A novel manifestation of $α$-clustering: new '$α$ + $^{208}$Pb' states in $^{212}$Po revealed by their enhanced E1 decays

Abstract

Excited states in $^{212}$Po were populated by $α$ transfer using the $^{208}$Pb($^{18}$O, $^{14}$C) reaction and their deexcitation $γ$-rays were studied with the Euroball array. Several levels were found to decay by a unique E1 transition (E$_γ<$ 1 MeV) populating the yrast state with the same spin value. Their lifetimes were measured by the DSAM method. The values, found in the range [0.1-1.4] ps, lead to very enhanced transitions, B(E1) = 2$\times10^{-2}$ - 1$\times10^{-3}$ W.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 '$α$ + $^{208}$Pb' case. Such states due to the oscillatory motion of the $α$-core distance are observed for the first time.