J.C. Mein

Probing fusion barrier distributions with quasi-elastic scattering

Excitation functions have been measured for quasi-elastic back-scattering of 16O on 92Zr, 144Sm, 154Sm and 186W targets. A new procedure to extract representations of fusion barrier distributions is suggested and applied. The results are compared with barrier distributions obtained from fusion data and with coupled-channel calculations. The representations from quasi-elastic scattering are consistent with those from fusion below the average barrier energy. However, the effect of transfer smooths the quasi-elastic representations at higher energies, so that the sensitivity to the barrier distribution is reduced.

Strong dependence of sub-barrier fusion on the nuclear hexadecapole deformation

Abstract Fusion cross-sections σ for the reaction 186 W + 16 O have been measured with high precision at small energy intervals, for energies E spanning the barrier region. The distribution of fusion barriers, derived from the curvature of Eσ , has the shape expected classically from a target nucleus with a negative hexadecapole deformation. Comparison with the previously measured distribution for 154 Sm + 16 O demonstrates the strong sensitivity of fusion to the hexadecapole deformation. The magnitudes of the deformation parameters for 186 W, extracted using a purely geometrical description, are different from those determined from non-fusion measurements. This is interpreted as reflecting the sensitivity of the excitation function to small additional couplings.

Competition between fusion-fission and quasi-fission in the reaction 28Si+208Pb

Abstract It has recently been shown that collisions of 16O with the tips of prolate deformed 238U nuclei result in quasi-fission, whilst collisions with the sides result in fusion-fission. Detailed measurements have now been made of the fusion barrier distribution and fission anisotropies for the reaction 28Si+208Pb, forming a compound nucleus of similar fissility, to investigate whether 28Si displays analogous effects of deformation to those of 238U, in both fusion and fission. Although the fusion barrier distribution shows a large influence due to the 28Si, not inconsistent with anoblate deformation, the fission anisotropies decrease steadily as the bombarding energy falls through the fusion barrier region. In contrast, the 16O+238U reaction shows a dramatic rise at energies below the average barrier. This difference is interpreted in terms of the transition from the sudden (fusion) potential to the adiabatic (fission) potential.

Fusion excitation function measurements for the 16O+58Ni and 16O+62Ni systems

Abstract High precision fusion excitation functions have been measured for the 16 O+ 58 Ni and 16 O+ 62 Ni systems from which fusion barrier distributions have been evaluated. Coupled-reaction-channels (CRC) calculations, which describe elastic and quasi-elastic scattering, also satisfactorily reproduce the fusion cross sections and barrier distributions. The small value of Z 1 Z 2 in this case leads to barrier distributions with relatively little structure. However, in conjunction with the detailed elastic scattering data for these systems, this allows us to elucidate the role of previously ignored states in 16 O in pushing the entire distribution to lower energies. These shifts are consistent with derived magnitudes of polarization potentials for both systems.

Barrier distributions from elastic scattering

Abstract A new representation of the distribution of potential barriers present in heavy-ion reactions is defined in terms of the elastic scattering excitation function. Its validity is demonstrated for the systems 16 O + 144.154 Sm, 186 W, 208 Pb, for which precise measurements have been made. Compared with fusion barrier distributions, which show structures characteristic of collective inelastic couplings, the elastic distributions are less detailed. This appears to be due to couplings to weaker direct-reaction channels.

Barrier distributions and scattering

The extraction of representations of the fusion barrier distribution from backward-angle, quasi-elastic, elastic and transfer excitation functions is discussed. Such excitation functions have been measured for , and projectiles incident on a variety of targets. The results are compared with representations obtained from fusion excitation functions. Varying in their sensitivity, all representations show evidence of the barrier structure. Differences between the scattering and the fusion representations can be related to the effects of coupling to residual, weak reaction channels.

Fusion barrier distributions and fission anisotropies

Abstract Fusion excitation functions for 16,17 O + 144 Sm have been measured to high precision. The extracted fusion barrier distributions show a double-peaked structure interpreted in terms of coupling to inelastic collective excitations of the target. The effect of the positive Q-value neutron stripping channel is evident in the reaction with 17 O. Fission and evaporation residue cross-sections and excitation functions have been measured for the reaction of 16 O + 208 Pb and the fusion barrier distribution and fission anisotropies determined. It is found that the moments of the fusion l-distribution determined from the fusion and fission measurements are in good agreement.