Selectivity of the one-neutron knockout reaction on ${}^{45}\mathrm{Cl}$ and the collapse of the $N=28$ shell closure

Neutron hole states in the exotic $N=27$ isotope ${}^{44}\mathrm{Cl}$ have been populated using the intermediate-energy single-neutron knockout reaction ${}^9\mathrm{Be}({}^{45}\mathrm{Cl},{}^{44}\mathrm{Cl})\mathrm{X}$. The momentum distribution of the residual ${}^{44}\mathrm{Cl}$ nuclei after direct population of the ground state is consistent with removal of an $l=1$ neutron. This observation and comparison with a shell-model calculation imply that $p_{3/2}$ neutrons from above the $N=28$ major shell closure play important roles in the ground states of both ${}^{44}\mathrm{Cl}$ and ${}^{45}\mathrm{Cl}$. The present result is significant because ${}^{44}\mathrm{Cl}$ is even closer to the valley of stability than ${}^{43}\mathrm{S}$, where a similar result was recently obtained using a $g$-factor measurement.

Figure 1

Spectrum of $\gamma$ rays measured in coincidence with recoiling ${}^{44}\mathrm{Cl}$ particles. The solid curve is the GEANT4 fit described in the text. The dashed curve is the fit without the 475-keV $\gamma$ ray.

Figure 2

(a) Inclusive, (b) $\gamma$-gated, and (c) ground-state parallel momentum distributions of ${}^{44}\mathrm{Cl}$ particles produced in one-neutron knockout from ${}^{45}\mathrm{Cl}$. The dashed (dotted) curves are theoretical distributions for $l=1$ ($l=3$) neutron removal. The solid curve in panel (a) is the fit described in the text.