André Lejeune

Many Body Theory of Nuclear Matter

Abstract We combine the many-body theory and the low-density expansion developed by Brueckner, Bethe and others to investigate several properties of the ground state and of single-particle excited states of symmetric nuclear matter. We calculate the following quantities from Reids hard core nucleon-nucleon interaction: strength, energy-dependence, nonlocality and density-dependence of the real and of the imaginary parts of the optical-model potential, momentum distribution in the interacting ground state, dependence on density and momentum of the norm of a quasiparticle and of the effective mass, spectral function for particle states, saturation density and average binding energy per nucleon. No free parameter is adjusted in the calculation; good agreement is obtained with empirical values. It is shown that the effective mass has a narrow maximum at the Fermi surface; this is investigated in the framework of analytical models.

Hot nuclear matter in an extended Brueckner approach

Abstract The properties of cold and hot nuclear matter are studied in the frame of the Brueckner theory, extended to finite temperature. The basic task is the evaluation of the two-hole line diagram using the Paris potential supplemented by the introduction of three-body forces, coming from the exchange of π and ρ mesons. The latter have an important saturating effect, but not sufficient to reach correct saturation. The latter is achieved by a phenomenological treatment. The properties of hot nuclear matter, for temperatures around 10 MeV, are investigated. Particular attention is paid to one-body properties. The density and temperature dependence of many quantities, like the single-particle energy spectrum, the optical potential, the effective mass, the non-locality of the single-particle field, the mean free path, is displayed and analyzed. The relative importance of the temperature dependence of the g -matrix and of phase space is investigated, especially in relation with the imaginary part of the optical potential and the mean free path. The temperature dependence of the effective mass is particularly studied. It is shown that the peak due to the so-called core polarization effect disappears rapidly as the matter is heated. The evaluation of the entropy and of the level density parameter a , which are closely related, is discussed, and the failure of the Hartree-Fock approach to reproduce the value of a correctly is explained. Two-body properties are also investigated. The temperature and density dependence of the two-body correlations are displayed. Particular attention is paid to the temperature dependence of the effective interaction. The latter is exhibited in a simple manner. It is shown that the effective force felt by low-energy nucleons does not change by more than a few percent when the temperature goes from 0 to 10 MeV. For high-energy nucleons, the change may be as large as ten percent.

Superfluidity in neutron matter and nuclear matter with realistic interactions

Abstract The 1S0 superfluidity of neutron matter and nuclear matter is studied by solving the gap equation exactly for two realistic nuclear potentials, namely the Paris and the Argonne v14 potential. For neutron matter, the predicted domain of superfluidity is very close to previous results, whereas differences appear in the predicted value of the maximum gap. The results are, however, very close to each other for the two potentials mentioned above. The role of the large momentum component is underlined and the accuracy of several approximations is discussed. The temperature dependence is exhibited. For nuclear matter, the superfluidity disappears at smaller density. The gap is rather small for equilibrium density. The condensation energy estimated through a local-density approximation is dominated by the surface contribution. The definition of the pairing interaction is discussed and an illustrative calculation for an effective interaction is presented.

Medium Polarization Effects on Neutron Matter Superfluidity

Abstract We solve the 1 S 0 gap equation for neutron matter using an effective interaction based on the Argonne V 14 potential that includes effects of the polarization (RPA) graphs. We find a substantial reduction of the gap, and a slight extension of the domain of superfluidity to larger densities. We demonstrate the inadequacy of using the weak coupling approximation.

Nuclear mean field with correlations at finite temperature

Abstract Zero and finite temperature contributions of ground state correlations to the nuclear mean field are studied in nuclear matter at normal density. The framework is the nonrelativistic hole line expansion with the Paris potential as the bare NN interaction. For different temperatures we calculate single particle properties including correlation contributions in the self-consistent determination of the single-particle energies. We evaluate the nucleon effective mass and the energy mass. Their temperature dependence is studied and related to that of the level density parameter. We also calculate the momentum distribution of nucleons and discuss its behaviour at large momenta. In the present approach the spectral function and the lifetime of hole state can be obtained directly. We present our first results and analyze them briefly. Finally, we examine the important aspects of the conserving character of the approximations made in the course of this study.

Proton and neutron superfluidity in neutron star matter

Abstract The 3 P 2 neutron superfluidity and the 1 S 0 proton superfluidity in neutron star matter are investigated by solving the gap equation exactly for a realistic nucleon-nucleon potential, namely the Argonne v 14 potential. For the 3 P 2 case, our results point to a nearly isotropic pairing function, in close analogy with the rigorous result obtained by Balian and Werthamer for S = 1 pairing without J -dependence. The proton abundance and the 1 S 0 proton gap are calculated and their relationship with the symmetry energy is discussed.

Sciatic Nerve Regeneration through Venous or Nervous Grafts in the Rat

This study analyses the interest of isologous venous grafts filled with saline or with Schwann cells versus nerve grafts as guides for regeneration of the sciatic nerve in 35 Wistar rats. Electrophysiological parameters (conduction velocities and distal latencies of motor responses) and the functional index of De Medinacelli were measured several times from 1 month to 1 year after surgery. An histological analysis was performed on 2 control rats and on 3 rats killed 6 or 12 months after surgery: the total number of fibers was counted on a montage photoprint of the whole nerve, and the diameters of axons and the thickness of the myelin sheath were measured on digitized images. With a portion of nerve as guide, the regeneration is faster than with a vein. However, regeneration after 6 months is at least as good with a venous graft filled with Schwann cells, as assessed by electrophysiological, functional, and histological analysis. The addition of Schwann cells in grafted veins allows the nerve to regenerate through longer gaps than previously described (25 vs 15 mm). In order to assess the quality of nerve regeneration, functional, electrophysiological, and histological analysis are complementary.

Interplay of three-body interactions in the EOS of nuclear matter

Abstract The equation of state of symmetric nuclear matter has been investigated within Brueckner approach adopting the charge-dependent Argonne V 18 two-body force plus a microscopic three-body force based on a meson-exchange model. The effects on the equation of state of the individual processes giving rise to the three-body force are explored up to high baryonic density. It is found that the major role is played by the competition between the strongly repulsive ( σ , ω )-exchange term with virtual nucleon–antinucleon excitation and the large attractive contribution due to ( σ , ω ) exchange with N ∗ (1440) resonance excitation. The net result is a repulsive term which shifts the saturation density corresponding to the only two-body force much closer to the empirical value, while keeping constant the saturation energy per particle. The contribution from ( π , ρ )-exchange 3BF is shown to be attractive and rather small. The analysis of the separate three-body force contributions allows to make a comparison with the prediction of Dirac–Brueckner approach which is supposed to incorporate via the dressed Dirac spinors the same virtual nucleon–antinucleon excitations as in the present three-body force. The numerical results suggest that the three-body force components missing from the Dirac–Brueckner approach are not negligible, especially at high density. The calculation of the nuclear mean field and the effective mass shows that the three-body force affects to a limited extent such properties.

Non-toxic antiseptic irrigation with chlorhexidine in experimental revascularization in the rat.

The effect of different wound irrigation fluids upon femoral arteries and veins was investigated in the rat, using microsurgical techniques. Toxicity was evaluated by microscopical observation after selective staining of histological slides. Povidone-iodine, 10%, proved to be a very irritant solution, provoking an attack on the vascular endothelium and secondary thrombosis. Chlorhexidine at 0.05%, 0.02% and 0.001% was found by contrast to have a very low toxicity which was comparable to physiological saline. Experimental investigation of antiseptic solutions should not only include the determination of the antibacterial effect, but also the potential for cell toxicity, using an irrigation technique.

Microscopic optical model analyses of proton and neutron elastic scattering cross sections

Abstract In this paper we investigate the physical properties of a microscopic optical model potential (OMP) derived from a realistic nucleon-nucleon interaction through nuclear matter calculations, using the local density approximation for nuclei. We calculate nucleon elastic scattering cross sections for a wide range of energies and nuclei. Comparison with experimental data shows that the shape and depth of the real part of the OMP are quite satisfactory. However, the depth of the imaginary part turns out to be a little too large. We suggest some plausible physical arguments to explain this discrepancy. The overall argument is however quite encouraging and it suggests that the study of some microscopic effects, neglected previously, would be worthwhile.