Low-Lying Dipole Strength In 20O.

The availability of fast radioactive beams offers the possibility for studies of E1-strength in projectiles via Coulomb excitation. Theoretical calculations predict that a significant fraction of this strength is shifted towards lower excitation energies in neutron-rich systems (e.g., [1]). At the NSCL, virtual photon scattering was used to probe the discrete structure of both [sup 18]O and [sup 20]O for levels in the region between 1 and 8 MeV. Two 1[sup -] levels at 5.35(10) and 6.85(5) MeV were observed for the first time in [sup 20]O [2]. The observed γ-ray spectrum for [sup 20]O is, in fact, dominated by transitions resulting from E1 excitations to these states. The extracted B(E1) ↑ values of ∼0.062(16) e²fm² and ∼0.035(9) e&sup22fm² for the 5.35 and 6.85 MeV levels, respectively, are larger than shell model calculations predict [3, 4]. Such large dipole strengths are not observed for low-lying 1[sup -] states in [sup 18]O, indicating a shift of dipole strength towards lower energies as one approaches the neutron drip-line. [ABSTRACT FROM AUTHOR]