Fig. 1 Spin as a function of rotational frequency in the M1 bands of ⁸²Rb (left panel), ⁸³Rb (middle panel) and ⁸⁴Rb (right panel). Lines are drawn to guide the eye.

So far, the interpretation of the M1 bands was based on B(M1)/B(E2) ratios deduced from g-ray intensities. However, a detailed test of the predictions of the tilted-axis cranking model requires the knowledge of absolute M1 and E2 transition strengths. Therefore, we carried out a measurement of level lifetimes using the reaction ¹¹B + ⁷⁶Ge (E = 45 MeV) at the XTU tandem accelerator of the LNL Legnaro. Gamma rays were detected with the spectrometer GASP. A thick target was used to stop the recoil nuclei. Level lifetimes were deduced from a lineshape analysis applying the Doppler-shift-attenuation method.
Absolute experimental transition strengths within the M1 band of ⁸⁴Rb derived from the present lifetime measurement are compared with predictions of TAC calculations [2] in Fig. 2. The experimental B(M1) values show a smooth decrease with increasing rotational frequency which may indicate the shears mechanism, but is not as pronounced as the calculated behaviour. The experimental B(E2) values increase up to (^h/_2p)w » 0.6 MeV and decrease strongly towards higher frequency which may indicate a loss of collectivity due to a change of the structure. However, the calculated B(E2) values stay almost constant over the considered frequency range.
In contrast to the odd-odd isotopes, the negative-parity M1 sequence in the odd-even nucleus ⁸³Rb is not regular, which can be seen in Fig. 1. Moreover, the B(M1)/B(E2) ratios reveal a pronounced staggering that is not compatible with the shears mechanism [3] and cannot be described in the TAC model. Based on the present lifetime measurement we deduced absolute transition strengths within the negative-parity M1 sequence of ⁸³Rb which are shown in Fig. 3.

Fig. 2 Experimental and calculated B(M1) (left panel) and B(E2) (right panel) transition strengths versus the rotational frequency in the M1 band of ⁸⁴Rb.

Fig. 3 Experimental and calculated B(M1) (left panel) and B(E2) (right panel) transition strengths versus the rotational frequency in the negative-parity M1 sequence of ⁸³Rb.

The experimental B(M1) as well as the B(E2) values show an irregular behaviour which is considered as the consequence of the even neutron number of this isotope in difference to the two odd-odd neighbours ⁸²Rb₄₅ and ⁸⁴Rb₄₇. In ⁸³Rb, the breakup of a neutron pair is necessary to generate excitations that include unpaired nucleons of both kinds. A possible configuration of negative parity is the 3-qp configuration p(g_9/2) n(g_9/2) n(fp). This configuration has one g_9/2 particle less than the 4-qp configuration assumed for the odd-odd isotopes and, therefore, less possibilities to generate high spins. This and the specific neutron orbitals obviously lead to a spin coupling which cannot realize the shears mechanism. A further possible configuration similar to the one in the odd-odd isotopes is the 4-qp configuration p(fp) p(g_9/2²) n(g_9/2²) which may occur at higher energy.
The different behaviour of the isotopes with odd numbers of N = 45, 47 on the one hand and an even number of N = 46 on the other hand demonstrates the delicate balance between irregular and coherent collective motion in this mass region.

¹ FZR and Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria
² II. Physikalisches Institut, Universität Göttingen, 37073 Göttingen
³ INFN, Laboratori Nazionali di Legnaro, 35020 Legnaro, Italy
⁴ INFN and Dipartimento di Fisica dell'Universita di Padova, 35131 Padova, Italy