Photoneutron cross-section measurements in the Bi209 ( $\gamma$,xn ) reaction with a new method of direct neutron-multiplicity sorting

Photoneutron cross sections were measured in the $^{209}\mathrm{Bi}$($\ensuremath{\gamma}, xn$) reaction with $x=1--4$ at energies from the neutron threshold to 40 MeV using quasimonochromatic laser Compton-scattering $\ensuremath{\gamma}$-ray beams. A novel technique of direct neutron-multiplicity sorting with a flat-efficiency detector was used. Results are compared with the positron in-flight annihilation data of Livermore and Saclay as well as with bremsstrahlung data. The present neutron yields are consistent with the latest photoactivation measurement with bremsstrahlung. The giant dipole resonance component of the total photoneutron cross section is reasonably reproduced by model calculations of the Hartree-Fock Bogoliubov plus quasiparticle random phase approximation based on the Gogny D1M interaction for both E1 and M1 components. In contrast, the quasideuteron component of the total photoneutron cross section requires a factor of $\ensuremath{\sim}3$ enhancement of the Levinger parameter. Furthermore, a survival of large $(\ensuremath{\gamma},n)$ cross sections above 20 MeV may indicate the surface effect on the two-component particle-hole state density. The threshold behavior of photoneutron emission is discussed in terms of the average neutron kinetic energy.