Low-spin non-yrast states in light tungsten isotopes and the evolution of shape coexistence

Excited states in W-170, W-172, W-174, W-176 and W-178 have been populated in the beta -decay of Re170-178 isotopes and studied using gamma -ray singl es and coincidence techniques, utilising the high efficiency of a Compton-s uppressed array, and conversion electron systems. Many new states at low ex citation energies have been identified, complementing the level schemes pre viously established from in-beam studies. A large body of data on decay pro perties, spins and parities, and relative E2 and E0 matrix elements has bee n obtained. The properties of the K-pi = 0(+) ("quasi"-beta) and the K-pi = 2(+) ("quasi"-gamma) bands are compared with similar bands observed in lig ht osmium, platinum and mercury isotopes, where shape coexistence has been established at low spin. (C) 2001 Elsevier Science B.V. All rights reserved .