An examination of the role of dipole transitions in determining the collectivity of nuclear excitations

The inelastic neutron scattering (INS) reaction has been shown to populate both collective and non-collective nuclear states in a statistical manner. Moreover, with this reaction, it is often possible to probe excited states that cannot be examined with other reactions. Recent advances in Doppler-shift attenuation method (DSAM) techniques following INS have permitted the author to determine lifetimes in the femtosecond regime in heavy nuclei, and these measured lifetimes have been used to identify possible low-spin collective excitations. In two mass regions, he obtained evidence for fast E1 transitions; these transitions are offered as evidence for octupole-octupole and quadrupole-octupole excitations. However, he found the occurrence of fast E1 transitions to be a more common phenomenon than expected and was forced to question whether such transitions can unambiguously be taken as evidence of collective character. In the rare earth region, he has been able to observe γ-ray de-excitation from M1 scissors mode states and to measure the lifetimes of these states