Disappearance of the $N=14$ shell

An excited state of ${}^{22}\mathrm{N}$ unbound with respect to neutron emission was observed in a stripping reaction from a 85 MeV/nucleon ${}^{26}\mathrm{F}$ beam. The observed decay energy of 650(50) keV places the level, which is interpreted to be the first $3^{-}$ state, at an excitation energy of 1.93(22) MeV. Together with the previously measured bound states of ${}^{22}\mathrm{N}$, reduction of the $N=14$ shell gap compared to less neutron-rich nitrogen isotopes at the neutron dripline is observed. Based on the magnitude of the reduction of the shell gap for ${}^{22}\mathrm{N}$, a disappearance of the gap and even a level inversion of the $\nu 1s_{1/2}$ and the $\nu 0d_{5/2}$ levels in the neutron-unbound nucleus ${}^{21}\mathrm{C}$ seems likely.

Figure 1

Neutron-decay energy spectrum of ${}^{22}\mathrm{N}$. The data with statistical error bars were measured by requiring a coincidence between an identified ${}^{21}\mathrm{N}$ fragment and a neutron. The solid line is the sum of the resonant (dotted) and nonresonant (dashed) contributions of a Monte Carlo simulation that takes into account the resolutions and acceptances of the experimental apparatus. The resonance line-shape is a Breit-Wigner curve with $E_{\mathrm{decay}}=0.65(5)$ MeV.

Figure 2

Level scheme of ${}^{22}\mathrm{N}$. The present experimental data are shown together with the data of Ref. [7] and are compared to WBTM and WBPM shell model calculations. The neutron separation energy as well as the ground and first excited state of ${}^{21}\mathrm{N}$ are also shown. The shaded area around the ${}^{21}\mathrm{N}$ ground state represents the uncertainty due to the neutron separation energy. The shaded area around the measured $3^{-}$ state corresponds to the excitation energy uncertainty which is dominated by the uncertainty of the neutron separation energy.

Figure 3

The “competition” between the $\nu 0d_{5/2}$ and $\nu 1s_{1/2}$ levels for the $N=15$ and $N=14$ isotones of carbon, nitrogen, and oxygen following the prescription of Ref. [10]. Experimental data for the levels of ${}^{23}\mathrm{O}$ and ${}^{22}\mathrm{N}$ are taken from Refs. [7, 8, 15], and this work. The $\nu 1s_{1/2}\text{−}\nu 0d_{5/2}$ gap for ${}^{22}\mathrm{O}$ and ${}^{21}\mathrm{N}$ is taken from Refs. [7, 8].