Novel Manifestation of α-Clustering Structures: New ``α+Pb208'' States in Po212 Revealed by Their Enhanced E1 Decays

Excited states in Po212 were populated by α transfer using the Pb208(O18,C14) reaction, and their deexcitation γ rays were studied with the Euroball array. Several levels were found to decay by a unique E1 transition (E_γ<1MeV) populating the yrast state with the same spin value. Their lifetimes were measured by the Doppler-shift attenuation method. The values, found in the range 0.1-1.4 ps, lead to very enhanced transitions, B(E1)=2×10^-2-1×10^-3W.u. These results are discussed in terms of an α-cluster structure which gives rise to states with non-natural-parity values, provided that the composite system cannot rotate collectively, as expected in the “α+Pb208” case. Such states due to the oscillatory motion of the α-core distance are observed for the first time.