No abstract

No abstract

Abstract. The total photofission cross section c%,F for Z35U and 238U has been measured in the energy range 50< E.y < 800 MeV at the 855 MeV Mainz Microtron MAMI using energy and time tagged photons (Glasgow Tagger) and a 47r arrangement of position sensitive fragment detectors. Besides the absolute photofission cross section c%,F, which almost completely exhausts the total photon absorption cross section for these nuclei, fragment mass distributions in this energy domain were determined via time of flight techniques (TOF). The results for the total photofission cross sections c%,F normalized to the atomic number A for both isotopes coincide, and agree in the A-resonance region, within the systematic errors, with the socalled "Universal Curve" C%,T/A of the total photon absorption cross section crT,T. At higher energies the cross sections exhibit a smooth behaviour. In particular, it is shown for the first time that there is n__0o resonancelike shape near the D13 resonance (at ~ 710 MeV) as observed for the free proton. This complete suppression of the DI3 resonance in complex nuclei is not yet understood on a microscopic level. The fragment mass distributions show a predominantly mass symmetric fission. However, contributions from mass asymmetric fission at some photon energies may give a hint of an increased mass asymmetric fission after the onset of the pion and two pion channels.

No abstract

Abstract. The cross sections for Bhabha and Moller scat-tering have been investigated in the energy range 2.1-2.4 MeV using monoenergetic positrons and electrons to search for hypothetical resonances superimposed to the continuum predicted by quantum electro dynamics. Bhabba-to-Mott, Moller-to-Mott and Moller-to-Bhabha cross section ratios were measured. The Bhabha-to-Mott ratios could be determined with statistical errors of typically 1% and remaining systematic errors not exceeding the statistical ones. No resonances in Bhabha scattering were observed. Limits for the intrinsic widths of hypothetical resonances are given. For the first time upper limits are deduced from our data for hypothetical resonances in the Moller scattering cross section in the MeV range. Additionally, Moller-to-Bhabha cross section ratios could be determined with good precision. A good agreement between the experimental and theoretical Moller-to-Bhabha ratios can be stated calling in question the recently predicted existence of series of narrow, unresolvable resonances in the Bhabha scattering cross section.