$N=32$ shell closure below calcium: Low-lying structure of ${}^{50}\mathrm{Ar}$

Low-lying excited states in the $N=32$ isotope ${}^{50}\mathrm{Ar}$ were investigated by in-beam $\gamma$-ray spectroscopy following proton- and neutron-knockout, multinucleon removal, and proton inelastic scattering at the RIKEN Radioactive Isotope Beam Factory. The energies of the two previously reported transitions have been confirmed, and five additional states are presented for the first time, including a candidate for a $3^{-}$ state. The level scheme built using $\gamma \gamma$ coincidences was compared to shell-model calculations in the $sd\text{−}pf$ model space and to ab initio predictions based on chiral two- and three-nucleon interactions. Theoretical proton- and neutron-knockout cross sections suggest that two of the new transitions correspond to $2^{+}$ states, while the previously proposed $4_1^{+}$ state could also correspond to a $2^{+}$ state.

Figure 1

(a) Doppler-corrected $\gamma$-ray spectrum obtained for ${}^{50}\mathrm{Ar}$ populated from multinucleon removal reactions. The dashed blue lines represent the simulated responses of ${\text{DALI2}}^{+}$ to the different transitions, the dashed black line shows the fitted double-exponential background, and the solid red line shows the total fit. (b) $\gamma$-ray spectrum gated at $\approx 1150\mathrm{keV}$. The best fit is shown by the solid red line while the expected counts are shown by the darker line.

Figure 2

Same as Fig. 1, but for the ${}^{51}\mathrm{K}(p,2p){}^{50}\mathrm{Ar}$ reaction.

Figure 3

Same as Fig. 1, but for the ${}^{51}\mathrm{Ar}(p,pn){}^{50}\mathrm{Ar}$ reaction.

Figure 4

Same as Fig. 1, but for the ${}^{50}\mathrm{Ar}(p,p^{\prime }){}^{50}\mathrm{Ar}$ reaction.

Figure 5

(a) Experimental level scheme for ${}^{50}\mathrm{Ar}$ deduced in the present work. Level and transition energies are given by the italic and regular fonts, respectively. The calculated neutron separation energy, $S_n$, is indicated [39]. Uncertainties in the energy levels are displayed as shaded areas. Parts (b) and (c) display predictions for low-lying states in ${}^{50}\mathrm{Ar}$ by the SDPF-MU shell model and VS-IMSRG calculations, respectively.

Figure 6

Systematics of $E\left(3^{-}\right)$ for even-even $N=32$ isotones. The circles represent the available data [42, 61], including the value for ${}^{50}\mathrm{Ar}$ reported in this work. The solid line shows the SDPF-MU calculations.