Rotational structures in nuclei can be characterized by the signature quantum number a which defines the admissible spin sequence for a band through the relation I = a+ 2n (n = 0, 1, ...). This is a consequence of the well-known D2-symmetry of deformed intrinsic shapes reflected by the appearance of two signature partner bands with a = 0 or 1 in even-mass nuclei. By investigating the relative position of the signature partners, important information about the structural changes ocurring in the nucleus with the increasing spin can be gained.

Fig. 1 Energy difference between the states with spin I and spin (I - 1), divided by 2I, for the observed (upper panel) and calculated (lower panel) negative-parity bands in ⁷²Br.

A peculiar feature of the negative-parity bands in ⁷²Br is the signature splitting that can be seen in the upper panel of Fig. 1. The a = 1 branch is favoured up to I » 16 where a crossing of the branches takes place such that the a = 0 branch is favoured at higher spins. This signature inversion in the negative-parity bands is unique in the chain of the odd-odd Br isotopes. In contrast, the ^74,76,78Br nuclei display signature inversions in the positive-parity bands at lower spin values of 9,10,11, respectively.
To understand the microscopic origin of the signature inversion in the negative-parity bands of ⁷²Br we performed calculations using the configuration-dependent cranked Nilsson-Strutinsky (CNS) [1] approach. A fully consistent calculation was performed, where the equilibrium deformation in the potential energy surface was searched for at each spin value separately. A comparison of the calculated signature splitting with the experimental one is presented in Fig. 1. This comparison shows that the main features, especially the energetic order of the signature partners and the spin, where the crossing takes place, are correctly reproduced. However, it should be mentioned that the signature inversion can not be reproduced in calculations with a fixed g deformation [2]. The equilibrium deformation at low spin corresponds to negative g deformation and gradually changes with increasing spin to positive values at the highest observed spins. This shows that the signature inversion is a consequence of changing triaxial deformation.

¹ FZR and Horia Hulubei NIPNE, Bucharest, P.O. Box MG-6, Romania
² Department of Mathematical Physics, Lund Institute of Technology, P.O. Box 118, 22100 Lund, Sweden
³ INFN, Laboratori Nazionali di Legnaro, 35020 Legnaro, Italy
⁴ Institut für Kernphysik, Universität zu Köln, 50937 Köln
⁵ II. Physikalisches Institut, Universität Göttingen, 37073 Göttingen
⁶ University of York, Physics Department, Heslington, York Y01 5DD

References

[1] I. Ragnarsson et al., Phys. Rev. Lett. 74, 3935 (1995)

[2] C. Plettner et al., Phys. Rev. Lett. 85, 2454 (2000)  IKH 06/22/01 © C. Plettner