Radiative capture of fast neutrons by 89Y and 140Ce

The γ-ray spectra from the reactions ⁸⁹Y(n, γ)⁹⁰Y and ¹⁴⁰Ce(n, γ)¹⁴¹Ce have been measured in the neutron energy range of 6.2–15.6 MeV. The pulse-height spectra were recorded with NaI(Tl) spectrometers and time-of-flight techniques were used to improve signal-to-background ratio. Capture cross sections were determined for γ-ray transitions to the two $\text{2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ levels at 0 and 203 keV of ⁹⁰Y and to the $\text{2}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ ground state of ¹⁴¹Ce as well as integrated cross sections to bound states in these nuclei. The observed γ-ray spectra and partial radiative capture cross sections were compared with predictions of the direct-semidirect capture theory. The resonance behaviour with neutron energy of both the ground-state and integrated partial capture cross sections shows the validity of the semidirect model for ⁸⁹Y and ¹⁴⁰Ce in the region of neutron energy encompassing the giant-dipole resonance. The observed symmetry of the cross sections about the peak of the resonance argues strongly for the complex form of the particle-vibration coupling interaction. A detailed comparison of the predictions of the DSD model using the complex coupling interaction shows that the capture cross sections are relatively insensitive to the real part of the interaction.