Absolute photofission cross section of 197Au, natPb, 209Bi, 232Th, 238U, and 235U nuclei by 69-MeV monochromatic and polarized photons.

Abstract

Absolute cross-section measurements for the photofission reactions of $^{197}\mathrm{Au}$, $^{\mathrm{nat}}\mathrm{Pb}$, $^{209}\mathrm{Bi}$, $^{232}\mathrm{Th}$, $^{238}\mathrm{U}$, and $^{235}\mathrm{U}$ nuclei have been performed at an incident photon energy of 69 MeV using monochromatic and polarized photon beams and dielectric fission-track detectors. Nuclear fissility values have been obtained and results are in agreement with those from other laboratories, although in some cases discrepancies are observed between one other. For nuclei in the region of the actinides the fissility result is \ensuremath{\gtrsim}0.4, while for Au, Pb, and Bi nuclei it only is \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}3}$--${10}^{\mathrm{\ensuremath{-}}2}$. Results have been interpreted in terms of the primary Levinger's quasideuteron nuclear photoabsorption followed by a mechanism of evaporation-fission competition for the excited nuclei. Shell effects have been taken into account, and they are clearly manifested when fissility is evaluated. The influence of photon polarization on photofission of $^{238}\mathrm{U}$ also has been investigated, and results have shown isotropy in the fragment azimuthal distribution.