Damping of Quantum Vibrations Revealed in Deep Sub-barrier Fusion

TL;DR

This paper reveals that quantum vibrations in colliding nuclei are damped near the touching point, which explains fusion hindrance in deep sub-barrier reactions, using RPA and coupled-channel calculations to match experimental data.

Contribution

First application of RPA to two-body nuclear systems showing damping of quantum vibrations near touching point, elucidating fusion hindrance mechanisms.

Findings

Quantum vibrations are damped near touching point.

Damping explains fusion hindrance in deep sub-barrier reactions.

Calculated fusion cross sections match experimental data.

Abstract

We demonstrate that when two colliding nuclei approach each other, their quantum vibrations are damped near the touching point. We show that this damping is responsible for the fusion hindrance phenomena measured in the deep sub-barrier fusion reactions. To show those, we for the first time apply the random-phase-approximation (RPA) method to the two-body $^{16}$O + $^{16}$O and $^{40}$Ca + $^{40}$Ca systems. We calculate the octupole transition strengths for the two nuclei adiabatically approaching each other. The calculated transition strength drastically decreases near the touching point, strongly suggesting the vanishing of the quantum couplings between the relative motion and the vibrational intrinsic degrees of freedom of each nucleus. Based on this picture, we also calculate the fusion cross section for the $^{40}$Ca + $^{40}$Ca system using the coupled-channel method with the damping factor simulating the vanishing of the couplings. The calculated results reproduce well the experimental data, indicating that the smooth transition from the sudden to adiabatic processes indeed occurs in the deep sub-barrier fusion reactions.