The isovector E2 resonance in bismuth excited by neutron radiative capture

To investigate the isovector giant quadrupole resonance in the A=208 mass region, gamma-ray spectra from the reaction 209Bi(n, γ)210Bi were recorded at five neutron energies in the energy range 17.7 to 22.0 MeV at 55deg and 125deg. The measured fore-after asymmetries of the angular distributions for transitions to bound single-particle states are attributed to interference between radiation from the isovector giant quadrupole resonance and radiation of opposite parity (from the high-energy tail of the giant dipole resonance and direct E1 capture). A comparison of the asymmetry data obtained in the present work with calculations based on the direct-semidirect capture model shows that the data are compatible with an excitation energy of 22.5±1 MeV, a width of 6±3 MeV and complete exhaustion of the energy-weighted sum rule for the isovector giant quadrupole resonance. Ratios of cross sections to single-particle states are also compared with direct-semidirect model calculations. In general, agreement between experimental data and model calculations is observed. For a special case, effects of a possible decay of the isovector giant quadrupole resonance to the low-lying octupole state are investigated. (orig.)