$p$-$sd$ 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,$alpha$) at Florida State University. 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 $alpha$-$gamma$ and $alpha$-$gamma$-$gamma$ 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.