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

TL;DR

This paper reports the discovery of new $oldsymbol{ ext{alpha}}$-cluster states in $oldsymbol{^{212} ext{Po}}$ characterized by enhanced E1 decays, revealing oscillatory $oldsymbol{ ext{alpha}}$-core motion and non-natural parity states.

Contribution

It presents the first observation of $oldsymbol{ ext{alpha}}$ + $oldsymbol{^{208} ext{Pb}}$ states with enhanced E1 decays and non-natural parity, indicating oscillatory $oldsymbol{ ext{alpha}}$-core motion in $oldsymbol{^{212} ext{Po}}$.

Findings

Enhanced E1 transitions with B(E1) = 2×10^{-2} - 1×10^{-3} W.u.

Observation of non-natural parity states due to $oldsymbol{ ext{alpha}}$-cluster oscillations.

First experimental evidence of $oldsymbol{ ext{alpha}}$-core oscillatory motion in this nucleus.

Abstract

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.