Photoneutron Cross-Section Measurements on Gold Using nearly Monochromatic Photons

Continuously variable monochromatic gamma rays with an energy spread of 3% have been produced using the annihilation in flight of positrons accelerated in a linear electron accelerator. These gamma rays have been employed to measure the ($\gamma , n$) and ($\gamma , 2n$) cross sections for gold. A neutron counting method was used to determine the neutron multiplicity. The photoneutron yield for Au has a maximum of 535±37 mb at 13.7±0.3 MeV. The ($\gamma , 2n$) cross section was observed to have a threshold at 14.5±0.5 MeV and a maximum of approximately 100 mb at 18 MeV. The integrated ($\gamma , n$) and ($\gamma , 2n$) cross sections, up to 25 MeV, were found to be 2.14±0.15 and 0.83±0.16 MeV-b, respectively. The value of $\int 0^{\infty }\sigma E^{-2\mathrm{}}\mathrm{dE}$ was determined to be 15.3±1.5 mb/MeV. The level density parameter $a$ was found to be 17.1±0.5 Me${\mathrm{V}}^{-1\mathrm{}}$. The total cross section $\sigma (\gamma , n)+\sigma (\gamma , 2n)+\sigma (\gamma , pn)$ can be fitted with a single Lorentz curve having a peak cross section of 535 mb at 13.90 MeV and a width of 4.2 MeV.