Inelastic electron scattering and the electromagnetic properties of 9Be

Abstract

The simple Nilsson model was calculated out to higher values of the deformation parameter η, and this was shown to produce the intrinsic state sequence, g.s. 3 2 −, 1 2 + at 1.67 MeV, and 1 2 − at about 3 MeV, for not unreasonably large values of η. Rotational bands based on the intrinsic states of the Nilsson model were used to predict the energy spectrum. Electromagnetic transition rates and neutron decay widths have been calculated, and compared with the measured (γ, n) cross section. The low- q extrapolation of electron scattering data provided evidence for an induced octupole deformation of the 9Be surface. This simple Nilsson model was extended to include coupling between the major shells N, N+2, N+4, and N+6, and the same low-lying intrinsic state sequence was obtained for a lower value of η. The implications of using the wave functions belonging to the larger basis set are briefly discussed. Elastic and inelastic electron scattering from 9Be were measured at q-values of 0.6, 0.8 and 0.9 fm −1 and angles 120° and 154°. Form factors for the states at 1.67,2.43 and 3 MeV were analyzed into longitudinal and transverse contributions, and a similar separation made for the continuum scattering up to 10 MeV. Wave functions from the extended Nilsson model calculations were used to compare with the experimental data. Good fits to the ground state charge and magnetic form factors were obtained, but the calculated C2 form factor for the 5 2 − 2.43 MeV state was low. The transverse form factor in the 3 MeV region was shown to be in good agreement with the model calculation provided Ml coupling to a broad 1 2 − state at 3 MeV was included. The data and calculations on the 1.67 MeV 1 2 + state were in disagreement. However, the transverse form factor in the region 4.5 to 6.5 MeV was reasonably well predicted by the Nilsson model.