Shell structure of ${}^{43}\mathrm{S}$ and collapse of the $N=28$ shell closure

The single-particle structure of the $N=27$ isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly magic ${}^{48}\mathrm{Ca}$ toward the drip line. ${}^{43}\mathrm{S}$ was studied employing the one-neutron knockout reaction from a radioactive ${}^{44}\mathrm{S}$ beam. Using a combination of prompt and delayed $\gamma$-ray spectroscopy the level structure of ${}^{43}\mathrm{S}$ was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the ${}^{44}\mathrm{S}$ ground-state configuration has a strong intruder component. The results were confronted with shell-model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the $2p_{3/2}$ orbital in the experiment indicates that the breakdown of the $N=28$ magic number is more rapid than the theoretical calculations suggest.

Figure 1

Particle-identification plot of reaction residues detected in the S800 spectrograph. A gate on incoming ${}^{44}\mathrm{S}$ ions is applied. The dashed line is the outgoing ${}^{43}\mathrm{S}$ gate for the further analysis.

Figure 2

Prompt, Doppler-corrected $\gamma$-ray energy spectrum for the one-neutron knockout reaction from ${}^{44}\mathrm{S}$ to ${}^{43}\mathrm{S}$. The peaks are labeled with the transition energy and uncertainty in keV. The background around 500 keV includes transitions from neutron-induced reactions on Ge and Al.

Figure 3

Background subtracted $\gamma \text{−}\gamma$ coincidence spectra measured in GRETINA. (a)–(d) show the spectra gated on the prompt 1155, 625, 850, and 977 keV transitions.

Figure 4

$\gamma$-ray energy spectrum measured by IsoTagger. A gate on ${}^{43}\mathrm{S}$ has been applied. The isomeric decay of the 320 keV state is observed. The inset shows the prompt, Doppler-corrected $\gamma$-ray energy spectrum measured with GRETINA gated on the delayed 320 keV transition.

Figure 5

Level scheme of ${}^{43}\mathrm{S}$ determined from the present experimental results and predicted by shell-model calculations. The width of the arrows reflects the measured $\gamma$-ray yields. Gray, dashed transitions are place based on the energy differences of established levels, the 228 keV state is placed based on the comparison to the shell-model calculations. The levels are labeled with the spin and parity assignments derived from the measured momentum distributions and the partial cross section for each state (in mb). Spins and parities of predicted $1/2^{-}, 3/2^{-}, 5/2^{-}$, and $7/2^{-}$ states are indicated in black, red, green, and blue, respectively. Theoretical cross sections (in mb) include the calculated spectroscopic factors and the reaction model calculations for the single-particle cross sections (see text for details).

Figure 6

Parallel momentum distributions of the one-neutron knockout reaction for several states in ${}^{43}\mathrm{S}$. Each panel shows the experimental parallel momentum distribution obtained by gating on $\gamma$-ray transitions in black, compared to theoretical eikonal reaction model calculations for removal of a neutron from the $p$ (red), $d$ (green), and $f$ (blue) orbital. (f) is the momentum distribution extracted in coincidence with the isomeric transition measured in IsoTagger and all others are obtained by gating prompt transitions measured in GRETINA.