Structure of 19O from measurement and R-matrix analysis of σ(θ) for 18O(n, n) 18O and 18O(n, n′) 18O ∗(1.98 MeV)

Abstract

Differential cross sections for neutrons elastically scattered from 18O and inelastically scattered to the first excited state 18O ∗ (1.98 MeV) have been measured at forty-two incident energies for 5.0 < E n < 7.5 MeV corresponding to excitation energies in 19O of 8.7 to 11.1 MeV. The cross sections were measured at eleven laboratory angles per energy from 20° to 160° and show considerable resonance structure. These new elastic and inelastic 1.98 MeV level data have been analyzed together with previously published cross sections for 0 < E n < 2.5 MeV using a multilevel-multichannel R-matrix program to determine J, π, E λ and γ λc for states in 19O. The new assignments are compared to previous work where possible. Several previous assignments for resonances in the 0 < E n < 2.5 MeV region were verified, and disagreements over the J π assignments for several levels in this region have been resolved. A total of fourteen new J π assignments were made in the 5 < E n < 7.5 MeV region. An attempt was made to identify T = 3 2 analog states in 19F for the newly assigned levels in 19O. On the basis of spin, parity, excitation energy and a comparison of reduced neutron widths in 19O with reduced proton widths in 19F, six previously-identified T = 3 2 analogs were verified and eight new T = 3 2 analogs were tentatively assigned. Comparisons with existing theoretical work are discussed.