Structure of 12B from measurement and R-matrix analysis of σ(θ) for 11B(n, n) 11B and 11B(n, n') 11B ∗(2.12 MeV), and shell-model calculations

Abstract

Differential cross sections for neutrons elastically scattered from 11B and inelastically scattered to the first excited state 11B∗ (2.12 MeV) have been measured at 13 incident energies for 4.8 ≦ E n ≦ 7.6 MeV corresponding to excitation energies in 12B of 7.8 to 10.3 MeV. The cross sections were measured at nine laboratory angles per energy from 20° to 160° and show considerable resonance structure. Differential inelastic cross sections were also measured for the 4.45 and 5.02 MeV levels of 11B for 2 to 9 angles at several incident energies. These new elastic and inelastic 2.12 MeV level data have been analyzed together with previously published cross sections for 2 ≦ E n ≦ 7.5 MeV using a multilevel-multichannel R-matrix program to determine J, π, E λ and γ λc for states in 12B. The shell model was used to calculate states in 12B as well as spectroscopic amplitudes for reactions leading to these states. The results of this model calculation are compared to those of the R-matrix analysis. Much of the structure observed in the experimental work is predicted by the model for E x ≲ 7 MeV. For levels of higher excitation the agreement is not as good. The experimental data are also compared to continuum shell-model calculations.