Triple configuration coexistence in ${}^{44}\mathrm{S}$

The neutron-rich $N=28$ nucleus ${}^{44}\mathrm{S}$ was studied using the two-proton knockout reaction from ${}^{46}\mathrm{Ar}$ at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4${}^{+}$ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the “intruder” configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in ${}^{44}\mathrm{S}$, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations.

Figure 1

Spectrum of residual nuclei identified in the S800 spectrograph. The particles labeled ${}^{46}\mathrm{Ar}$ correspond to scattered beam particles in the hydrogen-like charge state. The vertical axis displays the energy loss of nuclides measured in the focal plane, and the horizontal axis represents a path-corrected time-of-flight signal measured between a focal plane detector and the accelerator RF reference.

Figure 2

Doppler-corrected ($v/c=0.4211$) energy of $\gamma$ rays detected in coincidence with the ${}^{44}\mathrm{S}$ residues. Inset shows a closeup of the 1700–2400 keV region where three less intense photopeaks are identified.

Figure 3

Doppler-corrected energy of $\gamma$ rays detected in coincidence with the 1319-keV $\gamma$ rays emitted from a ${}^{44}\mathrm{S}$ nucleus. Inset: Projected spectrum of $\gamma \gamma$ events showing energy coincidence window in dotted lines.

Figure 4

(Left) Experimental ${}^{44}\mathrm{S}$ level scheme. The arrow widths are proportional to the observed intensity of the transitions. (Right) Level and transition scheme predicted by the shell model (see text). The arrow widths represent the calculated $\gamma$ intensities based on population cross sections and subsequent $\gamma$-decay branching ratios, normalized to the strength of the $2_1^{+}\to 0_1^{+}$ transition. We show all predicted transitions calculated larger than 3$\%$ of the $2_1^{+}\to 0_1^{+}$ transition intensity.

Figure 5

Calculated and observed longitudinal momentum distributions for the direct population of the 1319-keV state (2${}_1^{+}$) (top), the 2268-keV state (2${}_3^{+}$) (middle), and the 2447-keV state (4${}_1^{+}$) (bottom). The experimental histogram is compared to the distributions calculated from the two-proton knockout mechanism for different final state spin hypotheses (see text), which were corrected for the broadening induced by beam particle and reaction residue energy losses in the target.