Photofission of americium isotopes in the energy range 6–12 MeV

Abstract

By means of the reference method, the cross sections for the fission of the 241Am, 242mAm, and 243Am isotopes were measured at the microtron of the Institute of Physics and Power Engineering (Obninsk). These measurements, which employed the cross section for 238U photofission as a reference, covered the energy range 6–12 MeV scanned with a variable step of 50 to 200 keV. Data on 242mAm photofission in the energy range 6–12 MeV and 243Am photofission in the region between 6 and 7 MeV were obtained for the first time. New results for 241Am reveal that the cross sections for 241Am photofission from previous studies of the present author were exaggerated. The new results for this isotope comply well with data of other authors. It is not confirmed that the fissility of 241Am is less than the fissility of 243Am. The energy dependences of the fissilities of the americium isotopes from photofission data are compared with those that were obtained for these fissilities from data on direct reactions like 240Pu(3He, df)241Am. The results of this comparison show that the observed fission thresholds and the plateaulike dependences at energies above 7.5 MeV from the two types of studies comply well. The present data show evidence that, in the energy region around 6 MeV, the photofission cross section has a maximum, which is associated, in all probability, with the low-energy resonance structure in the cross section for dipole photoabsorption.