A study of the spin dependence of the nuclear level density by means of the 89Y(n, γ)90g, 90mY reactions with fast neutrons

Abstract

The 89Y(n, γ)90mY and 89Y(n, γ)90gY cross sections have been determined at 12 neutron energies between 0.14 and 1.22 MeV. Neutrons were produced by the 7Li(p, n)7Be reaction. The full neutron energy spread was about 65 keV. The activation cross sections were measured relative to both the 235U(n, f) cross section and the 197Au(n, γ)198Au activation cross section. The results for the isomer ratios have been compared with the predictions of the statistical theory of nuclear reactions together with a Fermi gas, level-spacing formula in order to obtain the spin cut-off factor σ in the spin dependence of the nuclear level density. The effects of variations in the parameters in the theory have been studied. Very good agreement was obtained with σ = 3.3, assuming a γ-ray multiplicity of 4. The 89Y(n, γ)90mY cross section has also been measured at 14.8 MeV to be 0.50±0.07 mb. Finally, the half-life of 90mY has been found to be 3.19±0.01 h.