Photofission of Pu-238, Pu-240, and Pu-242 in the energy range 5-10 MeV

The cross sections for the photofission of plutonium isotopes (238)pU, Pu-2 40, and Pu-242 in the energy range 5-10 MeV have been measured by using a b eam of bremsstrahlung photons from the microtron installed at the Institute of Physics and Power Engineering (Obninsk). The energy regions below the f ission barrier and above 6 MeV have been scanned with pitches of 0.05 and 0 .1 MeV, respectively. In deriving the absolute cross section for Pu-238 pho tofission, U-238 photofission has been employed as a reference reaction. In measurements involving Pu-240 and Pu-242 nuclei, the neptunium isotope Np- 237, which is characterized by a more regular dependence of the photofissio n cross section on excitation energy than U-238, has been chosen for the fi rst time as a reference nucleus. The measured cross sections as functions o f energy show resonance structures in the vicinity of the fission threshold that are consistent with those previously observed in the energy dependenc es of fissilities for corresponding direct reactions. The partial-wave cros s sections for the J(pi)K = 1(-)0, 1(-)1, and 2(+)0 photofission channels h ave been determined as functions of energy. At energies below some 5.5 MeV, the total cross section for photofission of each plutonium isotope being s tudied receives a significant contribution from quadrupole interaction. Wit hin the one-dimensional model of a two-humped fission barrier, the paramete rs of the barriers for Pu-238, Pu-240, and Pu-242 have been extracted from data and have then been compared with estimates based on previous measureme nts. (C) 2000 MAIK "Nauka/Interperiodica".