Microscopic Study of Spin Transfer in Near-Barrier Nuclear Reactions

TL;DR

This paper uses microscopic simulations to analyze how angular momentum is transferred to nuclear fragments during near-barrier reactions, revealing mechanisms that challenge previous macroscopic models.

Contribution

It introduces a TDDFT-based method to study spin transfer dynamics, highlighting the roles of nucleon transfer and neck formation in angular momentum sharing.

Findings

Spin transfer does not always increase during collisions.

Nucleon transfer and neck formation significantly influence spin transfer.

Some mechanisms contradict previous macroscopic assumptions.

Abstract

In quasi-fission, it is unclear what the interaction between the relative orbital angular momentum and the spin of the fragments is from a microscopic perspective. In macroscopic approaches, it is expected that the large value of the relative orbital angular momentum, of the order of 100~$\hbar$ is transferred through tangential dissipation to the fragments' intrinsic spin by sliding and rolling friction. The goal is to investigate the angular momentum transfer from the initial relative orbital angular momentum to the fragments' spin. How is the transferred spin shared between the fragments? What is the time scale associated with the different mechanisms? How does deformation play a role? A TDDFT simulation in the TDHF-Skyrme framework is used to describe several reactions at different impact parameters with increasing complexity. A method is proposed to study the evolution of the fragments' total spin as a function of time and angular velocity. The increasingly complex reaction allows for careful analysis of all the mechanisms responsible for the transfer of spin. In particular, it is shown that the transfer of nucleons, and neck formation can significantly affect the transfer of spin through tangential friction. Several mechanisms are in contradiction with previous macroscopic calculations. In particular, the spin of the fragments does not always increase during the collision which prevents it from being used to estimate the collision time of the reaction.