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

By means of the reference method, the cross sections for the fission of the Am-241, Am-242m, and Am-243 isotopes were measured at the microtron of the Institute of Physics and Power Engineering (Obninsk). These measurements, which employed the cross section for U-238 photofission as a reference, cov ered the energy range 6-12 MeV scanned with a variable step of 50 to 200 ke V. Data on Am-242m photofission in the energy range 6-12 MeV and Am-243 pho tofission in the region between 6 and 7 MeV were obtained for the first tim e. New results for Am-241 reveal that the cross sections for Am-241 photofi ssion 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 Am-241 is less than the fissility of Am-24 3. Th, energy dependences of the fissilities of the americium isotopes from photofission data are compared with those that were obtained for these fis silities from data on direct reactions like Pu-240(He-3, df)Am-241. The res ults of this comparison show that the observed fission thresholds and the p lateaulike dependences at energies above 7.5 MeV from the two types of stud ies comply well. The present data show evidence that, in the energy region around 6 MeV, the photofission cross section has a maximum, which is associ ated, in ail probability, with the low-energy resonance structure in the cr oss section for dipole photoadsorption. (C) 2001 MAIK "Nauka/Interperiodica ".