抄録
Measurements of the photofission cross section, fragment angular distribution and fragment single kinetic energy distribution for 238U and 232Th over an energy range from 200 MeV to 1150 MeV are described. Semiconductor detectros were used for the measurements. It is found from a photon difference method that the photofission cross section shows a resonance at a photon energy of about 330 MeV and 350 MeV with a peak value of ∼120 mb and ∼40 mb for U and Th respectively. It is evident that this resonance can be attributed to the well known (3, 3) resonance in the pion photoproduction from a free nucleon. Furthermore, the result of the photofission cross section measurements was analyzed, by assuming that nuclear excitation resulting from the reabsorption of the created pions and the quasi-deuteron absorption within a nucleus induces the photofission in the heavy nucleus. From this analysis it is shown that these assumptions explain qualitatively the general properties of the photofission in the energy region studied. The fragment single kinetic energy distribution seems to be consistent with this conclusion. A preliminary result for the fragment angular distribution is shown.
