Unbound excited states of the $N=16$ closed shell nucleus ${}^{24}\mathrm{O}$

Two low-lying neutron-unbound excited states of ${}^{24}\mathrm{O}$, populated by proton-knockout reactions on ${}^{26}\mathrm{F}$, have been measured using the MoNA and LISA arrays in combination with the Sweeper Magnet at the Coupled Cyclotron Facility at the NSCL using invariant mass spectroscopy. The current measurement confirms the separate identity of two states with decay energies 0.51(5) MeV and 1.20(7) MeV, and provides support for theoretical model calculations, which predict a $2^{+}$ first excited state and a $1^{+}$ higher-energy state. The measured excitation energies for these states, 4.70(15) MeV for the $2^{+}$ level and 5.39(16) MeV for the $1^{+}$ level, are consistent with previous lower-resolution measurements, and are compared with five recent model predictions.

Figure 1

Decay energy spectrum of ${}^{24}\mathrm{O}$. The experimental data are not corrected for detector acceptances and efficiencies. The data were fitted with two asymmetric Breit-Wigner resonances (blue dotted and red dashed curves) and a nonresonant background (green dash-dotted curve) determined through Monte Carlo simulation and ${\chi }^2$ minimization.

Figure 2

Measured decay energies for the ${}^{24}\mathrm{O}\left(2^{+}\right)\to {}^{23}\text{O}$(g.s.) + $n$ (lower blue points) and the ${}^{24}\mathrm{O}\left(1^{+}\right)\to {}^{23}\text{O}$(g.s.) + $n$ (upper red points) neutron decays. Current measurements (diamond) and the previous NSCL (square) [12] and RIKEN (triangle) [15] results are shown. The current results are generally consistent with the two previous measurements.

Figure 3

A comparison of the measured $2^{+}$ (lower blue) and $1^{+}$ (upper red) energy levels in ${}^{24}\mathrm{O}$ (including those from Hoffmann 2009 [12] and Tshoo 2012 [15] adjusted for the separation energy $S_n$ used in this paper [22]). Included also are predictions from Brown 2015 (left to right: USD, USD-05A, and USD-05B) [24], Tsukiyama 2010 CCSM [14], and Volya 2014 CSM [25] shell model calculations, as well as ab initio calculations from Jansen 2014 CCEI model [26], and Bogner 2014 IM-SRG model (left to right: ${\mathrm{\Lambda }}_{3N}=400,400,500,500$ MeV and ${\lambda }_{SRG}=1.88,2.24,1.88,2.24{\mathrm{fm}}^{-1}$, respectively) [27]. Hatched regions correspond to level energy ranges reported in the original citations.

Figure 4

Evolution of the ${}^{23}\mathrm{O}$ and ${}^{24}\mathrm{O}$ mass measurements over time. Experimental determinations (solid circles) of the ${}^{24}\mathrm{O}$ and ${}^{23}\mathrm{O}$ mass measurements, as well as the $1-n$ separation energies $S_n$ are shown along with the AME recommended values (open circles). Measurement values are from Refs. [23, 28, 29, 30, 31, 32] and the recommended values are from Refs. [22, 23, 33, 34].

Figure 5

Difference in energy between the $1^{+}$ and $2^{+}$ states in ${}^{24}\mathrm{O}$ for the three measurements (blue) and values from theory (red). References are the same as those described in Fig. 3.