Photoneutron Cross-Section Measurements on Gold Using nearly Monochromatic Photons

Abstract

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 ($\ensuremath{\gamma}, n$) and ($\ensuremath{\gamma}, 2n$) 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\ifmmode\pm\else\textpm\fi{}37 mb at 13.7\ifmmode\pm\else\textpm\fi{}0.3 MeV. The ($\ensuremath{\gamma}, 2n$) cross section was observed to have a threshold at 14.5\ifmmode\pm\else\textpm\fi{}0.5 MeV and a maximum of approximately 100 mb at 18 MeV. The integrated ($\ensuremath{\gamma}, n$) and ($\ensuremath{\gamma}, 2n$) cross sections, up to 25 MeV, were found to be 2.14\ifmmode\pm\else\textpm\fi{}0.15 and 0.83\ifmmode\pm\else\textpm\fi{}0.16 MeV-b, respectively. The value of $\ensuremath{\int}{0}^{\ensuremath{\infty}}\ensuremath{\sigma}{E}^{\ensuremath{-}2}\mathrm{dE}$ was determined to be 15.3\ifmmode\pm\else\textpm\fi{}1.5 mb/MeV. The level density parameter $a$ was found to be 17.1\ifmmode\pm\else\textpm\fi{}0.5 Me${\mathrm{V}}^{\ensuremath{-}1}$. The total cross section $\ensuremath{\sigma}(\ensuremath{\gamma}, n)+\ensuremath{\sigma}(\ensuremath{\gamma}, 2n)+\ensuremath{\sigma}(\ensuremath{\gamma}, pn)$ 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.