The structure of nuclei in the vicinity of N = Z in the mass region A » 70 - 80 reveals a rich variety of structural phenomena and provides a good test for nuclear models. The competition between proton and neutron shell gaps occurring at large oblate and prolate deformations near the nucleon numbers 36 and 38 results in a coexistence of different nuclear shapes [1]. The purpose of the present study is to extend the knowledge about the structure of nuclei close to N = Z in the mass region A » 70 - 80. In particular, the information about high-spin states in such nuclei is scarce, since they can be produced in heavy-ion fusion reactions with small cross sections only. With the advent of large highly-efficient spectrometers as EUROBALL there are new prospects for the study of nuclei close to N = Z which are populated in weak reaction channels.

Fig. 1 Level scheme of ⁷⁰Se deduced from the present experiment.

Excited states in ⁷⁰Se were populated via the 2a2p exit channel of the reaction ⁴⁰Ca + ⁴⁰Ca at a beam energy of 185 MeV. The ⁴⁰Ca beam was provided by the XTU tandem accelerator at the Laboratori Nazionali di Legnaro. Gamma rays were measured with the EUROBALL III array consisting of 15 Cluster and 26 Clover detectors. Charged particles were detected with the silicon ball ISIS consisting of 40 DE-E telescopes. In the forward hemisphere, 50 neutron detector units filled with liquid scintillator BC501A were mounted.
The level scheme of ⁷⁰Se based on the present work is presented in Fig. 1. Excited states known from previous studies [2] were confirmed in our experiment. The positive-parity bands labelled A and C in Fig. 1 were considerably extended and a new positive-parity band (labelled B) was observed. Two new negative-parity bands (labelled D and E) were established on top of the irregular low-spin sequences.
The bands C, D and E are interpreted in terms of the configuration-dependent cranked Nilsson-Strutinsky model [3]. The potential energy surfaces show different minima that develop from configurations with the same numbers of nucleons lifted from the fp shell to the g_9/2 orbital. The newly observed negative-parity bands are thought to belong to such different minima. Thus, they represent a coexistence of different nuclear shapes at high spin.

¹ FZR and Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria
² Department of Mathematical Physics, Lund Institute of Technology, 22100 Lund, Sweden
³ Institut für Kernphysik, Universität zu Köln, 50937 Köln, Germany
⁴ II. Physikalisches Institut, Universität Göttingen, 37073 Göttingen, Germany
⁵ INFN, Laboratori Nazionali di Legnaro, 35020 Legnaro, Italy
⁶ University of York, Physical Department, Heslington, Y015DD York, United Kingdom
⁷ FZR and Institute for Nuclear Research and Nuclear Energy, BAS, 1784 Sofia, Bulgaria

References

[1] W. Nazarewicz et. al, Nucl. Phys. A 435, 397 (1985)
[2] T. Mylaeus et. al, J. Phys. G 15, L135 (1989)
[3] A.V. Afanasjev et. al, Phys. Rep. 322, 1 (1999)  IKH 06/21/01 © G. Rainovki