Increase in |S_L| induced by channel coupling: the case of deuteron scattering

For deuteron scattering from 58Ni at laboratory energies of 56, 79, and 120 MeV, we study the dynamic polarization potentials (DPPs) induced by S-wave and D-wave breakup (BU), separately and together, in order to gain insight into the nature of the DPP as well as a counterintuitive property: the existence of L values for which the BU coupling increases |SL|, a ‘wrong-way’ effect. The effect is associated with the existence of emissive regions in the imaginary DPP, suggesting a connection with the nonlocal nature of the DPP. The same relationship was previously found for 6Li scattering, indicating a generic effect bearing on the dynamics of nuclear reactions

Determination of Li-6 -- He-4 interaction from multi-energy scattering data

We present the first successful potential model description of Li-6 -- He-4 scattering. The differential cross-sections for three energies and the vector analyzing powers for two energies were fitted by a single potential with energy dependent imaginary components. An essential ingredient is a set of Majorana terms in each component. The potential was determined using a recently developed direct data-to-potential inversion method which is a generalisation of the IP S-matrix-to-potential inversion algorithm. We discuss the problems related to this phenomenological approach, and discuss the relationship of our results to existing and future theories.Comment: 9 pages plain LaTeX, 6 postscript figue

Reaction channel coupling effects for nucleons on <b>¹⁶O</b>: Induced undularity and proton-neutron potential differences

Background: Precise fitting of scattering observables suggests that the nucleon-nucleus interaction is l dependent. Such l dependence has been shown to be S-matrix equivalent to an undulatory l-independent potential. The undulations include radial regions where the imaginary term is emissive. Purpose: To study the dynamical polarization potential (DPP) generated in proton-16O and neutron-16O interaction potentials by coupling to pickup channels. Undulatory features occurring in these DPPs can be compared with corresponding features of empirical optical model potentials (OMPs). Furthermore, the additional inclusion of coupling to vibrational states of the target will provide evidence for dynamically generated nonlocality. Methods: The FRESCO code provides the elastic channel S-matrix Slj for chosen channel couplings. Inversion, Slj -> V(r)+ 1 ⋅ s VSO (r), followed by subtraction of the bare potential, yields an l-independent and local representation of the DPP due to the chosen couplings. Results: The DPPs have strongly undulatory features, including radial regions of emissivity. Certain features of empirical DPPs appear, e.g., the full inverted potential has emissive regions. The DPPs for different collective states are additive except near the nuclear center, whereas the collective and reaction channel DPPs are distinctly nonadditive over a considerable radial range, indicating dynamical nonlocality. Substantial differences between the DPPs due to pickup coupling for protons and neutrons occur; these imply a greater difference between proton and neutron OMPs than the standard phenomenological prescription. Conclusions: The onus is on those who object to undularity in the local and l-independent representation of nucleon elastic scattering to show why such undulations do not occur. This work suggests that it is not legitimate to halt model-independent fits to high-quality data at the appearance of undularity

Breakup coupling Effects on Near-Barrier 6Li, 7Be and 8B + 58Ni Elastic Scattering Compared

Submitted to Nucl. Phys. ANew data for near-barrier 6Li, 7Be and 8B + 58Ni elastic scattering enable a comparison of breakup coupling effects for these loosely-bound projectiles. Coupled Discretised Con- tinuum Channels (CDCC) calculations suggest that the large total reaction cross sections for 8B + 58Ni are dominated by breakup at near-barrier energies, unlike 6Li and 7Be where breakup makes a small contribution. In spite of this, the CDCC calculations show a small coupling influence due to breakup for 8B, in contrast to the situation for 6Li and 7Be. An examination of the S matrices gives a clue to this counter-intuitive behaviour

Deuteron -- α\alpha interaction by inversion of RGM S-matrix: determination of spin-orbit potential for spin-1 projectile

The iterative-perturbative (IP) procedure for S-matrix to potential inversion is applied to spin-one projectiles for the restricted case of vector spin-orbit interaction only. In order to evaluate this extension of IP inversion we have inverted the multi-channel RGM $S_{lj}$ of Kanada et al for deuterons scattering from $^4$He with deuteron distortion and then compared the central components with those derived from RGM with spin set to zero. Attention is given to the question of how well the resulting potentials are established. Reliable spin-1 inversion is demonstrated. Results relating to inversion, to deuteron-nucleus interactions and to RGM are presented and suggest the range of nuclear interaction information which the procedure makes possible. Unusual non-locality and parity dependence effects are found; these are of possible relevance to generic properties of nuclear potentials.Comment: 15 pages LaTeX plus 6 postscript figure

Elastic Scattering Phenomenology

We argue that, in many situations, fits to elastic scattering data that were historically, and frequently still are, considered “good”, are not justifiably so describable. Information about the dynamics of nucleon-nucleus and nucleus-nucleus scattering is lost when elastic scattering phenomenology is insufficiently ambitious. It is argued that in many situations, an alternative approach is appropriate for the phenomenology of nuclear elastic scattering of nucleons and other light nuclei. The approach affords an appropriate means of evaluating folding models, one that fully exploits available empirical data. It is particularly applicable for nucleons and other light ions

Reaction channel contributions to proton scattering at 65 MeV

Background: Well-established coupled channel (CC) and coupled reaction channel (CRC) processes make contributions to elastic scattering that are absent from local density folding models. Purpose: To establish and characterize the contribution to the proton optical model potential (OMP) made by the coupling to neutron pickup channels, in particular the proton OMP for 65 MeV protons on 48Ca and 40Ca. Also to relate this contribution to results for 40Ca at lower energies; to investigate the dynamical nonlocality of this contribution; to characterize the effect on the OMP of breakup of the deuteron. Methods: CRC calculations of neutron pickup and CC calculations of collective states, provide the elastic channel S matrix Slj. Inversion of Slj produces a local potential that yields, in a single channel calculation, the elastic scattering observables from the CC/CRC calculation. Subtracting the bare potential yields a local and l-independent representation of the dynamical polarization potential, DPP. From the DPPs due to a selection of channel couplings the influence of dynamically generated nonlocality can be identified. The effect of coupling to the deuteron breakup continuum is also identified. Results: For 40Ca, coupling to pickup channels has an effect on observables somewhat weaker than that at 30 MeV, and much less than for pickup coupling for 48Ca. The DPPs have similar general properties in each case, but are much larger in magnitude for 48Ca. Subsequent breakup of the deuteron makes a large contribution to the DPP, and hence to the OMP. The formal DPPs due to pickup coupling exhibit dynamical nonlocality. Conclusions: The DPPs challenge local density folding models for elastic scattering. The breakup of the deuteron must henceforth be included in calculations of the DPP due to neutron pickup in proton scattering. Pickup coupling effects are still substantial at 65 MeV. No smoothly varying global OMP could fit proton elastic scattering from both 40Ca and 48Ca

Elastic transfer and parity dependence of the nucleus-nucleus optical potential

Background: A recent coupled-reaction-channel (CRC) study shows that the enhanced oscillation of the elastic 16O + 12C section at backward angles is due mainly to the elastic α transfer or the core exchange. Such a process gives rise to a parity-dependent term in the total elastic S matrix, an indication of the parity dependence of the 16O + 12C optical potential (OP). Purpose: To explicitly determine the core exchange potential (CEP) induced by the symmetric exchange of the two 12C cores in the elastic sup>16O + 12C scattering at Elab = 132 and 300 MeV and explore its parity dependence. Method: S matrix generated by CRC description of the elastic 16O + 12C scattering is used as the input for the inversion calculation to obtain the effective local OP that contains both the Wigner and Majorana terms. Results: The high-precision inversion results show a strong contribution by the complex Majorana term in the total OP of the 16O + 12C system and thus provide for the first time a direct estimation of the parity-dependent CEP. Conclusions: The elastic α transfer or exchange of the two 12C cores in the 16O + 12C system gives rise to a complex parity dependence of the total OP. This should be a general feature of the OP for the light heavy-ion systems that contain two identical cores

Cosmogenic nuclides constrain surface fluctuations of an East Antarctic outlet glacier since the Pliocene

Understanding past changes in the Antarctic ice sheets provides insight into how they might respond to future climate warming. During the Pliocene and Pleistocene, geological data show that the East Antarctic Ice Sheet responded to glacial and interglacial cycles by remaining relatively stable in its interior, but oscillating at its marine-based margin. It is currently not clear how outlet glaciers, which connect the ice sheet interior to its margin, responded to these orbitally-paced climate cycles. Here we report new ice surface constraints from Skelton Glacier, an outlet of the East Antarctic ice sheet, which drains into the Ross Ice Shelf. Our multiple-isotope (10Be and 26Al) cosmogenic nuclide data indicate that currently ice-free areas adjacent to the glacier underwent substantial periods of exposure and ice cover in the past. We use an exposure-burial model driven by orbitally-paced glacial–interglacial cycles to determine the probable ice surface history implied by our data. This analysis shows that: 1) the glacier surface has likely fluctuated since at least the Pliocene; 2) the ice surface was >200 m higher than today during glacial periods, and the glacier has been thicker than present for ∼75–90% of each glacial–interglacial cycle; and 3) ice cover at higher elevations possibly occurred for a relatively shorter time per Pliocene cycle than Pleistocene cycle. Our multiple-nuclide approach demonstrates the magnitude of ice surface fluctuations during the Pliocene and Pleistocene that are linked to marine-based ice margin variability