ALTERNATING PARITY BANDS IN DOUBLY ODD AC-218(129) AND OCTUPOLE INSTABILITY IN THE LIGHT ACTINIDE REGION

High-spin states of doubly odd Ac-218(129) have been investigated usin g in-beam alpha, gamma and conversion electron spectroscopy techniques through the Bi-209(C-12(13),3(4)n) fusion-evaporation reactions. The level scheme of Ac-218 shows three interconnected, alternating parity structures, each one linked by strong E1 transitions. Two of these ban ds are strikingly similar to the known scheme of the isotone Ra-217 an d this analogy is shown to persist for the pair Ac-220, Ra-219. The re lation between the structure of Ac-218 and its neighbors is thoroughly discussed showing that the first signals of octupole collectivity app ear for N = 129.