Browsing by Author "Umar, A.S."

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    Enhanced dynamics in fusion of neutron-rich oxygen nuclei at above-barrier energies
    (2020-04-28) Hudan, S.; DeSouza, R.; Umar, A.S.; Lin, Z.; Horowitz, C.J.
    Above-barrier fusion cross-sections for an isotopic chain of oxygen isotopes with A=16-19 incident on a $^{12}$C target are presented. Experimental data are compared with both static and dynamical microscopic calculations. These calculations are unable to explain the ∼37% increase in the average above-barrier fusion cross-section observed for $^{19}$O as compared to β-stable oxygen isotopes. This result suggests that for neutron-rich nuclei existing time-dependent Hartree-Fock calculations underpredict the role of dynamics at near-barrier energies. High-quality measurement of above-barrier fusion for an isotopic chain of increasingly neutron-rich nuclei provides an effective means to probe this fusion dynamics.
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    Microscopic sub-barrier fusion calculations for the neutron star crust
    (American Physical Society, 2012) Umar, A.S.; Oberacker, V.E.; Horowitz, C.J.
    Fusion of very neutron-rich nuclei may be important to determine the composition and heating of the crust of accreting neutronstars. Fusion cross sections are calculated using time-dependent Hartree-Fock theory coupled with density-constrained Hartree-Fock calculations to deduce an effective potential. Systems studied include 16O+16O, 16O+24O, 24O+24O, 12C+16O, and 12C+24O. We find remarkable agreement with experimental cross sections for the fusion of stable nuclei. Our simulations use the SLy4 Skyrme force that has been previously fit to the properties of stable nuclei, and no parameters have been fit to fusion data. We compare our results to the simple São Paulo static barrier penetration model. For the asymmetric systems 12C+24O or 16O+24O we predict an order of magnitude larger cross section than those predicted by the São Paulo model. This is likely due to the transfer of neutrons from the very neutron rich nucleus to the stable nucleus and dynamical rearrangements of the nuclear densities during the collision process. These effects are not included in potential models. This enhancement of fusion cross sections, for veryneutron rich nuclei, can be tested in the laboratory with radioactive beams.