br rotational structures were measured which persist to the extremely high spin value I=65/2(^h/_2p) . The theoretical analysis has shown that one of those bands is observed up to its limiting spin where all particles of a given configuration have fully aligned their angular momenta along the nuclear symmetry axis. This is the first case of a so-called band termination in the mass region around A=80.
• In collaboration with the ISOL group at the GSI Darmstadt the nuclei ⁵²Fe and ⁹³Pd were studied. The detailed properties of these very neutron deficient nuclei are important for the understanding of the nucleosynthesis in rp-processes.
• Concerning the progress in nuclear structure theory further developments of the 3-dimensional cranking model were made to describe the chiral rotation which incorporates the highest degree of symmetry breaking occurring for the spin orientation. In fact, according to the latest experimental findings there exists an island of odd-odd isotones with N=75 around ¹³⁴Pr where this peculiar type of rotation shows up through the predicted signature of parallel sister bands.
• The integration method developed to calculate the RPA correlation energy was applied for realistic systems. These results demonstrate that quantal vibrations may considerably influence the rotational properties. Remarkably, these effects are sensitive against the specific features of the effective interaction.

Collaborations

We were benefiting from the successful and permanent cooperation with the spectroscopy groups from the German universities at Stuttgart, Köln, Göttingen and Darmstadt as well as with ISOL group of the GSI Darmstadt. We acknowledge also the fruitful cooperation with the Italian colleagues which were experimenting with us at the EUROBALL Legnaro, the colleagues from the Sofia University and University of Leuven. We are also grateful to the theoretical support from our colleagues at the University of Tennessee, and Notre Dame University, both in the US.  IKH 06/27/01 © F. Dönau