A novel manifestation of α clustering discovered in 212Po - 26 March 2010
Alain Astier (CSNSM-France)
11h -GANIL room 105
When a nucleus clusterizes into fragments with different charge to mass ratios, its centre of mass does not coincide anymore with its centre of charge, and a sizeable static E1 moment may arise in the intrinsic frame, leading to large values of the B(E1) transition probabilities.
This feature is well documented in deformed systems, such as light nuclei described in terms of a “α-core” system rotating around its centre of mass, or heavy nuclei displaying octupole deformation. However, if the spherical core is heavy enough, the centre of mass of the system is very close to the core centre, and the composite system cannot rotate collectively. Such a case has never been studied.
In this seminar, we will present new experimental results obtained for the 212Po isotope. Excited states in 212Po were populated by α transfer using the 208Pb(18O,14C) reaction and their deexcitation γ-rays were studied with the Euroball array. Several levels were found to decay by a unique E1 transition (Eγ < 1 MeV) populating the yrast state with the same spin value. Their lifetimes, measured by the DSAM method, are in the range [0.1-1.4] ps; they lead to very-enhanced E1 transitions: B(E1) = 2×10-2 – 1×10-3 W.u. (i.e. up to 1000 times more than typical B(E1) values).
These states, which coexist with those expected by the shell model, are the fingerprints of the “α-208Pb” structure of 212Po. They are interpreted as being due to the oscillatory motion of the α-core distance around its equilibrium position. Such a motion, observed for the first time in the whole nuclear chart, is a novel manifestation of α clustering in nuclei.
