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

Astier, A.
Petkov, P.
Porquet, M. -G.
Delion, D. S.
Schuck, P.
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Excited states in $^{212}$Po were populated by $\alpha$ transfer using the $^{208}$Pb($^{18}$O, $^{14}$C) reaction and their deexcitation $\gamma$-rays were studied with the Euroball array. Several levels were found to decay by a unique E1 transition (E$_\gamma <$ 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 $\alpha$-cluster structure which gives rise to states with non-natural parity values, provided that the composite system cannot rotate collectively, as expected in the '$\alpha$ + $^{208}$Pb' case. Such states due to the oscillatory motion of the $\alpha$-core distance are observed for the first time.
Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Lett
Nuclear Experiment, Nuclear Theory