Cross sections for the photofission of 243Cm, 245Cm, 249Bk, and 249Cf isotopes in the energy range from the threshold to 10–12 MeV

Abstract

The relative method was used to measure the photofission cross sections for 243Cm and 249Cf isotopes in the energy range from 6 to 12 MeV, for 245Cm in the energy range from 5 to 10 MeV, and for 249Bk in the energy range from 5.5 to 10 MeV. The measurements were performed with an energy step of 50 to 200 keV by using the microtron installed at the Institute of Physics and Power Engineering (Obninsk). The cross section for 238U photofission was used as a reference in these measurements. Data on the cross sections for 243Cm, 245Cm, and 249Bk photofission were obtained for the first time, while data on the cross section for 249Cf photofission were obtained for the first time only in the energyregion E<10 MeV. The data on the 245Cm nucleus suggest that, in the energy region around 6 MeV, the cross section for its photofission has a maximum, which is likely to be due to the low-energy resonance structure of the dipole-photoabsorption cross section. For 249Cf, an anomalously large value of the photo fission cross section is observed in the region of the first maximum of the giant dipole resonance (E≈11 MeV). By comparing the energy dependences obtained for the fissilities of the 243Cm and 249Bk isotopes from photofission data with the fissilities from direct-reaction data, it is found that the observed fission thresholds agree and that there is a plateau-like dependence at energies above 7.5 MeV. For the 245Cm and 249Cf nuclei, there are no similar data for performing such a comparison. Data on the fissilities as obtained from the present series of relative measurements that employ the microtron bremsstrahlung spectrum are analyzed.