$p\mathrm{\text{−}}sd$ Shell Gap Reduction in Neutron-Rich Systems and Cross-Shell Excitations in ${}^{20}\mathrm{O}$

Excited states in ${}^{20}\mathrm{O}$ were populated in the reaction ${}^{10}\mathrm{Be}({}^{14}\mathrm{C},\alpha )$ at Florida State University (FSU). Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and $\gamma$ radiation was detected with the FSU $\gamma$ detector array. Five new states were observed below 6 MeV from the $\alpha \mathrm{\text{−}}\gamma$ and $\alpha \mathrm{\text{−}}\gamma \mathrm{\text{−}}\gamma$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited $1p\mathrm{\text{−}}1h$ excitations across the $N=Z=8$ shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the $p\mathrm{\text{−}}sd$ shell gap as neutrons are added.

Figure 1

Transitions in coincidence with $\alpha$ particles and the 1674 keV $\gamma$ ray ($2_1^{+}\to 0_1^{+}$). The lines in this spectrum marked by their energies correspond to decays in ${}^{20}\mathrm{O}$.

Figure 2

Spectra in coincidence with $\alpha$ particles and newly observed transitions in ${}^{20}\mathrm{O}$.

Figure 3

$\alpha$ spectra for two $\gamma$-ray pairs originating from states with similar excitation energies. The spectra are labeled by their energy (in keV) of the $\gamma$-ray gate.

Figure 4

The level scheme as deduced from current and previous work. The widths of the lines indicate their intensities normalized to the 1674 keV transition. Branching ratios are given where applicable. States and transitions marked with solid circles are new additions. $\gamma$ transitions with open circles were tentatively reported in Ref. 12 and are confirmed in the present work.

Figure 5

Systematic reduction of the $p\mathrm{\text{−}}sd$ shell gap as neutrons are added to O and F nuclei. Shell model calculations were performed and the shell gap adjusted based on the comparison to available experimental data from the current work and Refs. [18, 26, 28]. The error bars reflect the current experimental knowledge available for the respective nuclei.

Figure 6

Comparison of calculated and experimentally observed states in ${}^{18}\mathrm{O}$ and ${}^{20}\mathrm{O}$. States in blue indicate positive-parity states ($0p\mathrm{\text{−}}0h$ and $2p\mathrm{\text{−}}2h$); states in red are negative-parity states ($1p\mathrm{\text{−}}1h$). The levels in black at 3895, 4353, 4598, 5115, and 5873 keV are new additions in ${}^{20}\mathrm{O}$. Also shown in black is the previously known 6555 keV state where no parity assignment is available. For ${}^{18}\mathrm{O}$ the standard ${}^{16}\mathrm{O}$ closed shell single-particle energies are used, whereas for ${}^{20}\mathrm{O}$ the$p\mathrm{\text{−}}sd$ shell gap was reduced by 850 keV.