P. J. Dortmans

Alternative evaluations of halos in nuclei

Data for the scattering of {sup 6}He, {sup 8}He, {sup 9}Li, and {sup 11}Li from hydrogen are analyzed within a fully microscopic folding model of proton-nucleus scattering. Current data suggest that of these only {sup 11}Li has a noticeable halo. For {sup 6}He, we have also analyzed the complementary reaction {sup 6}Li({gamma},{pi}{sup +}){sup 6}He{sub g.s.}. The available data for that reaction support the hypothesis that {sup 6}He 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 {sup 6}He. (c) 2000 The American Physical Society.

Fully microscopic model analyses of the elastic scattering of 200 MeV protons from targets of diverse mass

Non-local optical potentials for the scattering of 200 MeV protons from 22 nuclei, ranging in mass from to , have been defined by folding a complex, medium-dependent effective interaction with 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 bound-state single-particle wavefunctions that specify the nuclear density matrices are Woods - Saxon functions for and harmonic oscillator functions thereafter. The resulting differential cross sections, analysing powers and spin rotations all compare well with the known data.

Medium corrections to nucleon-nucleon interactions

The Beth-Goldstone equations have been solved for both negative and positive energies to specify two-nucleon G matrices fully off the energy shell. Medium correction effects of Pauli blocking and of the auxiliary potential are included in infinite matter systems characterized by Fermi momenta in the range 0.5 fm-1 to 1.8 fm-1. The Paris interaction is used as the starting potential in most calculations. Medium corrections are shown to be very significant over a large range of energies and densities. On-the-energy-shell values of G matrices vary markedly from those of free two-nucleon (NN) t matrices which have been solved by way of the Lippmann-Schwinger equation. Off the energy shell, however, the free and medium-corrected Kowalski-Noyes f ratios are quite similar suggesting that useful models of medium-corrected G matrices are appropriately scaled free NN t matrices. The choice of auxiliary potential form is also shown to play a decisive role in the negative energy regime, especially when the saturation of nuclear matter is considered.

Inelastic proton scattering analyses and ambiguities in spin-parity assignments of states in 12 C

Data from the excitation of the 13·35, 18·35, 19·20 and 19·40 MeV states in 12C by the inelastic scattering of protons have been analysed using a fully microscopic distorted wave approximation to determine optimal assignments from amongst the current and ambiguous J π; T values of each state involved. While not conclusive, the present analysis suggests the assignments of 4-1; 0, 2-2; 0, 4-2; 0 and 22-2; 1 respectively.

Cross sections, spin observables, and helicity amplitudes of the reaction at the isobar resonance and below

We have calculated the reaction cross sections and spin observables from threshold to the resonance by considering three types of mechanisms; the impulse (absorption) mechanism, rescattering and s-wave rescattering. The effect of final-state interactions has also been included. While traditionally the first two mechanisms have been included within a sound theoretical basis, the s-wave rescattering generally has been treated phenomenologically using a coupling constant fitted to the low-energy pion - nucleon data. Herein, we treat the isovector component of such a pion rescattering process as being mediated by the -exchange. Also, in those absorption mechanisms where the pion enters directly without rescattering, we employ a phenomenological cut-off scaling governed by the nucleon coordinates, since at the pion - nucleon vertex the nucleon itself may be off mass shell. The energy dependence of the total cross section is well explained by the combined effects of the three mechanisms, but we find that this observable is not sensitive enough to give useful insight into fine details of the reaction. We calculated the spin observables of at a variety of energies spanning the resonance, finding such sensitivity to fine details of the calculation that we cannot obtain a complete agreement with the vast experimental data base. A better understanding of the importance of the various aspects of the reaction was obtained when the calculated helicity amplitudes were compared with those extracted via phase-shift analyses of the reaction data.

Signatures of medium effects on NN interactions in proton scattering from nuclei

Effective two nucleon (NN) interactions in the nuclear medium have been defined from an accurate mapping of NN g matrices obtained by solving the Brueckner–Bethe–Goldstone (BBG) equations for infinite nuclear matter. Those effective interactions have been used in fully microscopic calculations of proton-light nuclei (nonlocal) effective interactions from which predictions of the elastic scattering differential cross sections and analysing powers have been obtained. Results for incident proton energies of 65 and 200 MeV are considered in particular herein. The associated relative motion wave functions have been used as the distorted waves in distorted wave approximation (DWA) studies of select inelastic scattering events. The same effective interactions were used as the transition operators in those calculations. The relevant nuclear spectroscopy for the elastic and DWA (p, p′) calculations has been found from full (0 + 2)hϖ shell model evaluations of the nuclear structure; wave functions of which give good descriptions of form factors obtained from electron scattering.