Fusion Reactions Between Neutron-rich Nuclei

Fusion reactions between neutron-rich nuclei have been proposed as a potentially important heat source in the outer-crust of an accreting neutron star, possibly triggering an X-ray superburst.  The underpinning element of this hypothesis is that the fusion of neutron-rich nuclei is enhanced relative to their beta-stable counterparts. Such an enhancement could arise from the increased polarizability of the neutron-rich nucleus due to the decreased average binding of the outermost neutrons and the existence of low-energy collective modes.  This increased polarizability, the prelude to neutron transfer, is schematically illustrated in Fig. 1. Examining the evolution of the fusion cross-section with increasing neutron number for an isotopic chain provides the best means to address this question.

The experiment was conducted at the ReA3 facility where beams of 39K and 47K at near-barrier energies were incident on a 28Si target. Fusion products were identified by measuring their energy and time-of-flight using a simple yet highly efficient experimental setup. Use of a highly efficient setup is mandated by the low intensity associated with radioactive beams. Presented in Fig. 2 is the dependence on incident energy of the fusion cross-section for 47K + 28Si as compared to 39K + 28Si. The relative fusion cross-section s(47K)/s(39K) clearly increases with decreasing incident energy. This increase in the relative cross section with decreasing energy reflects both the larger size of the neutron-rich nucleus, specifically the increased extent of its neutron density distribution, and the dynamics associated with the additional neutrons. [J. Vadas et al., Phys. Rev. C 97 031601 (R) (2018)]


Fig. 1: Schematic illustrating the fusion of 39K and 47K nuclei with 28Si nuclei. The influence of polarization on the fusion barrier is indicated.




Fig. 2: Dependence of the relative cross-section for fusion of 47K and 39K with 28Si nuclei on incident energy. With decreasing incident energy the relative cross-section increases rapidly. Polarization is included in the CCFULL and CCFULL+ n-Trans calculations. The role of mutual excitation of the two nuclei and neutron transfer is evident.