Browsing by Author "Cowley, A. A."

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    Alpha-cluster structure in the ground state of 40Ca displayed in a (p,pa) knockout reaction
    (IOP Publishing, 2013) Cowley, A. A.
    The analyzing power is very sensitive to details of the reaction mechanism of (p, pα) knockout reactions in the incident energy range of approximately 100 MeV and higher. Whereas distorted wave impulse approximation calculations in the past proved to give an excellent reproduction of analyzing power angular distributions for quasifree (p, pα) reactions on light targets such as 6Li, 9Be and 12C, the situation for 40Ca was not as simple. It is now shown that the theory also offers good agreement with the experimental distribution of the heaviest target nucleus if care is taken to use proper distorted waves which treat α−36Ar properly as a system for which α–elastic scattering is anomalous. Thus it is shown that 40Ca reveals its ground state α–cluster structure in an unambiguous way similar to the light target nuclei.
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    Distorted-wave Born approximation study of the 11Li(p,t) 9Li reaction
    (IOP Publishing, 2016) Cowley, A. A.
    The reaction 11Li(p,t) 9Li(gs) at an incident energy of 4 MeV is treated in terms of a simplistic distorted-wave Born approximation transfer. The halo neutrons involved in the reaction are treated as a di-neutron cluster transferred in a simultaneous process. This appears to be a good approximation of the mechanism. The dominant contribution to the reaction comes from the known (1s1/2) 2 structure component of the ground state of 11Li, and the cross section angular distribution seems to be relatively insensitive to the fact that 11Li has an anomalously large radius due to its Borromean halo properties. Significantly this simple treatment of the reaction is in much better agreement with the experimental angular distribution than a more sophisticated calculation.
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    Influence of nuclear cluster structure in proton-induced pre-equilibrium composite particle emission
    (IOP Publishing, 2017) Cowley, A. A.
    Recent statistical multistep analyses of the pre-equilibrium reaction⁹³Nb(p, α) at incident energies of 65 and 100 MeV are reviewed. It is shown that the α-cluster structure of the target nucleus is prominently displayed at the lower incident energy, whereas at the higher value its presence is obscured by the dominance of three-nucleon pickup. This result appears to be simply associated with a difference in the details of the effective momentum mismatch experienced by a cluster knockout mechanism, as opposed to a pickup process.
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    Interplay of dynamical and structure effects in the observables for 12C(p,2p) near 400 MeV with polarized and unpolarized beams
    (Elsevier, 2019-10-01) Mecca, A.; Cravo, E.; Deltuva, A.; Crespo, R.; Cowley, A. A.; Arriaga, A.; Wiringa, R. B.; Noro, T.
    We investigate the interplay of reaction mechanism and structure effects in the calculated cross sections and polarization observables for the direct 12C(p, 2p)reaction near 400 MeV around the Quasi Free Scattering (QFS) kinematical condition. We do the first consistent comparison between the scattering observables obtained from solutions of three-body Faddeev/Alt-Grassberger-Sandhas (F/AGS) equations and from the Distorted-Wave Impulse Approximation (DWIA). We explore structure effects on the calculated observables, making use of one-nucleon spectroscopic overlaps obtained from the quantum Monte Carlo (QMC) many-body wave functions and using a Woods-Saxon parametrization with parameters adjusted to experimental (p, 2p) data. We show, for the first time, that the two reaction formalisms exhibit a distinct behavior depending upon the kinematic conditions. We also show that the agreement between the experimental data and the theoretical results depends on the reaction formalism, kinematical conditions and optical model parametrizations in addition to the spectroscopic factors (SFs). The agreement between the data and predictions using QMC wave functions diminishes prominently for transitions to excited states of 11B.
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    Relevance of neutron excess in nuclear matter to proton-induced composite-particle pre-equilibrium emission
    (Italian Physical Society, 2019) Cowley, A. A.
    Formulation of the equation of state (EoS) of nuclear matter has far-reaching implications for issues ranging from the atomic nucleus to crucial aspects of astrophysics. Recent investigations of the influence of the formation of light ion clusters on the development of the EoS are of interest. A study based on a generalized relativistic mean-field model revealed an isotope-dependant quenching of α-clustering on the surface of Sn as the nuclear system becomes more asymmetric. This trend of decreasing α-clustering with increasing mass number of Sn was previously found to be consistent with existing experimental results of an α- pickup reaction. The implications of predictions of the EoS are discussed as being of profound interest to proton-induced α-particle pre-equilibrium emission at incident energies above about 100 MeV.