Measurements of the 16C + 12C and 16C + 13C Fusion Cross Sections with Implications for Astrophysics

Abstract

The fusion of neutron-rich nuclei is of interest to nuclear astrophysics and nuclear structure. X-ray superbursts are powered by runaway thermonuclear burning deep inside of a neutron star, where heating from the pycnonuclear fusion of neutron-rich isotopes is an important heat source. Experimental measurements of fusion cross sections of neutron-rich isotopes have provided insights regarding nucleon transfer and nuclear structure properties affecting fusion. Recently, the 15C + 12C total fusion cross section was measured using a 15C beam produced by the in-flight beam production facility, which is part of the Argonne Tandem LINAC Accelerator System (ATLAS) at Argonne National Laboratory (ANL). As an extension of that study to more neutron-rich systems, the 16C + 12C and 16C + 13C total fusion cross sections were measured. This dissertation presents the first fusion cross section measurements made with a 16C radioactive beam. The beam was produced using the newly upgraded RAdioactive Ion SeparatOR (RAISOR) facility at ANL. The total fusion cross sections were measured with 12C and 13C targets in the active target MUlti-Sampling Ionization Chamber (MUSIC) detector filled with natural methane gas and 99.9% enriched 13C methane gas, respectively. This is the most neutron-rich carbon fusion system that has been studied experimentally to date. The 16C + 12C and 16C + 13C cross sections were measured for EC.M. = 8 - 22 MeV. The measured cross sections show good agreement with theoretical models developed using the barrier-penetration formalism with the Sao Paulo potential and theoretical models using the selective resonant tunneling model and a complex square-well nuclear potential. Despite the significantly larger RMS radius of 16C, the 16C + 12, 13C cross sections are measured to be smaller than the 15C + 12C cross section. This indicates that an enhanced s-wave 15C wave function might be increasing the 15C fusion cross section or that neutron pairing effects in 16C may reduce the 16C cross sections.