Photofissility of ²³²Th measured with tagged photons from 250 to 1200 MeV

The photofission cross section of ²³²Th was measured in the energy range 250–1200 MeV using a monochromatic tagged photon beam and a parallel-plate avalanche detector. The ²³²Th nuclear photofissility was obtained by measuring simultaneously and in the same energy range the photofission cross section of ²³⁸U and assuming a photofissility equal to one for ²³⁸U. It was found that, contrary to the case for heavier actinides, the ²³²Th photofissility value does not saturate up to 1200 MeV. Rather, it lies between 0.6 and 0.8, and shows a weak energy dependence: it increases by about 15% over an interval of 1 GeV. A comparison of these experimental findings with the predictions of an intranuclear-cascade Monte Carlo calculation is given.