Migration of nuclear shell gaps studied in the d(24Ne,p gamma)25Ne reaction

TL;DR

This study investigates the migration of neutron shell gaps in neutron-rich neon isotopes by measuring the (d,p) reaction on 24Ne, revealing a reduction in the N=20 shell gap and challenging existing shell model assumptions.

Contribution

It provides experimental confirmation of the migration of the neutron shell gap from N=20 to N=16 in 25Ne, highlighting the need for revised shell model interactions.

Findings

Confirmation of the raised 3/2+ level in 25Ne

Evidence for reduction of the N=20 shell gap

Breakdown of the USD shell model in this region

Abstract

The transfer of neutrons onto 24Ne has been measured using a reaccelerated radioactive beam of 24Ne to study the (d,p) reaction in inverse kinematics. The unusual raising of the first 3/2+ level in 25Ne and its significance in terms of the migration of the neutron magic number from N=20 to N=16 is put on a firm footing by confirmation of this state's identity. The raised 3/2+ level is observed simultaneously with the intruder negative parity 7/2- and 3/2- levels, providing evidence for the reduction in the N=20 gap. The coincident gamma-ray decays allowed the assignment of spins as well as the transferred orbital angular momentum. The excitation energy of the 3/2+ state shows that the established USD shell model breaks down well within the sd model space and requires a revised treatment of the proton-neutron monopole interaction.