Low-Lying Dipole Strength In ~(20)O

摘要

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 ~(18)O and ~(20)O for levels in the region between 1 and 8 MeV. Two 1~- levels at 5.35(10) and 6.85(5) MeV were observed for the first time in ~(20)O. The observed γ-ray spectrum for ~(20)O is, in fact, dominated by transitions resulting from E1 excitations to these states. The extracted B(E1) ↑ values of ～0.062(16) e~2fm~2 and ～0.035(9) e~2fm~2 for the 5.35 and 6.85 MeV levels, respectively, are larger than shell model calculations predict. Such large dipole strengths are not observed for low-lying 1~- states in ~(18)O, indicating a shift of dipole strength towards lower energies as one approaches the neutron drip-line.