129 research outputs found

### Self-energy Effects in the Superfluidity of Neutron Matter

The superfluidity of neutron matter in the channel $^1 S_0$ is studied by
taking into account the effect of the ground-state correlations in the
self-energy. To this purpose the gap equation has been solved within the
generalized Gorkov approach. A sizeable suppression of the energy gap is driven
by the quasi-particle strength around the Fermi surface.Comment: 8 pages and 3 figure

### Nuclear Transparency to Intermediate-Energy Protons

Nuclear transparency in the (e,e'p) reaction for 135 < Tp < 800 MeV is
investigated using the distorted wave approximation. Calculations using
density-dependent effective interactions are compared with phenomenological
optical potentials. Nuclear transparency is well correlated with proton
absorption and neutron total cross sections. For Tp < 300 MeV there is
considerable sensitivity to the choice of optical model, with the empirical
effective interaction providing the best agreement with transparency data. For
Tp > 300 MeV there is much less difference between optical models, but the
calculations substantially underpredict transparency data and the discrepancy
increases with A. The differences between Glauber and optical model
calculations are related to their respective definitions of the semi-inclusive
cross section. By using a more inclusive summation over final states the
Glauber model emphasizes nucleon-nucleon inelasticity, whereas with a more
restrictive summation the optical model emphasizes nucleon-nucleus
inelasticity; experimental definitions of the semi-inclusive cross section lie
between these extremes.Comment: uuencoded gz-compressed tar file containing revtex and bbl files and
5 postscript figures, totalling 31 pages. Uses psfi

### Towards a fully self-consistent spectral function of the nucleon in nuclear matter

We present a calculation of nuclear matter which goes beyond the usual
quasi-particle approximation in that it includes part of the off-shell
dependence of the self-energy in the self-consistent solution of the
single-particle spectrum. The spectral function is separated in contributions
for energies above and below the chemical potential. For holes we approximate
the spectral function for energies below the chemical potential by a
$\delta$-function at the quasi-particle peak and retain the standard form for
energies above the chemical potential. For particles a similar procedure is
followed. The approximated spectral function is consistently used at all levels
of the calculation. Results for a model calculation are presented, the main
conclusion is that although several observables are affected by the inclusion
of the continuum contributions the physical consistency of the model does not
improve with the improved self-consistency of the solution method. This in
contrast to expectations based on the crucial role of self-consistency in the
proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.

### Proton capture cross section of Sr isotopes and their importance for nucleosynthesis of proton-rich nuclides

The (p,$\gamma$) cross sections of three stable Sr isotopes have been
measured in the astrophysically relevant energy range. These reactions are
important for the $p$-process in stellar nucleosynthesis and, in addition, the
reaction cross sections in the mass region up to 100 are also of importance
concerning the $rp$-process associated with explosive hydrogen and helium
burning. It is speculated that this $rp$-process could be responsible for a
certain amount of $p$-nuclei in this mass region. The (p,$\gamma$) cross
sections of $^{84,86,87}$Sr isotopes were determined using an activation
technique. The measurements were carried out at the 5 MV Van de Graaff
accelerator of the ATOMKI, Debrecen. The resulting cross sections are compared
with the predictions of statistical model calculations. The predictions are in
good agreement with the experimental results for $^{84}$Sr(p,$\gamma$)$^{85}$Y
whereas the other two reactions exhibit differences that increase with mass
number. The corresponding astrophysical reaction rates have also been computed.Comment: Phys. Rev. C in pres

### Asymptotic normalization coefficient of ^{8}B from breakup reactions and the S_{17} astrophysical factor

We show that asymptotic normalization coefficients (ANC) can be extracted
from one nucleon breakup reactions of loosely bound nuclei at 30-300 MeV/u. In
particular, the breakup of ^{8}B is described in terms of an extended Glauber
model. The 8B ANC extracted for the ground state of this nucleus from breakup
data at several energies and on different targets, C^2 = 0.450+/-0.039} fm^-1,
leads to the astrophysical factor S_{17}(0)= 17.4+/-1.5 eVb for the key
reaction for solar neutrino production 7Be(p,gamma)8B. The procedure described
here is more general, providing an indirect method to determine reaction rates
of astrophysical interest with beams of loosely bound radioactive nuclei.Comment: 4 pages, RevTex, 3 figures revised version to appear in Phys Rev Let

### Strange hadron matter and SU(3) symmetry

We calculate saturation curves for strange hadron matter using recently
constructed baryon-baryon potentials which are constrained by SU(3) symmetry.
All possible interaction channels within the baryon octet (consisting of $N$,
$\Lambda$, $\Sigma$, and $\Xi$) are considered. It is found that a small
$\Lambda$ fraction in nuclear matter slightly increases binding, but that
larger fractions ($>10%$) rapidly cause a decrease. Charge-neutral
${N,\Lambda,\Xi}$ systems, with equal densities for nucleons and cascades, are
only very weakly bound. The dependence of the binding energies on the
strangeness per baryon, $f_s$, is predicted for various ${N,\Lambda,\Xi}$ and
${N,\Lambda,\Sigma,\Xi}$ systems. The implications of our results in
relativistic heavy-ion collisions and the core of a dense star are discussed.
We also discuss the differences between our results and previous hadron matter
calculations.Comment: 14 pages RevTeX, 7 postscript figure

### Momentum--dependent nuclear mean fields and collective flow in heavy ion collisions

We use the Boltzmann-Uehling-Uhlenbeck model to simulate the dynamical
evolution of heavy ion collisions and to compare the effects of two
parametrizations of the momentum--dependent nuclear mean field that have
identical properties in cold nuclear matter. We compare with recent data on
nuclear flow, as characterized by transverse momentum distributions and flow
($F$) variables for symmetric and asymmetric systems. We find that the precise
functional dependence of the nuclear mean field on the particle momentum is
important. With our approach, we also confirm that the difference between
symmetric and asymmetric systems can be used to pin down the density and
momentum dependence of the nuclear self consistent one--body potential,
independently. All the data can be reproduced very well with a
momentum--dependent interaction with compressibility K = 210 MeV.Comment: 15 pages in ReVTeX 3.0; 12 postscript figures uuencoded; McGill/94-1

### Comparison of Transfer-to-Continuum and Eikonal Models of Projectile Fragmentation Reactions

Spectroscopic properties of nuclei are accessible with projectile
fragmentation reactions, but approximations made in the reaction theory can
limit the accuracy of the determinations. We examine here two models that have
rather different approximations for the nucleon wave function, the target
interaction, and the treatment of the finite duration of the reaction. The
nucleon-target interaction is treated differently in the eikonal and the
transfer-to-continuum model, but the differences are more significant for light
targets. We propose a new parameterization with that in mind. We also propose a
new formula to calculate the amplitude that combines the better treatment of
the wave function in the eikonal model with the better treatment of the target
interaction in the transfer-to-continuum model.Comment: 21 pages, latex file including 3 tables. 5 figures. Submitted to
Phys. Rev.

### Maximum Azimuthal Anisotropy of Neutrons from Nb-Nb Collisions at 400 AMeV and the Nuclear Equation of State

We measured the first azimuthal distributions of triple--differential cross
sections of neutrons emitted in heavy-ion collisions, and compared their
maximum azimuthal anisotropy ratios with Boltzmann--Uehling--Uhlenbeck (BUU)
calculations with a momentum-dependent interaction. The BUU calculations agree
with the triple- and double-differential cross sections for positive rapidity
neutrons emitted at polar angles from 7 to 27 degrees; however, the maximum
azimuthal anisotropy ratio for these free neutrons is insensitive to the size
of the nuclear incompressibility modulus K characterizing the nuclear matter
equation of state.Comment: Typeset using ReVTeX, with 3 ps figs., uuencoded and appende

### Sensitivity of nucleon-nucleus scattering to the off-shell behavior of on-shell equivalent NN potentials

The sensitivity of nucleon-nucleus elastic scattering to the off-shell
behavior of realistic nucleon-nucleon interactions is investigated when
on-shell equivalent nucleon-nucleon potentials are used. The study is based on
applications of the full-folding optical model potential for an explicit
treatment of the off-shell behavior of the nucleon-nucleon effective
interaction. Applications were made at beam energies between 40 and 500 MeV for
proton scattering from 40Ca and 208Pb. We use the momentum-dependent Paris
potential and its local on-shell equivalent as obtained with the
Gelfand-Levitan and Marchenko inversion formalism for the two nucleon
Schroedinger equation. Full-folding calculations for nucleon-nucleus scattering
show small fluctuations in the corresponding observables. This implies that
off-shell features of the NN interaction cannot be unambiguously identified
with these processes. Inversion potentials were also constructed directly from
NN phase-shift data (SM94) in the 0-1.3 GeV energy range. Their use in
proton-nucleus scattering above 200 MeV provide a superior description of the
observables relative to those obtained from current realistic NN potentials.
Limitations and scope of our findings are presented and discussed.Comment: 17 pages tightened REVTeX, 8 .ps figures, submitted to Phys. Rev.

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