Hope College Department of Physics and Engineering
Research Experiences for Undergraduates
Summer 2003
Project Summary


Project Title: Using Fusion Cross Sections to Understand the Nuclear Structure of 8Li
Student Name: Carol J. Guess
Studentís home institution: Hope College
Research Advisor(s): Dr. P. A. DeYoung, Dr. G. F. Peaslee
Source of Support: NSF-RUI

In some isotopes, excess neutrons form a skin around the nucleus. The presence or absence of a skin affects many nuclear properties and is thus an integral part of understanding the nucleus. A skin increases the nuclear radius, and the outer neutrons allow the nucleus to come closer to other nuclei without an increase in the Coulomb repulsion. This increases the cross section of fusion. An experiment was done at the University of Notre Dame's Nuclear Structure Laboratory to investigate the nuclear structure of 8Li. A beam of 8Li was produced from the collision between 7Li and 9Be. The 8Li then fused with 208Pb to yield 216At. Between two to five neutrons quickly evaporated. The most common evaporation formed 212At, which underwent alpha decay to 208Bi. Alpha particles were emitted at 7.68 MeV and 7.84 MeV from the ground and metastable states of 212At. Determining how the ratio of these states changes with beam energies will give information about the angular momentum of the original 216At. Choosing beam energies from 36 to 42 MeV allowed exploration of the energy dependence of the reaction. Information from alpha decay permits calculation of the fusion cross section for 216At. This will be compared to the theoretical cross section, and the results will yield information on the structure of the 8Li nucleus. An important feature of the experiment is that the beam was cycled on and off. When the beam is on the target, many types of particles hit the detectors: alpha particles of many energies, scattered beam particles, and electrons. When the beam is off, only products of radioactive decay reach the detectors. The beam was cycled so alpha particles of the correct energy could be more readily distinguished from other particles emerging from the target. Detector efficiency has been modeled by Monte Carlo simulations, and currently differential equations are being developed to model the effects of the cycled beam on the detection of 212At decay products. Future goals include the calculation and comparison of both the theoretical and experimental cross sections.

Slide show of Carol Guess' work (Requires Microsoft PowerPoint or a PowerPoint viewer.
Publications and Presentations:
“Hindrance of Complete Fusion in the 8Li+208Pb System at Above-Barrier Energies.” E.F. Aguilera, E. Martinez-Quiroz, P. Rosales, J.J. Kolata, P.A. DeYoung, P.J. Mears*, C. Guess*, F.D. Becchetti, J.H. Lupton, and Yu Chen. Phys. Rev. C 80, 044605 (2009).
“Protons from 8B+58Ni.” E.f. Aguilera, E. Martinez-Quiroz, H. Garcia-martinez, D. Lizcano, J.J. Kolato, L.O. Lamm, G. Rogachev, P.A. DeYoung, C. Guess*, U. Khadka*, P.J. Mears*, F.D. Becchetti, Y. Chen, H. Jiang, J.D. Hinnefeld, and G.F. Peaslee. Revista Mexicana de Fisica 52, 41 (2006).
“Two-neutron Transfer in the 6He+209Bi Reaction Near the Coulomb Barrier.” P.A. DeYoung, P.J. Mears*, J.J. Kolata, E.F. Aguilera, F.D. Beccetti, Y. Chen, M. Cloughesy, H. Griffin, C. Guess*, J.D. Hinnefeld, H. Jiang, Scott R. Jones, U. Khadka*, D. Lizcano, E. Martinez-Quiroz, M. Ojaniega, G.F. Peaslee, A. Peña*, S. VanDenDrieddshe, and J. Zimmerman. Phys. Rev. C 71, 051601 (2005).
“Understanding the Nuclear Structure of 8Li with a Radioactive Nuclear Beam.” Carol J. Guess*, P.A. DeYoung, G.F. Peaslee, E.F. Aguilera, E. Martinex Quiroz, J.J. Kolata, and J.D. Hinnefeld. Poster CK.025. Division of Nuclear Physics Fall Meeting. Chicago IL, October 2004