Systematics of near-barrier nuclear reactions using quasi-elastic scattering
Creators: name: Evers, Maurits Meindert
Publisher: name: The Australian National University
Year: 2010
Subjects: subject: QC794.6.S3 E94 2010
subject: Elastic scattering (Physics)
subject: Neutrons Scattering
subject: Nuclear reactions

description: The sensitivity of total quasi-elastic scattering yields to the nuclear potential in the surface region has recently been shown to allow a precise determination of the shape of the nuclear potential in terms of the diffuseness value ao of the Woods-Saxon parametrization of the nuclear potential. Measurements of back-angle quasi-elastic scattering using beams of {u2081}{u2086}O and {u00B3}{u00B2}S with an unprecedented precision are presented in this work, and diffuseness values for each measured reaction are derived within the coupled-channels framework. Results strongly indicate a diffuseness value of a{u2080} ~ 0.66 fm, which is considerably smaller than diffuseness values of 0.9 - 1.4 fm required to reproduce the measured fusion cross sections particularly at energies above the barrier. Furthermore, by exploiting an interference phenomenon in the excitation function of the first excited state in {u2082}{u2080}{u2088}Pb, the consistency of a small diffuseness value is confirmed. Due to its complimentary nature to nuclear fusion, quasi-elastic scattering might also provide an independent tool to investigate the physical mechanisms leading to the suppression of fusion at sub-barrier and above-barrier energies. The hindrance of fusion at energies above the fusion barrier has been related to deep inelastic processes depleting the probability for fusion. All recorded back-angle spectra presented in this work show the presence of many more excited states than are included in the traditional coupled-channels framework, even at energies below the fusion barrier. Many of these states correspond to transfer events where the detected projectile-like fragments show large excitation energies ~ 15 MeV for the reaction {u2081}{u2086}O+{u2082}{u2080}{u2088}Pb measured at an energy near the fusion barrier. Excitation energies following transfer processes in reactions involving {u00B3}{u00B2}S are even higher, ~ 30 MeV. These high excitation energies are associated with large kinetic energy losses of the projectile-like fragments. Measured probabilities for various transfer processes for the reaction {u2081}{u2086}O+{u2082}{u2080}{u2088}Pb are analyzed within this work. It is shown that traditional calculations within the coherent coupled channels formalism or the microscopic time-dependent Hartree-Fock model are not able to reproduce the extracted excitation functions of processes where large kinetic energy losses of the projectile-like fragments are observed. This points towards a missing element in current models describing nuclear reaction dynamics, and may be related to the loss of coherence in the superposition of the quantum states describing the degrees of freedom of the interacting nuclei. Decoherence as part of deep inelastic processes may therefore result in energy dissipation and a reduction of the tunneling probability even at energies below the barrier.