Large B(M1) staggering at high spins in Zr86: Broken boson pairs in the four-quasiparticle regime

The transitional nucleus Zr86 has been studied by the Ni60(30Si,2p2n) reaction, with γ-γ coincidence, angular correlation, Doppler-shift-attenuation and measurements. Band structures exhibiting strongly staggered M1 transition rates were found at high spins, with large B(M1) values but no significant E2 collectivity. The unusual behavior of this nucleus is interpreted within the framework of an interacting boson model incorporating two broken boson pairs. This new model for high-spin states is seen to be remarkably successful in describing the electromagnetic decay properties in the four-quasiparticle regime.