Experimental Nuclear Structure
The Ursinus College Nuclear Structure Group collaborates with nuclear scientists on experiments at accelerator laboratories like the John D. Fox Laboratory at Florida State University (FSU) and the Facility for Rare Isotope Beams at Michigan State University (FRIB). Our experimental work is focused on understanding the interplay between collective nuclear behavior, such as vibrations and rotations of the entire system, and the behavior of the individual protons and neutrons within it.
Our work is currently funded by National Science Foundation Grant Nos. PHY-2405485, “MRI: A New CeBr3 Gamma-Ray Detection Array (CeBrA) for Particle-Gamma Coincidence Experiments at the FSU Super-Enge Split-Pole Spectrograph” and PHY-2609627: “RUI: Collective Excitations and Shell Structure in Neutron-Rich Nuclei”.
Transfer Reactions at John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University (FSU)
For several years, we have used the Super Enge Split-Pole Spectrograph (SPS) at FSU to measure high-resolution proton momentum spectra and proton angular distributions from (d,p) reactions, as well as gamma-ray energies, relative intensities, and angular distributions in an effort to investigate the structure of neutron orbitals beyond the N=28 shell and how they vary with proton number.
In summer 2026, Hanna Ross (UC Physics ‘28) finalized the model of the CeBrA array of CeBr3 scintillator detectors in the UCCeBrA geant4 simulation code used for the planning and analysis of experiments at FSU. Following the close of the Summer Fellows program, we delivered 9 new CeBr3 detectors to FSU to complete CeBrA. We ran a commissioning experiment, single neutron transfer from a deuteron beam to a thin 54Fe target, adding gamma-ray information to our earlier measuremend made with just the spectrograph. Hanna plans to analyze this data set.
Hanna Ross (UC Physics ‘28) and Dr. Mark Spieker (FSU) installing newly-delivered cerium bromide detectors around the target chamber in preparation for the commissioning experiment in summer 2026.
Inverse-Kinematics Proton Scattering and Coulomb Excitation at the NSCL/FRIB
We have been studying the systematic behavior of proton and neutron contributions to collective excitations of N=28 isotones “south” of 48Ca since 2003. Most recently, we have been using the GRETINA gamma-ray tracking array and the NSCL/Ursinus liquid hydrogen target for these studies. There significant experimental evidence for the erosion of the N=28 shell approaching 42Si. This work culminated in Coulomb excitation and proton scattering measurements of 42Si at FRIB in 2024, in which five Ursinus undergraduates participated. There are two major findings. First, protons are “punching above their weight” in 42Si, despite the fact that it contains twice as many neutrons as protons. Second, 42Si behaves as a deformed rotor, providing another piece of evidence that the N=28 neutron shell closure has vanished in silicon.
The systematic behavior of the neutron and proton transition matrix elements in the neutron-rich sulfur and silocon isotopes. We see that the matrix elements are consistent with each other within uncertainty in 42Si, but there are twice as many neutrons as protons in this nucleus, which has 28 neutrons and only 14 protons. Similar matrix elements means protons are dominant in the excitation.
Determination of proton and neutron contributions to the 0+g.s. → 2+1 excitations in 42Si and 44S using inelastic proton scattering in inverse kinematics and intermediate-energy Coulomb excitation, L. A. Riley, I. Conroy, A. M. Himmelreich, M. Heinze, J. Kosa, B. McNulty, P. D. Cottle, M. Spieker, A. Volya, A. L. Conley, D. Houlihan, B. Kelly, K. W. Kemper, Sk M. Ali, T. Beck, S. A. Gillespie, M. Hausmann, S. Noji, J. Pereira, and D. Weisshaar, J. Chung-Jung, P. Farris, A. Gade, G. Grauvogel, A. M. Hill, Z. Rahman, and R. G. T. Zegers, B. Longfellow, N. D. Pathirana, Phys. Rev. C 112, 014331 (2025), arXiv:2508.00703.
