One-neutron knockout reactions on proton-rich nuclei with $N=16$

One-neutron knockout reactions from the deeply bound $N=16$ isotones with $Z=16,17,$ and 18 have been studied in inverse kinematics with intermediate-energy beams. $\gamma$-ray spectroscopy in coincidence with the detection of knockout residues allowed for an investigation of the one-neutron removal leading to individual excited states. Spectroscopic factors are deduced in the framework of the sudden and eikonal approximations and are compared to USD shell-model predictions. The momentum distributions observed in the experiment are used to identify the angular momentum $l$ carried by the knockedout neutron by comparing with calculations based on a black-disk reaction model. The systematics of reduced single-particle occupancies attributed to the effect of short-range correlations, observed so far for stable and near-magic nuclei in $(e,e^{\prime }p)$ and $\left(d,{}^{\mathrm{3}}\mathrm{He}\right)$ reactions and in one-nucleon knockout on light deeply bound systems, are extended in this work.

Figure 1

$\gamma$ spectra event-by-event Doppler reconstructed into the projectile frame detected in coincidence with the residues ${}^{33}\mathrm{Ar}$ (upper panel) and ${}^{31}\mathrm{S}$ (lower panel). The $\gamma$ rays observed depopulate known excited states in these $N=15$ isotones. The excess of counts around $1740\text{keV}$ in the ${}^{31}\mathrm{S}$ spectrum does not correspond to a known transition in ${}^{31}\mathrm{S}$ and might be attributed to one of the higher-lying fragmented $5∕2^{+}$ states expected to be populated in one-neutron knockout according to shell-model calculations (see text for details).

Figure 2

$\gamma$-ray spectrum detected in the $37°$ ring of SeGA in coincidence with ${}^{32}\mathrm{Cl}$ event-by-event Doppler reconstructed into the projectile frame. The $90\text{keV}$ $\gamma$ ray is detectable only because it is Doppler boosted well above the electronics threshold at detector angles of $37°$. In the $90°$ ring its energy was below the detection limit. The comparison with the mirror nucleus ${}^{32}\mathrm{P}$ is given in the inset.

Figure 3

Inclusive longitudinal momentum distributions of ${}^{33}\mathrm{Ar}$ knockout residues (a) and the ${}^{34}\mathrm{Ar}$ “unreacted” beam (b). In focus mode the beam is dispersed in the focal plane. The shape of the ${}^{34}\mathrm{Ar}$ momentum distribution is nicely reproduced by a superposition of error functions. The theoretical momentum distributions were then folded with this curve to take into account the broadening induced by the S800 optics setting.

Figure 4

Momentum distribution of ${}^{31}\mathrm{S}$ decomposed into knockout to the ground state (a) and one-neutron removal leading to excited states (b) in the knockout residue. The excited-state distribution nicely fits with the assumption of $l=2$ (dashed-dotted line), while the distribution for the knockout to the ground state is best described by $68\%$ $l=0$ and $32\%$ $l=2$ suggesting unobserved feeding from higher-lying $5∕2^{+}$ fragments into the ground-state distribution which is expected to be of pure $l=0$ character corresponding to the knockout of a neutron out of the $s_{1∕2}$ orbit.

Figure 5

Ground-state (a) and excited-state (b) momentum distributions for the one-neutron knockout residues ${}^{32}\mathrm{Cl}$. The shapes are compared to pure $l=2$ (dashed-dotted line), pure $l=0$ (solid line), as well as to a superposition of both (dotted line) motivated by the estimates discussed in the text.

Figure 6

Ground-state (a) and excited-state momentum distributions (b) for ${}^{33}\mathrm{Ar}$ compared to calculations assuming $l=0$ (solid line) and $l=2$ (dashed-dotted), respectively. The narrow distribution for the knockout to the ${}^{33}\mathrm{Ar}$ ground state is in good agreement with the calculated momentum distribution for $l=0$ knockout consistent with the expected $s_{1∕2}$ configuration for the ground state of ${}^{33}\mathrm{Ar}$. The wider excited-state momentum distribution is nicely reproduced by $l=2$ proving that predominantly excited states with $d_{3∕2}$ and $d_{5∕2}$ configuration are populated in agreement with the level scheme observed.

Figure 7

Excited states observed in ${}^{33}\mathrm{Ar}$ compared to the mirror nucleus ${}^{33}\mathrm{P}$. The second $5∕2^{+}$ state is proton unbound. This level scheme is in agreement with a recent in-beam fragmentation experiment [48].

Figure 8

Reduction factors $R_s$ and inclusive cross sections for the one-neutron removal reactions ${}^9\mathrm{Be}\left({}^{32}\mathrm{S},{}^{31}\mathrm{S}+\gamma \right)\mathrm{X},{}^9\mathrm{Be}\left({}^{33}\mathrm{Cl},{}^{32}\mathrm{Cl}+\gamma \right)\mathrm{X},$ and ${}^9\mathrm{Be}({}^{34}\mathrm{Ar},{}^{33}\mathrm{Ar}+\gamma )\mathrm{X}$. Two sets of error bars are given for $R_s$, solely the experimental uncertainty (solid line style) and error bars with a $15\%$ uncertainty added in quadrature attributed to the reaction theory and its sensitivity to the Woods-Saxon potential and the core radius (dotted line style).