< Lew Riley Nuclear Structure Research

The Ursinus College Nuclear Structure Group

Experiments that probe nuclear structure are generally done at large accelerator facilities. We do our experimental work at the accelerator facilities at Florida State University (FSU) and the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. At the NSCL, we study exotic nuclei using fast beams (> 50 MeV/nucleon) in reactions including inverse-kinematics proton scattering, Coulomb excitation, single-nucleon knockout, and fragmentation. These reactions often produce nuclei in excited states which rapidly de-excite by emitting gamma rays. We measure the intensities and energies of these gamma rays to deduce properties of the excited states.

We study shell structure by measuring proton and neutron contributions to collective nuclear excitations. Collective excitations are excited states in which many protons and neutrons participate. The cartoon at right is an illustration of a collective quadrupole shape vibration, a common low-lying collective excitation. In the simplest collective picture, the nucleus us an incompressible "liquid drop" which undergoes collective shape vibrations and rotations. This purely collective model predicts equal proton and neutron contributions to excitations. However, the nuclear shell model, which focuses on single particle behavior, predicts that protons and neutrons can be trapped in closed shells and that valence nucleons dominate low-lying excited states. Hence, differences between proton and neutron behavior in collective excitations are evidence of underlying shell structure.

This material is based upon work supported by the National Science Foundation under Grant Nos. PHY-0355129, PHY-0098774/ PHY-0342281, PHY-0653323, PHY-0922615, and PHY-0969002. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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