mercredi 5 février 2014 à 11:20
Amphi G. Besse
Light neutron-rich nuclei lying at the frontier of particle stability, represent an important benchmark to probe the evolution of nuclear structure for increasing N/Z ratios. Structural information on these systems can be obtained either by nuclear reaction studies and exotic decay modes, providing complementary observables and different theoretical approaches. For nuclei located at the drip-line, large Qβ values and low breakup thresholds in the daughters imply many open particle-decay channels and cluster decays. Beta-delayed particle-emission channels can carry an important part of the GT strength for these nuclei, and precise measurements of their branching ratios provide insight on the original wave function and level schemes of the daughters.
On the other hand, neutron-unbound systems lying beyond the drip-line, can give us information about the structure evolution in neutron-rich nuclei with respect to the normal shell ordering, and help us to constrain the modeling of three-body systems like the two-neutron halo nuclei.
In the first part of this seminar, I will present some new experimental results obtained using one- and multi-nucleon removal reactions to study the neutron-unbound systems 10Li and 13Be. Using a fragmentation beam of 14B impinging on a carbon target, the decay energy spectra of the fragmentneutron systems have been measured using the invariant mass technique. New resonant states have been observed for 13Be, which have been interpreted with the help of simulations, based on theoretical calculations and on the selectivity of the reaction.
In the second part, I will talk about beta-delayed charged-particle emission in light exotic nuclei like 11Li and 14Be, measured by direct implantation on a thin, highly segmented silicon strip detector. This calorimetric method allows to suppress the beta-particle background in the decay energy spectra and provides an accurate normalization. These characteristics, coupled with a half-life discrimination of the daughters by means of a time stamp, make possible to measure accurately the spectral shape and the branching ratios of very rare charged-particle decay channels.
I will present some preliminary results concerning the 11Li particle-emission spectrum, which presents several beta-delayed emission channels in the continuum, proceeding through sequential decays towards many-body final states. This spectrum has been interpreted using the single-level, multiplechannel R-Matrix formalism. Finally, perspectives on some new measurements are discussed, such as the beta-delayed α-particle emission of 16N recently measured at KVI, of particular interest to constrain the fraction of E1 gamma transitions in the astrophysical reaction 12C(α,γ)16O.