arXiv subjects
Publications and source records attributed to K. Amos.
Total cross sections for neutron scattering with energies between 10 and 600 MeV and from nine nuclei spanning the mass range from 6Li to 238U have been analyzed using a simple function of three parameters. The values of those parameters with which neutron total cross section data are replicated vary smoothly with energy and target mass and may themselves be represented by functions of energy and mass.
The fusion of two interacting heavy ions traditionally has been interpreted in terms of the penetration of the projectile into the target. Observed rates well below the Coulomb barrier are considerably lower than estimates obtained from penetration factors. One approach in the analysis of the data invokes coupling to non-elastic channels in the scattering as the source of the depletion. Another is to analyze those data in terms of tunneling in semi-classical models, with the observed depletion being taken as evidence of a ``friction'' under the barrier. A complementary approach is to consider such tunneling in terms of a fully quantal model. We investigate tunneling with both one-dimensional and three-dimensional models in a fully quantal approach to investigate possible sources for such a friction. We find that the observed phenomenon may not be explained by friction. However, we find that under certain conditions tunneling may be enhanced or diminished by up to 50%, which finds analogy with observation, without the invocation of a friction under the barrier.
Proton and neutron densities have been obtained for the even-even isotopes of Sn from $^{100}$Sn to $^{176}$Sn using a Hartree-Fock-Bogoliubov model with a Skyrme interaction. The matter densities so defined have been used with realistic nucleon-nucleon interactions in a folding model to specify optical potentials for the elastic scattering of protons with energies in the range 40 to 200 MeV. Those potentials have been used to make predictions of the differential cross sections and spin observables for proton scattering. As the target mass increases, the emergence of the neutron skin in the Sn isotopes is revealed by marked effects in the differential cross section. Comparisons with available data show how similar scattering data for the neutron-rich isotopes may provide constraints for the model structures.
Recently, we published a paper (Nucl. Phys. A 728 (2003) 65; see nucl-th/0306068) presenting a new calculational method for nucleon-nucleus elastic scattering at low energies. That method is particularly appropriate for analyses in the region of narrow resonances. The method is based upon the sturmian representation of the S-matrix, and allows inclusion of nonlocality effects due to Pauli principle. It also provides a systematic identification of narrow-resonance spectra and subthreshold bound states. A phenomenological test calculation for low-energy (below 4 MeV) neutrons on 12-C (including the first two excitations of the target) was presented to illustrate the validity of the approach. The model calculation received a violent criticism (see nucl-th/0312038) by the developer of a method (ECIS) which to date cannot handle nonlocality effects and cannot be used easily to identify all narrow resonances. We demonstrate that Raynal's opposition to our development is not well founded by the arguments he presents. Indeed the work we published shows, on rewording the title of nucl-th/0312038, that it is ``aberrant'' phenomenologically to analyze resonant low-energy nucleon-nucleus processes with coupled-channel methods without taking into account the nonlocalities due to the Pauli principle; problems typically encountered in the ECIS formulation.
Compound resonances in nucleon-nucleus scattering are related to the discrete spectrum of the target. Such resonances can be studied in a unified and general framework by a scattering model that uses sturmian expansions of postulated multichannel interactions between the colliding nuclei. Associated with such expanded multichannel interactions are algebraic multichannel scattering matrices. The matrix structure of the inherent Green functions not only facilitates extraction of the sub-threshold (compound nucleus) bound state spin-parity values and energies but also readily gives the energies and widths of resonances in the scattering regime. We exploited also the ability of the sturmian-expansion method to deal with non-local interactions to take into account the strong non-local effects introduced by the Pauli principle. As an example, we have used the collective model (to second order) to define a multichannel potential matrix for low energy neutron-C12 scattering allowing coupling between the 0+ (ground), 2+ (4.4389 MeV), and 0+ (7.64 MeV) states. The algebraic S matrix for this system has been evaluated and the sub-threshold bound states as well as cross sections and polarizations as functions of energy are predicted. The results are reflected in the actual measured data, and are shown to be consistent with expectations as may be based upon a shell model description of the target and of the compound nucleus.
Total reaction cross sections are predicted for proton scattering from various nuclei. A simple functional form has been used that reproduces the total reaction cross sections for the scattering of protons from (15) nuclei spanning the mass range from ${}^9$Be to ${}^{238}$U and for proton energies 10 to 300 MeV.
A simple functional form has been found that gives a good representation of the total reaction cross sections for the scattering of protons from (15) nuclei spanning the mass range ${}^{9}$Be to ${}^{238}$U and for proton energies ranging from 20 to 300 MeV.
Total reaction cross sections are predicted for nucleons scattering from various nuclei. Projectile energies to 300 MeV are considered. So also are mass variations of those cross sections at selected energies. All predictions have been obtained from coordinate space optical potentials formed by full folding effective two-nucleon (NN) interactions with one body density matrix elements (OBDME) of the nuclear ground states. Good comparisons with data result when effective NN interactions defined by medium modification of free NN t matrices are used. Coupled with analyses of differential cross sections, these results are sensitive to details of the model ground states used to describe nuclei.
We seek a measure of the neutron density of 208Pb from analyses of intermediate energy nucleon elastic scattering. The pertinent model for such analyses is based on coordinate space nonlocal optical potentials obtained from model nuclear ground state densities. Those potentials give predictions of integral observables and of angular distributions which show sensitivity to the neutron density. When compared with experiment, and correlated with analyses of electron scattering data, the results suggest that 208Pb has a neutron skin thickness ~0.17 fm.
Neutron total reaction cross sections at 45, 50, 55, 60, 65, and 75 MeV from nuclei 12C, 28Si, 56Fe, 90Zr, and 208Pb have been measured and are compared with (microscopic) optical model predictions. The optical potentials were obtained in coordinate space by full folding effective nucleon-nucleon interactions with realistic nuclear ground state density matrices. Good to excellent agreement is found.
A simple functional form has been found that gives a good representation of the total reaction cross sections for the scattering from ${}^{208}$Pb of protons with energies in the range 30 to 300 MeV.
Nucleon total reaction and neutron total cross sections to 300 MeV for 12C and 208Pb, and for 65 MeV spanning the mass range, are predicted using coordinate space optical potentials formed by full folding of effective nucleon-nucleon interactions with realistic nuclear ground state densities. Good to excellent agreement is found with existing data.
Fixed energy inverse scattering theory has been used to define central and spin-orbit Schrödinger potentials for the scattering of 5 eV polarized electrons from Xe atoms. The results are typical for a range of such data; including energies above threshold when the potentials become complex. The phase shifts obtained from an analysis of the measured differential cross section and analyzing power has been used as input data. Both semi-classical (WKB) and fully quantal inversion methods have been used to extract central and spin-orbit interactions. The analysis shows that information additional to the set of input phase shifts extracted from this (and similar) data may be needed to ascertain physical potentials.
A Newton-Sabatier fixed energy inversion scheme has been used to equate inherently non-local p-${}^{12}$C potentials at a variety of energies to pion threshold, with exactly phase equivalent local ones. Those energy dependent local potentials then have been recast in the form of non-local Frahn-Lemmer interactions.
The differential cross sections and analyzing powers from the elastic scattering of 25 and 40 MeV protons from many nuclei have been studied. Analyses have been made using a fully microscopic model of proton-nucleus scattering seeking to establish a means appropriate for use in analyses of radioactive beam scattering from hydrogen with ion energies 25A and 40A MeV.
Proton elastic scattering and inelastic scattering to the first excited state of 6He have been measured over a wide angular range using a 40.9A MeV 6He beam. The data have been analyzed with a fully microscopic model of proton-nucleus scattering using 6He wave functions generated from large space shell model calculations. The inelastic scattering data show a remarkable sensitivity to the halo structure of 6He.
Data for the scattering of 6He, 8He, 9Li, and 11Li from hydrogen are analyzed within a fully microscopic folding model of proton-nucleus scattering. Current data suggest that of these only 11Li has a noticeable halo. For 6He, we have also analysed the complementary reaction 6Li(gamma,pi)6He(gs). The available data for that reaction support the hypothesis that 6He may not be a halo nucleus. However, those data are scarce and there is clearly a need for more to elicit the microscopic structure of 6He.
Nonlocal coordinate space optical potentials for the scattering of 65 MeV protons from nuclei ranging in mass from 6Li to 238U have been defined by folding a complex, medium dependent effective interaction with the density matrix elements of each target. The effective interaction is based upon solutions of the Lippmann--Schwinger and Brueckner--Bethe--Goldstone equations having the Paris potential as input. The nuclear structure information required in our folding model are the one body density matrix elements for the target and the single nucleon bound state wave functions that they weight. For light mass nuclei, very large basis shell model calculations have been made to obtain the one body density matrix elements. For medium and heavy mass nuclei, a very simple shell model prescription has been used. The bound state single particle wave functions that complete the nuclear density matrices are either Woods--Saxon or harmonic oscillator functions. The former are employed in most cases when large basis structure is available. For light nuclei (A < 16) Woods--Saxon potential parameters and harmonic oscillator lengths are determined from fits to electron scattering form factors. For all other nuclei, we use harmonic oscillator functions with the oscillator lengths set from an A^{1/6} mass law. Using this microscopic model, optical potentials result from which differential cross sections, analyzing powers and spin rotations are found. In general the calculated results compare very well with data when the effective interactions are determined from a mapping of nucleon--nucleon g matrices. This is not the case when effective interactions built from a mapping of (free) t matrices are used.