Bulletin of the American Physical Society
2010 Fall Meeting of the APS Division of Nuclear Physics
Volume 55, Number 14
Tuesday–Saturday, November 2–6, 2010; Santa Fe, New Mexico
Session NC: Mini-symposium on Reaction Theory for FRIB II |
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Chair: Filomena Nunes, Michigan State University Room: Sweeny C |
Saturday, November 6, 2010 10:30AM - 10:42AM |
NC.00001: Extraction of Nuclear Radii from Interaction Cross Section Data Ivan Novikov, Yuli Shabelski The nuclear density distribution of light radioactive nuclei can be extracted from the experiments when radioactive isotopes are scattered from the stable nuclear target. To obtains distribution parameters or root-mean-square (r.m.s.) radii, the interaction cross-section $\sigma^{(I)}$, measured in these experiments, is usually compared with calculated reaction cross-section, $\sigma^{(r)}$. This difference has been estimated in some nuclei to be less than a few percent. However, that still can significantly affect extracted value for the r.m.s In the presented paper we calculate values of r.m.s. radii for nuclei with atomic weights A$<$40 in various theoretical approaches: the optical approximation, rigid target approximation and the exact expression of Glauber Theory. R.m.s. radii were extracted from comparison of experimentally measured and calculated interaction cross-section $\sigma^{(I)}$. [Preview Abstract] |
Saturday, November 6, 2010 10:42AM - 10:54AM |
NC.00002: Finite Range Effects in Knockout Reactions Bhushan N. Joshi Finite Range DWIA calculations have been performed for the first time. For the $(\alpha ,2\alpha )$ reactions, the calculations have indicated extreme sensitivity to the short range component of the t-matrix effective interaction. The vagaries of the energy dependent $\alpha $-spectroscopic factors, have been understood using well established nuclear data. Using repulsive core $\alpha -\alpha $ interaction two order of magnitude enhancement is explained. FR-DWIA calculation is a new tool to investigate the nature of nuclear potential. Heavy cluster knockout reaction such as ${ }^{16}O({ }^{12}C,2{ }^{12}C){ }^4He$has been performed for the first time indicating an order of magnitude enhancement compared to the $(\alpha ,2\alpha )$ results. The (C,2C) results support a short range repulsive core C-C potential. Similar study can be made to probe the short range behavior of $p-p$, $\pi -p$and $K^+n$systems to study the dibaryons, delta resonances and the pentaquarks. A new field of Heavy Cluster Knockout Reaction is opened up to study the core knockout of Halo nuclei. Our FR-DWIA formalism has applications in Atomic and Molecular Physics and neutron multiplication calculations for ADS also. [Preview Abstract] |
Saturday, November 6, 2010 10:54AM - 11:06AM |
NC.00003: (Multi-)nucleon transfer and its role in understanding the transition from quasi-elastic scattering to deep inelastic collisions in nuclear reactions Maurits Evers, David Hinde, Mahananda Dasgupta Measurements of (multi-)nucleon transfer excitation functions in backward-angle quasi-elastic scattering at near- barrier energies have been carried out, using beams of $^{16}$O and $^{32}$S [1]. A detailed comparison of the measured transfer probabilities within the coupled-channels framework indicates a major problem with the standard coupled-channels approach. The experimental results show events with large kinetic energy losses, which are not treated in the coherent coupled channels model. The presence of these deep inelastic collision processes already seen at sub-barrier energies, leads to questions of what the physical processes are behind the transition from quasi- elastic scattering to deep inelastic collisions, and how processes leading to large total kinetic energy losses may be included in nuclear reaction models. Our observations point to the need to include effects of quantum decoherence in a new nuclear reaction model [2], and promises a new understanding of nucleus-nucleus collisions, with implications for areas such as nuclear astrophysics and nucleosynthesis.\\ {\bf References}\\ \noindent $[1]$ M. Evers et al., Phys. Rev. \textbf{C78}, 034618 (2007)\\ $[2]$ D. Hinde et al., Nuc. Phys. {\bf A834}, 117c-122c (2010) [Preview Abstract] |
Saturday, November 6, 2010 11:06AM - 11:18AM |
NC.00004: Microscopic study of heavy-ion reactions with n-rich nuclei: dynamic excitation energy and capture Volker Oberacker, A.S. Umar Heavy-ion reactions at RIB facilities allow us to form new exotic neutron-rich nuclei. These experiments present numerous challenges for a microscopic theoretical description. We study reactions between neutron-rich $^{132}$Sn nuclei and $^{96}$Zr within a dynamic microscopic theory, and we compare the properties to those of the stable system $^{124}$Sn+$^{96}$Zr. The calculations are carried out on a 3-D lattice using the density-constrained Time-Dependent Hartree-Fock (DC-TDHF) method [1- 3]. In particular, we calculate the dynamic excitation energy $E^*(t)$ and the quadrupole moment of the dinuclear system $Q_{20}(t)$ during the initial stages of the collision. Regarding the heavy-ion interaction potential $V(R)$, we find that the fusion barrier height and width increase dramatically with increasing beam energy. The fusion barriers of the neutron-rich system $^{132}$Sn+$^{96}$Zr are systematically 1-2 MeV higher than those of the stable system. Large differences (9 MeV) are found in the interaction barriers of the two systems. Capture cross sections are analyzed in terms of dynamic effects and a comparison with recently measured capture-fission data is given. [1] Umar and Oberacker, PRC 76, 014614 (2007). [2] Umar, Oberacker, Maruhn, and Reinhard, PRC 80, 041601(R) (2009). [3] Umar, Maruhn, Itagaki, and Oberacker, PRL 104, 212503 (2010). [Preview Abstract] |
Saturday, November 6, 2010 11:18AM - 11:30AM |
NC.00005: Coupled-Channels Calculations of Nucleon-Induced Reaction Observables Gustavo Nobre, Ian Thompson, Jutta Escher, Frank Dietrich A microscopic calculation of the reaction cross-section for nucleon-nucleus scattering has been performed by explicitly coupling the elastic channel to all particle-hole excitation states in the target and to all one-nucleon pickup channels. These states may be regarded as {\em doorway states} through which the flux flows to more complicated configurations, and subsequently to long-lived compound nucleus resonances. Target excitations for $^{40,48}$Ca, $^{58}$Ni, $^{90}$Zr and $^{144}$Sm were described in a QRPA framework. Reaction cross sections calculated in this approach were compared to predictions of a fitted optical potential and to experimental data, reaching very good agreement. Couplings between inelastic states were found to be negligible, while the couplings to pickup channels contribute significantly. For the first time observed reaction cross-sections are completely accounted for by explicit channel coupling, for incident energies between 10 and 40 MeV. Elastic angular distributions were also obtained and compared to experimental data and results from optical model. The accuracy of the description of the observed cross sections was found to be sensitive to the nucleon-nucleon effective interaction, serving as a direct assessment of the structure models used. [Preview Abstract] |
Saturday, November 6, 2010 11:30AM - 11:42AM |
NC.00006: Three-body approach to direct nuclear reactions involving weakly bound systems Ant\'onio C. Fonseca The Faddeev type Alt, Grassberger and Sandhas (AGS) equations for transition operators were, in recent years, consistently applied to study direct nuclear reactions using realistic nucleon-nucleus optical potentials together with modern nucleon-nucleon interactions. The equations are solved numerically using momentum-space partial-wave basis. The Coulomb interaction between charged particles is included using a novel implementation of the screening and renormalization method. The AGS equations have been successfully used to study elastic, transfer, and breakup reactions in three-body-like nuclear systems. Examples are deuteron scattering on stable nuclei ${}^4$He, ${}^{10}$Be, ${}^{12}$C, ${}^{14}$C, ${}^{16}$O, ${}^{40}$Ca, and ${}^{58}$Ni and proton scattering on weakly bound two-body system such as ${}^{11}$Be, ${}^{13}$C, ${}^{15}$C, and ${}^{17}$O. These calculations allow to evaluate the accuracy of traditional approximate nuclear reaction approaches like the continuum-discretized coupled-channels (CDCC) method but also to test novel dynamical models such as energy dependent and nonlocal optical potentials. [Preview Abstract] |
Saturday, November 6, 2010 11:42AM - 11:54AM |
NC.00007: Low-energy neutron capture reactions via the surrogate method Jutta Escher, Jason Burke, Frank Dietrich, Jo Ressler, Nicholas Scielzo, Ian Thompson Indirect methods, such as the surrogate approach, play a crucial role in determining cross sections for reactions on unstable nuclei. In a surrogate experiment, the compound nucleus of interest is produced using a light-ion direct reaction on a target that is easier to produce. The decay of the compound nucleus is measured in coincidence with the outgoing direct-reaction particle and the coincidence probabilities are used to infer the desired cross sections. The method is expected to play an important role in cross-section measurements with radioactive ion beams (RIBs), since inverse-kinematics experiments cannot be performed on a neutron target. For example, if one measures (d,p$\gamma$) instead of (n,$\gamma$), one carries out a surrogate measurement. To make full use of radioactive-beam capabilities, the inverse-kinematics surrogate method needs to be developed further. This presentation summarizes recent applications of the surrogate approach to (n,$\gamma$) reactions. The limitations of frequently-employed approximation schemes will be discussed, as well as progress made in moving beyond these limitations. Results will be shown for the gadolinium region. [Preview Abstract] |
Saturday, November 6, 2010 11:54AM - 12:06PM |
NC.00008: Glauber method for alpha particle scattering M.A. Alvi, M.A. Abdulmomen, J.H. Madani, R.J. Peterson Using the Coulomb-modified Glauber model in the optical limit approximation, we have used known distributions of nucleons in complex nuclei and a new parameterized nucleon-nucleon (N-N) phase shift function to compute elastic alpha particle scattering at beam energies from 36 to 60 MeV per nucleon, as a first example of a program for heavy ion elastic scattering analyses. At each of three energies, the three parameters of our N-N phase shift function were adjusted to fit the data for one nucleus. It was found that calculations with these same N- N parameters for other nuclei at that energy also gave good agreement with the data. This method offers good insights into the role of the N-N interaction potential within nuclei. For instance, our N-N parameters give N-N total cross sections of 74, 79, and 97\% of the N-N free space total cross sections at alpha particle energies of 36, 43, and 60 MeV per nucleon. This method is readily suited to further applications to heavy ion elastic scattering, and can be expected to give useful insights into the relevant N-N interactions. [Preview Abstract] |
Saturday, November 6, 2010 12:06PM - 12:18PM |
NC.00009: Coupled-channels Calculations of Heavy-ion Fusion Reactions Henning Esbensen Heavy-ion fusion reactions are sensitive probes of the nuclear surfaces of the reacting nuclei. Roughly speaking, the height of the Coulomb barrier in the entrance channel is determined by the surface density profiles of the two nuclei, and the couplings to low-lying surface modes are responsible for lowering the effective barrier and enhancing the fusion cross section at sub-barrier energies. However, it is difficult to accurately predict the fusion cross from coupled-channels calculations, partly because of the influence of the polarization of high-lying states that are not considered explicitly, and partly because of transfer reactions in asymmetric collisions. These features are illustrated by analyzing new data for the fusion of symmetric and asymmetric combinations of calcium isotopes, and it is discussed how they affect the extrapolation of fusion cross sections to extreme sub-barrier energies. [Preview Abstract] |
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