Energy Spectra and Angular Distributions of Photoneutrons from Heavy Nuclei

Abstract

The energy spectra of photoneutrons from the absorption of 22-Mev bremsstrahlung by silver and bismuth have been measured with nuclear emulsions. Comparisons are made between the observed photoneutron spectra and those predicted on the basis of a statistical nuclear model. Energy level densities are assumed proportional to $\mathrm{exp}[{(a\ensuremath{\epsilon})}^{\frac{1}{2}}]$ and $\mathrm{ln}(\frac{A\ensuremath{\epsilon}}{20}+1)$, where $\ensuremath{\epsilon}$ is the excitation energy of the residual nucleus. The exponential form of the energy level density is in better agreement with the data than is the logarithmic form. However, for neutron energies greater than 4 Mev there are more photoneutrons observed than predicted by the exponential energy level density scheme.The angular distributions of photoneutrons from several elements have been measured with a variety of neutron detectors. The moderated neutron detector, which is equally sensitive to neutrons of most energies, indicates the photoneutron flux to be predominately isotropic. The small anisotropic component has a maximum at 90 degrees to the x-ray beam. Photoneutrons from bismuth have a larger anisotropic component than photoneutrons from any other element which was tested. In that case 21 percent of the neutron flux is in the anisotropic component.The $\mathrm{Al}(n, p)$ and $\mathrm{Si}(n, p)$ reactions have been used to measure the angular distributions of photoneutrons from several elements. These reactions have thresholds at 1.95 Mev and 2.69 Mev, respectively and are primarily sensitive to neutrons with more than 4-Mev energy. The angular distributions thus observed have a maximum at 90 degrees to the x-ray beam. In general, photoneutrons from the heavy elements deviate more from isotropy than do the light elements. An epithermal neutron detector indicates that the low-energy photoneutrons are emitted isotropically.It is concluded that most photoneutrons are generated in a manner consistant with the statistical nuclear model. The high-energy photoneutrons, which constitute a small fraction of the total photoneutron flux, are ejected predominately at right angles to the x-ray beam as expected by dipole absorption of the photons.