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

Abstract

<p><span>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&rsquo;s impact on protons. This work, conducted at RIKEN, Japan, presents the first determination of the charge-changing cross section (&sigma;_cc) for ²³N as well as the &sigma;_cc for ²¹N, with a secondary beam energy of around 250&Alpha; MeV. The measured &sigma;_cc values for ²¹N and ²³N were 752 &plusmn; 4 mb and 747 &plusmn; 3 mb, respectively. Their comparable &sigma;_cc 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 &sigma;_cc of ²²N, these results indicate a shell closure at &Nu; = 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.</span></p>