A generalized r-matrix interpretation of the 89Y + n total cross section at low incident energies

Abstract

The resonance structure observed in the 89Y(n, n) 89Y total cross-section measurements in the range of 0.9 to 1.2 MeV neutron energy is investigated using a comprehensive theory of nuclear reactions. A shell-model calculation which formed the initial stage of this study predicts satisfactorily the energies of the negative-parity states that contribute to the observed anomalies. The neutron decay widths for these resonances are evaluated using the model wave functions. The general trends in the energy dependence of the total cross section are satisfactorily reproduced by the theory. The factors that could contribute to the discrepancies between theory and experiment are discussed. The theoretical estimates of the damping widths for the two 1 − anomalies that occur in this region were within 20 to 25% of the experimental values and support the view that these are intermediate-type resonances. Their configurational structure as predicted by the model calculation suggests that they are the parent states of the T > components of the giant dipole resonance near 21.0 MeV in 90Zr. The distribution of E1 widths calculated for a proposed 1 − → 2 + (at 0.78 MeV) transition in 90Y indicates that an anomaly corresponding to these 1 − states can also be expected in the (n, γ) reaction.