Neutron Spectra from Monoenergetic Photons on Bismuth

Abstract

Spectra of photoneutrons from a ${\mathrm{Bi}}^{209}$ target were obtained in the electric dipole giant-resonance region (12-16 MeV). A $\ensuremath{\gamma}$-ray energy resolution of 1.5% was attained using a bremsstrahlung monochromator; the neutron energies were measured with a time-of-flight technique with a resolution which varied from 10% at 2 MeV to 15% at 6 MeV. About 88% of the neutrons ejected by 14-MeV $\ensuremath{\gamma}$ rays are statistical. The inferred level density of ${\mathrm{Bi}}^{208}$ near its ground state is not a simple function of excitation energy, but from 4 to 7 MeV it increases exponentially with energy; in that energy range the temperature is about 1 MeV. The nonstatistical high-energy neutrons leave the ${\mathrm{Bi}}^{208}$ nucleus at the excitation energies at which the states are well represented as single-neutron holes in ${\mathrm{Bi}}^{209}$. These high-energy neutrons are intimately connected with the giant resonance. If the giant resonance is dominated by a single dipole state, the absolute width which characterizes the decay of the dipole state into the compound nucleus must increase by a factor of about 3 between 12 and 16 MeV.