120 research outputs found
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
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
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.
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
-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.
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.
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 ,
, , and ) are considered. It is found that a small
fraction in nuclear matter slightly increases binding, but that
larger fractions () rapidly cause a decrease. Charge-neutral
systems, with equal densities for nucleons and cascades, are
only very weakly bound. The dependence of the binding energies on the
strangeness per baryon, , is predicted for various and
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
Neutrons from multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 AMeV
We measured neutron triple-differential cross sections from
multiplicity-selected Au-Au collisions at 150, 250, 400, and 650 \AMeV. The
reaction plane for each collision was estimated from the summed transverse
velocity vector of the charged fragments emitted in the collision. We examined
the azimuthal distribution of the triple-differential cross sections as a
function of the polar angle and the neutron rapidity. We extracted the average
in--plane transverse momentum and the normalized
observable , where is the neutron
transverse momentum, as a function of the neutron center-of-mass rapidity, and
we examined the dependence of these observables on beam energy. These
collective flow observables for neutrons, which are consistent with those of
protons plus bound nucleons from the Plastic Ball Group, agree with the
Boltzmann--Uehling--Uhlenbeck (BUU) calculations with a momentum--dependent
interaction. Also, we calculated the polar-angle-integrated maximum azimuthal
anisotropy ratio R from the value of .Comment: 20 LaTeX pages. 11 figures to be faxed on request, send email to
sender's addres
Medium Effects on Binary Collisions with the Delta Resonance
To facilitate the relativistic heavy-ion calculations based on transport
equations, the binary collisions involving a resonance in either the
entrance channel or the exit channel are investigated within a Hamiltonian
formulation of interactions. An averaging procedure is developed to
define a quasi-particle and to express the experimentally measured
cross section in terms of an effective cross section. In contrast to previous works, the main feature of
the present approach is that the mass and the momentum of the produced
's are calculated dynamically from the bare vertex interaction of the model Hamiltonian and are constrained by the
unitarity condition. The procedure is then extended to define the effective
cross sections for the experimentally inaccessible and reactions. The predicted cross
sections are significantly different from what are commonly assumed in
relativistic heavy-ion calculations. The potential in nuclear matter
has been calculated by using a Bruckner-Hartree-Fock approximation. By
including the mean-field effects on the propagation, the effective
cross sections of the , and reactions in nuclear matter are
predicted. It is demonstrated that the density dependence is most dramatic in
the energy region close to the pion production threshold.Comment: 20 pages, RevTe
A Measurement of the Interference Structure Function, R_LT, for the 12C(e,e'p) reaction in the Quasielastic Region
The coincidence cross-section and the interference structure function, R_LT,
were measured for the 12C(e,e'p) 11B reaction at quasielastic kinematics and
central momentum transfer of q=400 MeV/c. The measurement was at an opening
angle of theta_pq=11 degrees, covering a range in missing energy of E_m = 0 to
65 MeV. The R_LT structure function is found to be consistent with zero for E_m
> 50 MeV, confirming an earlier study which indicated that R_L vanishes in this
region. The integrated strengths of the p- and s-shell are compared with a
Distorted Wave Impulse Approximation calculation. The s-shell strength and
shape are compared with a Hartree Fock-Random Phase Approximation calculation.
The DWIA calculation overestimates the cross sections for p- and s-shell proton
knockout as expected, but surprisingly agrees with the extracted R_LT value for
both shells. The HF-RPA calculation describes the data more consistently, which
may be due to the inclusion of 2-body currents in this calculation.Comment: 8 Pages LaTex, 5 postscript figures. Submitted to Phys. Rev.
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