$p$-$sd$ 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. 
M. Wiedeking, S.L. Tabor, J. Pavan, A. Volya, A.L. Aguilar, I.J.   Calderin, D.B. Campbell, W.T. Cluff, E. Diffenderfer, J. ...