Lifetime Measurements and Triple Coexisting Band Structure in ${}^{43}\mathrm{S}$

Lifetime measurements of excited states in the neutron-rich nucleus ${}^{43}\mathrm{S}$ were performed by applying the recoil-distance method on fast rare-isotope beams in conjunction with the Gamma-Ray Energy Tracking In-beam Nuclear Array. The new data based on $\gamma \gamma$ coincidences and lifetime measurements resolve a doublet of $(3/2^{-})$ and $(5/2^{-})$ states at low excitation energies. Results were compared to the $\pi (sd)-\nu (pf)$ shell model and antisymmetrized molecular dynamics calculations. The consistency with the theoretical calculations identifies a possible appearance of three coexisting bands near the ground state of ${}^{43}\mathrm{S}$: the $K^{\pi }=1/2^{-}$ band built on a prolate-deformed ground state, a band built on an isomer with a $1f_{7/2}^{-1}$ character, and a suggested excited band built on a newly discovered doublet state. The latter further confirms the collapse of the $N=28$ shell closure in the neutron-rich region.

Figure 1

Doppler-shift-corrected $\gamma$-ray spectrum of ${}^{43}\mathrm{S}$ together with the partial level scheme. Data are compared to the simulated responses for observed transitions (red line). (Insets) Background-subtracted $\gamma \gamma$ spectra in coincidence with the 628 keV (top) and 977 keV (bottom) transitions.

Figure 2

(Top) Gamma-ray spectrum for the ${}^{43}\mathrm{S}$ 977 keV transition using 0-, 0.5-, and 1 mm target-to-degrader separations. Identification of the fast ($f$) and slow ($s$) components is shown. The data are compared to simulated spectra with the best-fit result of $\tau =6.2\mathrm{ps}$ with the adopted order of the $977\to 184\mathrm{keV}$ transition. The dashed blue line is given as a reference for the inverted order of the 184 keV ($\tau =15\mathrm{ps}$) $\to$ 977 keV ($\tau =0.1\mathrm{ps}$) decays. A peak around 920 keV is the fast component of the 854 keV transition. (Bottom) Gamma-ray spectrum for the ${}^{43}\mathrm{S}$ 184 keV transition. The data are compared to simulated spectra with $\tau =7.8\mathrm{ps}$ for the 184 keV state.

Figure 3

Gamma-ray spectrum for the ${}^{43}\mathrm{S}$ 1159 keV transition using 0-, 0.5-, and 1 mm target-to-degrader separations. The data are compared to simulated spectra with the upper limit of 1.5 ps obtained from this experiment and the reference simulated spectra with $\tau =6.2\mathrm{ps}$ that was adapted for the 977 keV transition (dashed curves).

Figure 4

Comparison of the experimental level scheme with results of the SDPF-U shell model calculations. The states are labeled by twice their spin value ($2J$). The 1787 keV level is considered as the theoretical $5/2_3^{-}$ state. Spectroscopic factors $C^{2}S$ for dominant ($C^2{S}_{\max }$) or strong ($C^{2}S>0.03$) knockout channels are shown on the far right together with the single-particle orbitals for proton removal. The $M1$ transitions are denoted by empty arrows. The $E2$ transitions are denoted by gray arrows.