Nuclear Level Densities in 208Bi and 209Po from the Neutron Spectra in the (p, n) Reactions on 208Pb and 209Bi Nuclei

The spectra of neutrons from the (p, n) reactions on the Pb-208 and Bi-209 nuclei were measured in the proton-energy range 8-11 MeV. These measurements were performed by using a time-of-flight spectrometer of fast neutrons on the basis of the pulsed tandem accelerator EGP-15 of the Institute of Physics and Power Engineering (Obninsk, Russian Federation). A high resolution and stability of the time-of-flight spectrometer made it possible to identify reliably low-lying discrete levels along with the continuum section of the neutron spectra. The measured data were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed by using the precise formalism ofHauser-Feshbach statistical theory together with the generalized model of a superfluid nucleus and the back-shifted Fermi gas model for the nuclear-level density. The nuclear-level densities in Bi-208 and Po-209 were determined along with their energy dependences and model parameters. Our results are discussed together with available experimental data and recommendations of model systematics.