Measurement of charge-changing cross section of neutronrich nitrogen isotopes for determining their proton radii

Abstract

Exploring neutron-rich nuclei near the drip-line exposes exotic phenomena, including neutron halo or skin and the (dis)appearance of the existing magic numbers. A systematic study of point proton radii along an isotopic chain provides insights into the extended neutron wavefunction’s impact on protons. This work, conducted at RIKEN, Japan, presents the first determination of the charge-changing cross section (σcc) for 23N as well as the σcc for 21N, with a secondary beam energy of around 250A MeV. The measured σcc values for 21N and 23N were 752 ± 4 mb and 747 ± 3 mb, respectively. Their comparable σcc suggests that the center-of-mass of the two valence neutrons in 23N is not spatially separated from that of the core. Combined with the previously reported σcc of 22N, these results indicate a shell closure at N = 16 for nitrogen isotopes. A higher two-neutron separation energy in 23N supports the presence of a shell closure at N = 16 and raises questions about the reported large matter radius of this drip-line nitrogen isotope.