Collectivity in light zirconium isotopes: Evolution with neutron number and angular momentum

Considerable data have recently been collected on light zirconium isotopes. We present new measurements on [sup 84]Zr and some unpublished results on [sup 82]Zr. We have performed calculations on [sup 82,84,86]Zr using an extension to the IBM which allows a collective core to couple to up to four unpaired fermions (two broken pairs). The role of vibrational collectivity is clear in the [ital N]=44 and [ital N]=46 nuclei, but collective rotation becomes the dominant mode below [ital N]=44. The relative positions of the neutron and proton Fermi surfaces are crucial in determining the complexity of the decay path down the yrast states, being dominated by proton configurations for [ital N][le]44 but showing strong competition between neutron and proton structures when [ital N][gt]44.