Novel Manifestation of $\alpha$-Clustering Structures: New “$\alpha +{}^{208}\mathrm{Pb}$” States in ${}^{212}\mathrm{Po}$ Revealed by Their Enhanced $E1$ Decays

Excited states in ${}^{212}\mathrm{Po}$ were populated by $\alpha$ transfer using the ${}^{208}\mathrm{Pb}({}^{18}\mathrm{O},{}^{14}\mathrm{C})$ reaction, and their deexcitation $\gamma$ rays were studied with the Euroball array. Several levels were found to decay by a unique $E1$ transition ($E_{\gamma }<1\mathrm{MeV}$) 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, $B(E1)=2\times {10}^{-2}–1\times {10}^{-3}\mathrm{W}.\mathrm{u}.$ 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 “$\alpha +{}^{208}\mathrm{Pb}$” case. Such states due to the oscillatory motion of the $\alpha$-core distance are observed for the first time.

Figure 1

Part of the level scheme of ${{}_{84}{}^{212}\mathrm{P}\mathrm{o}}_{128}$ obtained in this work. The long-lived isomeric state at 2922(15) keV excitation energy, a pure $\alpha$ emitter not observed in the present work, is drawn for the sake of completeness. The width of the arrows is representative of the intensity of the $\gamma$ rays. The transitions marked with an asterisk are emitted by states with $\tau <1.4\mathrm{ps}$ (see Table ). The colored states are also displayed in the bottom part of Fig. 3.

Figure 2

Left: energy of the Doppler-shifted line around 780 keV as a function of the detection angle. The curve is the best fit obtained for a relative velocity $\frac{v}{c}=1\%$ and a transition energy of 780.4 keV. Right: examples of line-shape analysis at forward (35°) and backward (148°) angles.

Figure 3

(a), (b) Excitation energy of the ${}^{212}\mathrm{Po}$ states as a function of angular momentum. Each state drawn with a filled circle decays by an enhanced $E1$ transition towards the yrast state having the same $I$ value (drawn with a filled square). (c) Experimental $B(E1)$ values (in W.u.) versus the energy difference (in keV) between $I^{\mp }$ levels (log-log plot). The empty circles are lowest limits of $B(E1)$ calculated from the limit values of the lifetimes given in the right-hand portion of Table .