$p$-$sd$ shell gap reduction in neutron-rich systems and cross-shell excitations in ${}^{20}$O

Abstract

Excited states in ${}^{20}$O were populated in the reaction ${}^{10}$Be(${}^{14}$C,$\hat{I}\pm$) at Florida State University. Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and ${\hat{I}}^3$ radiation was detected with the FSU ${\hat{I}}^3$ detector array. Five new states were observed below 6 MeV from the $\hat{I}\pm$-${\hat{I}}^3$ and $\hat{I}\pm$-${\hat{I}}^3$-${\hat{I}}^3$ 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.