arXiv subjects

K. Amos

Publications and source records attributed to K. Amos.

At least 91 records · Page 5Linked to original sources

arXiv2006

Collective-coupling analysis of spectra of mass-7 isobars: ^7He, ^7Li, ^7Be, ^7B

A nucleon-nucleus interaction model has been applied to ascertain the underlying character of the negative-parity spectra of four isobars of mass seven, from neutron-- to proton--emitter driplines. With one single nuclear potential defined by a simple coupled-channel model, a multichannel algebraic scattering approach (MCAS) has been used to determine the bound and resonant spectra of the four nuclides, of which ^7He and ^7B are particle unstable. Incorporation of Pauli blocking in the model enables a description of all known spin-parity states of the mass-7 isobars. We have also obtained spectra of similar quality by using a large space no-core shell model. Additionally, we have studied ^7Li and ^7Be using a dicluster model. We have found a dicluster-model potential that can reproduce the lowest four states of the two nuclei, as well as the relevant low-energy elastic scattering cross sections. But, with this model, the rest of the energy spectra cannot be obtained.

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arXiv2006

Electron scattering form factors from exotic nuclei

The elastic electron scattering form factors, longitudinal and transverse, from the He and Li isotopes and from 8B have been studied. Large space shell model functions have been assumed. The precise distribution of the neutron excess has little effect on the form factors of the isotopes though there is a mass dependence in the charge densities. However, the form factors of the proton-rich nucleus, 8B, are significantly changed by the presence of the proton halo.

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arXiv2006

Scattering as a tool to study nuclear structure toward the drip lines

Results are presented for the elastic scattering of electrons and protons from the exotic He and Li isotopes. Comparison with scattering results from the stable He and Li nuclei allows for an investigation into the effects that the extensive neutron distributions have on the charge density. For comparison we also consider the proton halo nucleus 8B. The consequences and possible suggestions for proposed electron scattering facilities for exotic nuclei are discussed.

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arXiv2006

Properties of effective interactions and the excitation of 6- states in 28Si

Cross-section and analyzing power data from (p,p') scattering to the 6- states at 11.58 and 14.35 MeV in 28Si, taken with energies of 80, 100, 134, and 180 MeV protons, have been analyzed using a distorted wave approximation with microscopically defined wave functions. The results, taken in conjunction with an analysis of an M6 electron scattering form factor, suggest that the two states exhaust respectively, ~50% and ~60% of the strengths of isoscalar and isovector particle-hole excitations from the ground state. The energy variation of data also suggests that the non-central components of the effective interactions at 80 and 100 MeV may need to be enhanced.

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arXiv2005

Low-energy neutron-12C analyzing powers: Results from a multichannel algebraic scattering theory

Analyzing powers in low-energy neutron scattering from 12C are calculated in an algebraic momentum-space coupled-channel formalism (MCAS). The results are compared with recently obtained experimental data. The channel-coupling potentials have been defined previously to reproduce the total cross section and sub-threshold bound states of the compound system. Without further adjustment, good agreement with data for the analyzing powers is obtained.

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arXiv2005

Comparison between two methods of solution of coupled equations for low-energy scattering

Cross sections from low-energy neutron-nucleus scattering have been evaluated using a coupled channel theory of scattering. Both a coordinate-space and a momentum-space formalism of that coupled-channel theory are considered.A simple rotational model of the channel interaction potentials is used to find results using two relevant codes, ECIS97 and MCAS, so that they may be compared. The very same model is then used in the MCAS approach to quantify the changes that occur when allowance is made for effects of the Pauli principle.

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arXiv2005

Predicting narrow states in the spectrum of a nucleus beyond the proton drip line

Properties of particle-unstable nuclei lying beyond the proton drip line can be ascertained by considering those (usually known) properties of its mirror neutron-rich system. We have used a multi-channel algebraic scattering theory to map the known properties of the neutron-${}^{14}$C system to those of the proton-${}^{14}$O one from which we deduce that the particle-unstable ${}^{15}$F will have a spectrum of two low lying broad resonances of positive parity and, at higher excitation, three narrow negative parity ones. A key feature is to use coupling to Pauli-hindered states in the target.

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arXiv2005

Solution of multi-channel problems using MCAS for spectra and scattering cross sections

A multi-channel algebraic scattering theory, to find solutions of coupled-channel scattering problems with interactions determined by collective models, has been structured to ensure that the Pauli principle is not violated. Positive (scattering) and negative (sub-threshold) solutions can be found to predict both the compound nucleus sub-threshold spectrum and all resonances due to coupled-channel effects that occur on a smooth energy varying background. The role of the Pauli principle is crucial in defining what interaction potentials are required to fit data. The theory also gives an algebraic form for the dynamic polarization potential which adds to the ground state interaction to define the optical potential that gives the same elastic scattering cross sections.

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arXiv2005

Low energy nuclear scattering and sub-threshold spectra from a multi-channel algebraic scattering theory

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arXiv2005

Complementary reaction analyses and the isospin mixing of the 4- states in 16O

Data from the inelastic scattering of electrons, and of intermediate energy protons and pions leading to ``stretched'' configuration 4- states near 19 MeV excitation in 16O as well as from charge exchange (p,n) scattering to an isobaric analogue (4-) state in 16F have been analyzed to ascertain the degree of isospin mixing contained within those states and of the amount of d_{5/2}-p_{3/2}^{-1} particle-hole excitation strength they exhaust. The electron and proton scattering data have been analyzed using microscopic models of the structure and reactions, with details constrained by analyses of elastic scattering data.

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arXiv2005

Comparison of optical model results from a microscopic Schrödinger approach to nucleon-nucleus elastic scattering with those from a global Dirac phenomenology

Comparisons are made between results of calculations for intermediate energy nucleon-nucleus scattering for 12C, 16O, 40Ca, 90Zr, and 208Pb, using optical potentials obtained from global Dirac phenomenology and from a microscopic Schrödinger model. Differential cross sections and spin observables for scattering from the set of five nuclei at 65 MeV and 200 MeV have been studied to assess the relative merits of each approach. Total reaction cross sections from proton-nucleus and total cross sections from neutron-nucleus scattering have been evaluated and compared with data for those five targets in the energy range 20 MeV to 800 MeV. The methods of analyses give results that compare well with experimental data in those energy regimes for which the procedures are suited.

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arXiv2005

Role of the Pauli principle in collective-model coupled-channels calculations

A multi-channel algebraic scattering theory, to find solutions of coupled-channel scattering problems with interactions determined by collective models, has been structured to ensure that the Pauli principle is not violated. By tracking the results in the zero coupling limit, a correct interpretation of the sub-threshold and resonant spectra of the compound system can be made. As an example, the neutron-12C system is studied defining properties of 13C to 10 MeV excitation. Accounting for the Pauli principle in collective coupled-channels models is crucial to the outcome.

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arXiv2005

Compound and quasi-compound states in low-energy scattering of nucleons from 12C

A multi-channel algebraic scattering theory has been used to study the properties of nucleon scattering from 12C and of the sub-threshold compound nuclear states, accounting for properties in the compound nuclei to ~10 MeV. All compound and quasi-compound resonances observed in total cross-section data are matched, and on seeking solutions of the method at negative energies, all sub-threshold states in 13C and 13N are predicted with the correct spin-parities and with reasonable values for their energies. A collective-model prescription has been used to define the initiating nucleon-12C interactions and via use of orthogonalizing pseudo-potentials, account is made of the Pauli principle. Information is extracted on the underlying structure of each state in the compound systems by investigating the zero-deformation limit of the results.

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arXiv2004

Local scale transformations and extended matter distributions in nuclei

Local scale transformations are made to vary the long range properties of harmonic oscillator orbitals conventionally used in model structure calculations of nuclear systems. The transformations ensure that those oscillator states asymptotically have exponentially decaying forms consistent with chosen single nucleon binding energies, leaving the structure essentially unchanged within the body of the nucleus. Application has been made to the radioactive nuclei 6,8He and 11Li and the resulting wave functions are used to generate g-folding optical potentials for elastic scattering of those ions from hydrogen. As a consistency test, application has been made to form wave functions for 40Ca and they have been used also to specify relevant proton-40Ca optical potentials with which elastic scattering has been predicted. The need for appropriate specifications of single particle binding energies in exotic nuclei is discussed.

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arXiv2004

On Raynal's posting in arXiv (nucl-th/0407060)

With nucl-th/0407060, Jacques Raynal uses the arXiv in a way which does not conform to standard professional practices. His posting contains many statements that are beyond the borders of acceptable scientific disputes, with the scope to defame colleagues by manifestly false or misleading statements. In this comment we reject the three ``critiques'' expressed by Raynal. 1. The fact that we possibly misquoted our references. 2. The role of the Pauli principle in these kind of calculations. 3. The nature and limits of our coupled-channel potential model. Raynal's postings unfairly detract from the importance of our work, which we published in Nuclear Physics A728, 65 (2003), on a new approach, Multi-Channel-Algebraic-Scattering (MCAS), for coupled-channel calculations. With the MCAS approach we were able to identify systematically all low-energy compound resonances, and to include effectively the Pauli principle in collective, geometrical-type, macroscopic models of multichannel interaction. This represents a clear advantage with respect to the current distribution of the ECIS formulation.

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arXiv2004

Simple function form for n+208Pb total cross section between 5 and 600 MeV

The total cross section for neutron scattering from 208Pb with energies between 5 and 600 MeV has been analyzed extending a previously defined simple function of three parameters to reveal a Ramsauer-like effect throughout the whole energy range. This effect can be parametrized in a simple way so that it may be anticipated that the complete function prescription will apply for total cross sections from other nuclei.

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arXiv2004

Simple functional forms for total cross sections from neutron-nucleus collisions

Neutron scattering total cross sections have been estimated from nuclei ranging in mass 6 to 238 and for neutron energies 10 MeV to 600 MeV using a simple function of three parameters. These total cross sections have also been calculated using 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. Adjusting the theoretically defined parameter values has enabled us to fit the actual measured data. The simple functional parameter values vary smoothly with mass and energy.

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arXiv2004

Predicting total and total reaction cross sections using a simple functional form

Total reaction cross sections have been predicted for neucleons scattering from nuclei ranging in mass 6 to 238 and for projectile energies from just above noticeable giant resonance excitation to 300 MeV. So also have been the mass variations of those cross sections at selected energies when they have been calculated using 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. However, there is a simple three parameter functional form that reproduces the partial wave total reaction cross section values determined from those optical potential calculations; a functional form also maps the total scattering cross section partial wave elements. Adjusting the theoretical defined parameter values has enabled us to fit the actual measured data values from scattering involving (15) nuclei spanning the mass range from 9Be to 238U and for proton energies from 10 to 300 MeV. Likewise total cross sections for neutron cross sections for neutron scattering from various nuclei can be equally well reproduced. Of import is that the three parameter values vary smoothly with mass and energy.

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