Interplay between quantum shells and orientation in quasi-fission

Abstract

The quasi-fission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in $^{40}$Ca$+^{238}$U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed $^{238}$U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of $^{238}$U produce quasi-fission fragments in the magic $Z=82$ region, whilst collisions with the side are the only one which may result in fusion.