Single-neutron excitations in ${}^{18}$N

States in ${}^{18}$N have been populated in the neutron-adding $(d,p)$ reaction on the short-lived ${}^{17}$N beam. Previously observed levels, found in this work at excitation energies of 0.12(1) MeV and 0.74(1) MeV, have been identified as those expected from a proton $0p_{1/2}$ hole coupled to the ${}^{19}$O $5/2^{+}$ ground state. A new state at 1.17(2) MeV is consistent with the coupling of the $1/2^{-}$ proton-hole state to the excited $1/2^{+}$ state in ${}^{19}$O. Orbital angular momentum assignments and spectroscopic factors were determined from the measured angular distributions through a distorted wave Born approximation analysis. Systematics for the ${\left(0d_{5/2}1s_{1/2}\right)}^3$ neutron configurations in the $N=11$ isotones, ${}^{17}$C, ${}^{18}$N, and ${}^{19}$O are discussed and comparisons to $p$-$sd$ shell-model calculations are made.

Figure 1

Proton energies ($E_p$) as a function of the longitudinal distance from the target ($z$) for the ${}^{17}$N($d,p$)${}^{18}$N reaction in inverse kinematics. The events shown required a coincidence in the recoil detector telescope with either ${}^{18}$N ions for bound states, or ${}^{17}$N for unbound ones.

Figure 2

The measured excitation-energy ($Q$-value) spectrum for the ${}^{17}$N($d,p$) reaction with the same data set as is in Fig. 1. An expanded region of the excitation energy below the neutron separation energy ($S_{\text{n}}$) is shown in the inset.

Figure 3

Measured angular distributions (data points) with the center-of-mass (${\theta }_{\text{c.m.}}$) angles representing the centers of the angle bins. Statistical uncertainties are given by vertical error bars. DWBA angular distributions are shown by the various lines (see text for details).

Figure 4

Experimental (a) and calculated (b)–(d) single-neutron spectroscopic strengths (note the nonlinear energy scale). Filled boxes represent $\ell =2$ strength, hatched boxes $\ell =0$, and the colors correspond to levels shown in Fig. 5. The experimental spectroscopic strengths for the $1^{-}$ ground state and the 0.587-MeV $\left(2^{-}\right)$ state are upper limits, and relative uncertainties are shown by the black error bars. The measured state at 1.17 MeV labeled as $\left(1^{-}\right)$ could also be a $0^{-}$, $1^{-}$ $\ell =0$ doublet.

Figure 5

Lowest-lying experimental $J^{\pi }=5/2^{+}$, $3/2^{+}$, and $1/2^{+}$ excitations in $N=11$ nuclei. Level energy centroids are given by the round data points, and all levels are plotted relative to the $5/2^{+}$ centroid energies. The grey box represents the possible $1/2^{+}$ centroid energy range in ${}^{18}$N. The solid black lines connect corresponding levels in ${}^{19}$O to ${}^{17}$C.