217 research outputs found
Phaseshift equivalent NN potentials and the deuteron
Different modern phase shift equivalent NN potentials are tested by
evaluating the partial wave decomposition of the kinetic and potential energy
of the deuteron. Significant differences are found, which are traced back to
the matrix elements of the potentials at medium and large momenta. The
influence of the localisation of the one-pion-exchange contribution to these
potentials is analyzed in detail.Comment: 11 pages, LaTeX, 4 figures include
Total Cross Sections for Neutron Scattering
Measurements of neutron total cross-sections are both extensive and extremely
accurate. Although they place a strong constraint on theoretically constructed
models, there are relatively few comparisons of predictions with experiment.
The total cross-sections for neutron scattering from O and Ca are
calculated as a function of energy from ~MeV laboratory energy with a
microscopic first order optical potential derived within the framework of the
Watson expansion. Although these results are already in qualitative agreement
with the data, the inclusion of medium corrections to the propagator is
essential to correctly predict the energy dependence given by the experiment.Comment: 10 pages (Revtex 3.0), 6 fig
Nuclear structure and elastic scattering observables obtained consistently with different NN interactions
Nucleon-nucleon () interactions based on chiral effective theories are
commonly used in ab initio calculations of light nuclei. Here we present a
study based on three different NN interactions (up to next-to-next-to-leading
order) for which structure and elastic proton scattering observables are
consistently calculated for He, C, and O. The interactions
are compared at the two-body level in terms of Wolfenstein amplitudes, and
their predictions for ground state energies, point-proton radii, and charge
form factors, as well as proton elastic scattering observables in the
leading-order spectator expansion in the energy range between 65 and 160 MeV
projectile energy are presented. To gain further insight into differences
visible in elastic scattering observables, we investigate the behavior of the
calculated effective nucleon-nucleus interactions for the C nucleus
based on the different interactions.Comment: 17 pages, 17 figure
Ab initio nucleon-nucleus elastic scattering with chiral effective field theory uncertainties
Background: Effective interactions for nucleon-nucleus () elastic
scattering from first principles require the use of the same nucleon-nucleon
() interaction in the structure and reaction calculations, and a consistent
treatment of the relevant operators at each order.
Purpose: Truncation uncertainties of chiral forces have been studied for
scattering observables in few-body systems and for bound state properties of
light nuclei. We extend this to elastic scattering.
Methods: With the spectator expansion of multiple scattering theory and the
no-core shell model, we use a chiral interaction from the LENPIC collaboration
to consistently calculate the leading order effective interaction up to
third chiral order (N2LO) and extract elastic scattering observables. We
quantify the chiral truncation error using pointwise and correlated methods.
Results: We analyze proton-O and neutron-C elastic scattering
observables between 65 and 185 MeV projectile kinetic energy. We find
qualitatively similar results for the chiral truncation uncertainties as in
few-body systems, which we assess using similar diagnostic tools. The
order-by-order convergence of the scattering observables for O and
C is reasonable near 100 MeV, but for higher energies the expansion
parameter becomes too large to converge. We find a near-perfect correlation
between the neutron differential cross section and the Wolfenstein
amplitudes for small momentum transfers.
Conclusions: The tools used to study the convergence of a chiral
interaction in few-body systems can be applied to scattering with minor
changes. The interaction used here gives a good description of O
and C scattering observables as low as 65 MeV. The very forward
direction of the neutron differential cross section mirrors the behavior of the
interaction amazingly well.Comment: 17 pages, 13 figures, 1 tabl
Ab initio Leading Order Effective Potentials for Elastic Nucleon-Nucleus Scattering
Background: Calculating microscopic effective interactions (optical
potentials) for elastic nucleon-nucleus scattering has already in the past led
to a large body of work. For first-order calculations a nucleon-nucleon
(\textit{NN}) interaction and a one-body density of the nucleus were taken as
input to rigorous calculations of microscopic full-folding calculations.
Purpose: Based on the spectator expansion of the multiple scattering series
we employ a chiral next-to-next-to-leading order (NNLO) nucleon-nucleon
interaction on the same footing in the structure as well as in the reaction
calculation to obtain an in leading-order consistent effective potential for
nucleon-nucleus elastic scattering, which includes the spin of the struck
target nucleon.
Methods: The first order effective folding potential is computed by first
deriving a nonlocal scalar density as well as a spin-projected momentum
distribution. Those are then integrated with the off-shell Wolfenstein
amplitudes , , and . The resulting nonlocal potential serves as input
to a momentum-space Lippmann-Schwinger equation, whose solutions are summed to
obtain the nucleon-nucleus scattering observables.
Results: We calculate elastic scattering observables for He, He,
He, C, and O in the energy regime between 100 and 200 MeV
projectile kinetic energy, and compare to available data. We also explore the
extension down to about 70 MeV, and study the effect of ignoring the spin of
the struck nucleon in the nucleus.
Conclusions: In our calculations we contrast elastic scattering off
closed-shell and open-shell nuclei. We find that for closed-shell nuclei the
approximation of ignoring the spin of the struck target nucleon is excellent.
We only see effects of the spin of the struck target nucleon when considering
He and He, which are nuclei with a ratio larger than 1.Comment: 13 pages, 13 figure
Nuclear spin features relevant to ab initio nucleon-nucleus elastic scattering
Background: Effective interactions for elastic nucleon-nucleus scattering
from first principles require the use of the same nucleon-nucleon interaction
in the structure and reaction calculations, as well as a consistent treatment
of the relevant operators at each order.
Purpose: Previous work using these interactions has shown good agreement with
available data. Here, we study the physical relevance of one of these
operators, which involves the spin of the struck nucleon, and examine the
interpretation of this quantity in a nuclear structure context.
Methods: Using the framework of the spectator expansion and the underlying
framework of the no-core shell model, we calculate and examine spin-projected,
one-body momentum distributions required for effective nucleon-nucleus
interactions in nuclear states.
Results: The calculated spin-projected, one-body momentum distributions for
He, He, and He display characteristic behavior based on the
occupation of protons and neutrons in single particle levels, with more
nucleons of one type yielding momentum distributions with larger values.
Additionally, we find this quantity is strongly correlated to the magnetic
moment of the excited state in the ground state rotational band for each
nucleus considered.
Conclusions: We find that spin-projected, one-body momentum distributions can
probe the spin content of a wave function. This feature may allow future
\textit{ab initio} nucleon-nucleus scattering studies to inform spin properties
of the underlying nucleon-nucleon interactions. The observed correlation to the
magnetic moment of excited states illustrates a previously unknown connection
between reaction observables such as the analyzing power and structure
observables like the magnetic moment.Comment: 13 pages, 7 figures, 1 tabl
Two-Nucleon Scattering without partial waves using a momentum space Argonne V18 interaction
We test the operator form of the Fourier transform of the Argonne V18
potential by computing selected scattering observables and all Wolfenstein
parameters for a variety of energies. These are compared to the GW-DAC database
and to partial wave calculations. We represent the interaction and transition
operators as expansions in a spin-momentum basis. In this representation the
Lippmann-Schwinger equation becomes a six channel integral equation in two
variables. Our calculations use different numbers of spin-momentum basis
elements to represent the on- and off-shell transition operators. This is
because different numbers of independent spin-momentum basis elements are
required to expand the on- and off-shell transition operators. The choice of on
and off-shell spin-momentum basis elements is made so that the coefficients of
the on-shell spin-momentum basis vectors are simply related to the
corresponding off-shell coefficients.Comment: 14 pages, 8 Figures, typos correcte
Energy Dependence of the NN t-matrix in the Optical Potential for Elastic Nucleon-Nucleus Scattering
The influence of the energy dependence of the free NN t-matrix on the optical
potential of nucleon-nucleus elastic scattering is investigated within the
context of a full-folding model based on the impulse approximation. The
treatment of the pole structure of the NN t-matrix, which has to be taken into
account when integrating to negative energies is described in detail. We
calculate proton-nucleus elastic scattering observables for O,
Ca, and Pb between 65 and 200 MeV laboratory energy and study
the effect of the energy dependence of the NN t-matrix. We compare this result
with experiment and with calculations where the center-of-mass energy of the NN
t-matrix is fixed at half the projectile energy. It is found that around 200
MeV the fixed energy approximation is a very good representation of the full
calculation, however deviations occur when going to lower energies (65 MeV).Comment: 11 pages (revtex), 6 postscript figure
Full-Folding Optical Potentials for Elastic Nucleon-Nucleus Scattering based on Realistic Densities
Optical model potentials for elastic nucleon nucleus scattering are
calculated for a number of target nuclides from a full-folding integral of two
different realistic target density matrices together with full off-shell
nucleon-nucleon t-matrices derived from two different Bonn meson exchange
models. Elastic proton and neutron scattering observables calculated from these
full-folding optical potentials are compared to those obtained from `optimum
factorized' approximations in the energy regime between 65 and 400 MeV
projectile energy. The optimum factorized form is found to provide a good
approximation to elastic scattering observables obtained from the full-folding
optical potentials, although the potentials differ somewhat in the structure of
their nonlocality.Comment: 21 pages, LaTeX, 17 postscript figure
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