Hope College Department of Physics and Engineering Research Experiences for Undergraduates Summer 2002 Project Summary

Project Title: Understanding ²⁰⁹Bi[⁶He,⁴He] Near The Coulomb Barrier
Student Name: Joseph Bychowski
Student’s home institution: Hope College
Research Advisor(s): P. A. DeYoung, G. F. Peaslee
Source of Support (NSF-REU, or other): NSF Grant: PHY-0098061

The radioactive nucleus, ⁶He, has been studied widely because its neutron skin greatly affects that manner in which it interacts with other nuclei. In a previous experiment, ⁶He+²⁰⁹Bi at energies near the Coulomb barrier¹, the cross section for alpha particle emission was found to be surprisingly large. There are three likely reactions that may produce the observed alpha particles; two-neutron transfer, one-neutron transfer, and projectile breakup. Each of these mechanisms results in a different angular distribution of associated neutrons. In order to determine the cross section for these different mechanisms, an experiment was performed to measure the angular distribution of neutrons in coincidence with alpha particles. A 22.9 MeV ⁶He beam was produced with the University of Notre Dame’s TwinSol radioactive nuclear beam facility and directed onto a thin ²⁰⁹Bi target. The alpha particles were detected with two  D E-E Si telescopes and the neutrons were measured with eight liquid scintillator detectors. The data shows a strong neutron angular dependence.

Monte Carlo simulations of both one-neutron transfer and projectile breakup reactions were done in order to better understand the angular dependence of the neutron distributions in the experiment.

It is clear that cause of the experimentally observed angular distribution of neutrons is a result of the combination of both projectile breakup and 1-n transfer. Work is being done to quantify the contribution of both models as a fraction of total ⁴He yield.

¹E.F. Aguilera, et al. Phys. Rev. Lett. 84, 5058 (2000).