High-energy photofission in 238U, 232Th and 209Bi

The mass-yield distributions from photofission of 238U and 232Th at 300-1100 MeV and of 209Bi at 700 MeV have been measured by using catcher-foil techniques combined with gamma-ray spectroscopy. By means of a subtraction method the high-energy single-humped mass distributions of uranium and thorium were calculated; they show a half-width (FWHM) between 17 and 22 mass units, similar to that obtained in the bismuth case. The cross-sections of symmetric and asymmetric photofission in uranium and thorium have been calculated from the peak-to-valley ratios obtained. The cross-sections showed resonances around 350 MeV which was interpreted as a pion-effect. (author)