I.C. Nascimento

Nuclear excitation by electrons and positrons

Existing measurements of the ratio σ−σ+, the nuclear excitation cross sections produced by electrons and positrons, are compared with the results of distorted wave calculations. The behaviour as a function of atomic number Z and electron energy E0 is shown to be principally due to Coulomb distortion. The value of σ−σ+ displays no clear structure corresponding to resonances in the photoabsorption cross section, except that it should be quite sensitive to the presence of quadrupole strength.

Electrofission of 234U, 236U and 238U: Angular distributions and E2 strength functions

Abstract The electrofission angular distributions for 234 U in the energy range 5.5 to 25 MeV were measured and are analyzed together with those obtained previously for 236 U and 238 U. The competition between the K = 0 and K = 1 fission channels following E2 excitation is established, showing a dominance of the K = 0 channel for near-barrier fission. The E2 fission strength functions for 234 U, 236 U and 238 U are deduced as well, and the E2 fission probabilities (at energies below the pairing gap) are estimated. A substantial concentration of E2 strength near the fission barrier is found, in good agreement with earlier photofission angular-distribution studies.

Formalism and applications of electrofission and photofission fragment angular distributions

Abstract A formalism for the joint analysis of angular distributions of electro- and photofission fragments is presented, utilizing the vitual-photon spectrum technique in DWBA. This formalism is applied to the study of angular distributions for the electrofission of 238 U, measured near the fission barrier, to obtain information about the low-lying levels ( J π , K ) of the transition nucleus. The (2 + , 0), (1 − , 0), and (1 − , 1) levels, previously detected in photofission experiments, were confirmed. Evidences of a significant contributions of (1 + , 1), (2 + , 1)and (2 + , 2) levels are also presented.

The electrofission of 208Pb and 209

Abstract The fission yields from the electrofission of 208Pb and 209Bi confirm the theoretically predicted large difference in the fission barrier energies of these nuclei. In addition the level density parameters at the fission saddle point were measured for 208Pb and 209Bi.