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

• Main Authors: Wiedeking, M, Tabor, S. L, Pavan, J, Volya, A, Aguilar, A. L, Calderin, I. J, Campbell, D. B, Cluff, W. T, Diffenderfer, E, Fridmann, J, Hoffman, C. R, Kemper, K. W, Lee, S, Riley, M. A, Roeder, B. T, Teal, C, Tripathi, V, Wiedenhöver, I
• Format: Journal Article
• Language: English
• Published: 13.12.2004
• Subjects: Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.nucl-ex/0412028

Phys.Rev.Lett. 94 (2005) 132501 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.