Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements

The ABC effect - a puzzling low-mass enhancement in the $\pi\pi$ invariant mass spectrum - is well-known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on first exclusive and kinematically complete measurements of the most basic double pionic fusion reaction $pn \to d \pi^0\pi^0$ at 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a $(\pi\pi)_{I=L=0}$ channel phenomenon associated with both a resonance-like energy dependence in the integral cross section and the formation of a $\Delta\Delta$ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data

Fission Mass Widths in 19 F + 232 Th, 16 O + 235,238 U Reactions at Near-barrier Energies

The fission fragment mass ratio distributions of fusion-fission events have been measured for the reactions of 19F + 232Th, 16O + 235U, and 16O + 238U at energies near and below the fusion barrier. It is found that the mass ratio widths follow a decreasing trend with decreasing energy, contrary to recent claims of anomalous mass widths attributed to a quasifission mechanism

Entrance channel dependence of quasifission in reactions forming 220 Th

Mass-angle correlations of binary fragments produced in O16+Pb204, S34+W186, and Ti48,50+Er166,170 reactions have been measured for a range of bombarding energies around their Coulomb barriers. At above-barrier energies, the width of the mass distributions for the fission-like fragments in the Ti50+Er170 reaction are found to be higher than those from the same compound system at similar excitation energies populated via the more mass asymmetric entrance channel reaction S34+W186, which in turn is higher than those for the O16+Pb204 system. The width of the mass distributions of the Ti+Er systems is found to increase with decreasing bombarding energies, in contrast with those of the O16+Pb204 and S34+W186 systems, which show a monotonic reduction in mass widths. This may be associated with the elongated contact configuration at sub-barrier energies