$p$-$sd$ shell gap reduction in neutron-rich systems and cross-shell excitations in $^{20}$O
Excited states in $^{20}$O were populated in the reaction $^{10}$Be($^{14}$C,$α$) at Florida State University. Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and $γ$ radiation was detected with the FSU $γ$ detector array. Five new states were observed below 6 MeV from the $α$-$γ$ and $α$-$γ$-$γ$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-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 $p$-$sd$ shell gap as neutrons are added.
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