Neutron-unbound excited states of ${}^{23}\mathrm{N}$

Neutron unbound states in ${}^{23}\mathrm{N}$ were populated via proton knockout from an 83.4 MeV/nucleon ${}^{24}\mathrm{O}$ beam on a liquid deuterium target. The two-body decay energy displays two peaks at $E_1\sim 100\mathrm{keV}$ and $E_2\sim 1\mathrm{MeV}$ with respect to the neutron separation energy. The data are consistent with shell model calculations predicting resonances at excitation energies of $\sim 3.6\mathrm{MeV}$ and $\sim 4.5\mathrm{MeV}$. The selectivity of the reaction implies that these states correspond to the first and second $3/2^{-}$ states. The energy of the first state is about 1.3 MeV lower than the first excited $2^{+}$ in ${}^{24}\mathrm{O}$. This decrease is largely due to coupling with the $\pi p_{3/2}^{-1}$ hole along with a small reduction of the $N=16$ shell gap in ${}^{23}\mathrm{N}$.

Figure 1

Two-body decay energy for ${}^{22}\mathrm{N}+1n$. The best fit includes two-channel Breit-Wigners resulting from two states at 1.1 MeV (dashed-red line) and 2.4 MeV(dot-dashed-blue line). Background contributions are in shaded gray. The efficiency and resolution are shown in the inset as the blue histogram (left scale) and red-dashed line (right scale), respectively.

Figure 2

A possible level ordering in ${}^{23}\mathrm{N}$ consistent with the observed spectrum. The arrows indicate transitions from the first- and second-excited $3/2^{-}$ state in ${}^{23}\mathrm{N}$ to various states in ${}^{22}\mathrm{N}$. The hatched areas indicate the experimental uncertainty given the assumptions discussed in the text. The colors correspond to the fit in Fig. 1. The branching from the $3/2^{-}$ states to the various excited states of ${}^{22}\mathrm{N}$ cannot be resolved without $\gamma$ detection. Shell model calculations for ${}^{23}\mathrm{N}$ are shown for comparison on the right.

Figure 3

Energy dependence of the first-excited $3/2^{-}$ state on the shift, $\mathrm{\Delta }$, on the energy of the $d_{3/2}$ orbital (solid-blue line) or $p_{1/2}$ orbit (dashed-red line). The dotted black lines denote the energies of the pure $1p-1h$ or $\pi p_{3/2}^{-1}$ configurations in the initial calculation ($\mathrm{\Delta }=0$). The experimental energy determined in this work is denoted by the black square.