The isovector quadrupole resonance in yttrium excited by neutron radiative capture

In order to investigate the properties of the isovector giant quadrupole resonance (ΔT = 1, ΔS = 0) in the A = 90 mass region, gamma-ray spectra from the reaction 89Y(n, γ)90Y were recorded at several neutron energies in the energy range 12 to 27 MeV at 55°, 90° and 125°. The measured fore-aft asymmetry for the ground-state transition is very small in the low-energy region, but becomes appreciable above a neutron energy of 18 MeV. The observed asymmetry is 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 El capture). The data obtained in the present work, interpreted in terms of the direct-semidirect capture model, indicate that the excitation energy of the isovector E2 resonance in 90Y is 26 ± 1 MeV. The data are consistent with a resonance width of 10 ± 2 MeV and with complete exhaustion of the energyweighted sum rule for the lower isospin component of the resonance.