2,362 research outputs found
An investigation of the basement complex aquifer system in Lofa county, Liberia, for the purpose of siting boreholes
Liberia is recovering from a 14 year civil war and only 51% of the rural population has access to safe drinking water. Little hydrogeological knowledge survives in Liberia, increasing the difficulty in successfully siting new boreholes. An understanding of the local hydrogeological environment is therefore needed to improve borehole site selection and increase success rates. This research provides a semi-quantitative characterization of the hydrogeological environment of the basement aquifer in Lofa county, Liberia. Based on literature review and analysis of borehole logs, the study has developed a conceptual hydrogeological model for the local conditions, which is further characterized using 2D geoelectrical sections. Groundwater is predominantly obtained from the saprolite and underlying fractured bedrock, but specific capacities (median 281 l h-1 m-1; 25th and 75th percentile of 179 and 490 l h-1 m-1, respectively) are constrained by the limited thickness of the saturated saprolite. This study has shown that the groundwater resources in the crystalline basement in this part of Liberia conform to the general conceptual model, allowing standard techniques used elsewhere for siting and developing groundwater to be used
Modern NN Force Predictions for the Total ND Cross Section up to 300 MeV
For several modern nucleon-nucleon potentials state-of-the-art Faddeev
calculations are carried out for the total cross section between 10 and
300 MeV projectile energy and compared to new high precision measurements. The
agreement between theory and data is rather good, with exception at the higher
energies where a 10% discrepancy builds up. In addition the convergence of the
multiple scattering series incorporated in the Faddeev scheme is studied
numerically with the result, that rescattering corrections remain important.
Based on this multiple scattering series the high energy limit of the total
cross section is also investigated analytically. In contrast to the naive
expectation that the total cross section is the sum of the and
total cross sections we find additional effects resulting from the rescattering
processes, which have different signs and different behavior as function of the
energy. A shadowing effect in the high energy limit only occurs for energies
higher than 300 MeV. The expressions in the high energy limit have
qualitatively a similar behavior as the exactly calculated expressions, but can
be expected to be valid quantitatively only at much higher energies.Comment: 22 pages, 5 figure
Ab initio Translationally Invariant Nonlocal One-body Densities from No-core Shell-model Theory
[Background:] It is well known that effective nuclear interactions are in
general nonlocal. Thus if nuclear densities obtained from {\it ab initio}
no-core-shell-model (NCSM) calculations are to be used in reaction
calculations, translationally invariant nonlocal densities must be available.
[Purpose:] Though it is standard to extract translationally invariant one-body
local densities from NCSM calculations to calculate local nuclear observables
like radii and transition amplitudes, the corresponding nonlocal one-body
densities have not been considered so far. A major reason for this is that the
procedure for removing the center-of-mass component from NCSM wavefunctions up
to now has only been developed for local densities. [Results:] A formulation
for removing center-of-mass contributions from nonlocal one-body densities
obtained from NCSM and symmetry-adapted NCSM (SA-NCSM) calculations is derived,
and applied to the ground state densities of He, Li, C, and
O. The nonlocality is studied as a function of angular momentum
components in momentum as well as coordinate space [Conclusions:] We find that
the nonlocality for the ground state densities of the nuclei under
consideration increases as a function of the angular momentum. The relative
magnitude of those contributions decreases with increasing angular momentum. In
general, the nonlocal structure of the one-body density matrices we studied is
given by the shell structure of the nucleus, and can not be described with
simple functional forms.Comment: 13 pages, 11 Figure
Subtractive renormalization of the NN interaction in chiral effective theory up to next-to-next-to-leading order: S waves
We extend our subtractive-renormalization method in order to evaluate the 1S0
and 3S1-3D1 NN scattering phase shifts up to next-to-next-to-leading order
(NNLO) in chiral effective theory. We show that, if energy-dependent contact
terms are employed in the NN potential, the 1S0 phase shift can be obtained by
carrying out two subtractions on the Lippmann-Schwinger equation. These
subtractions use knowledge of the the scattering length and the 1S0 phase shift
at a specific energy to eliminate the low-energy constants in the contact
interaction from the scattering equation. For the J=1 coupled channel, a
similar renormalization can be achieved by three subtractions that employ
knowledge of the 3S1 scattering length, the 3S1 phase shift at a specific
energy and the 3S1-3D1 generalized scattering length. In both channels a
similar method can be applied to a potential with momentum-dependent contact
terms, except that in that case one of the subtractions must be replaced by a
fit to one piece of experimental data.
This method allows the use of arbitrarily high cutoffs in the
Lippmann-Schwinger equation. We examine the NNLO S-wave phase shifts for
cutoffs as large as 5 GeV and show that the presence of linear energy
dependence in the NN potential creates spurious poles in the scattering
amplitude. In consequence the results are in conflict with empirical data over
appreciable portions of the considered cutoff range. We also identify problems
with the use of cutoffs greater than 1 GeV when momentum-dependent contact
interactions are employed. These problems are ameliorated, but not eliminated,
by the use of spectral-function regularization for the two-pion exchange part
of the NN potentialComment: 40 pages, 21 figure
Subtractive renormalization of the NN scattering amplitude at leading order in chiral effective theory
The leading-order nucleon-nucleon (NN) potential derived from chiral
perturbation theory consists of one-pion exchange plus a short-distance contact
interaction. We show that in the 1S0 and 3S1-3D1 channels renormalization of
the Lippmann-Schwinger equation for this potential can be achieved by
performing one subtraction. This subtraction requires as its only input
knowledge of the NN scattering lengths. This procedure leads to a set of
integral equations for the partial-wave NN t-matrix which give
cutoff-independent results for the corresponding NN phase shifts. This
reformulation of the NN scattering equation offers practical advantages,
because only observable quantities appear in the integral equation. The
scattering equation may then be analytically continued to negative energies,
where information on bound-state energies and wave functions can be extracted.Comment: 16 pages, 11 figure
Ab initio Folding Potentials for Nucleon-Nucleus Scattering based on NCSM One-Body Densities
Calculating microscopic optical potentials for elastic nucleon-nucleus
scattering has already led to large body of work in the past. For folding
first-order calculations the nucleon-nucleon (NN) interaction and the one-body
density of the nucleus were taken as input to rigorous calculations in a
spectator expansion of the multiple scattering series.
Based on the Watson expansion of the multiple scattering series we employ a
nonlocal translationally invariant nuclear density derived from a chiral
next-to-next-to-leading order (NNLO) and the very same interaction for
consistent full-folding calculation of the effective (optical) potential for
nucleon-nucleus scattering for light nuclei.
We calculate scattering observables, such as total, reaction, and
differential cross sections as well as the analyzing power and the
spin-rotation parameter, for elastic scattering of protons and neutrons from
He, He, C, and O, in the energy regime between 100 and
200~MeV projectile kinetic energy, and compare to available data.
Our calculations show that the effective nucleon-nucleus potential obtained
from the first-order term in the spectator expansion of the multiple scattering
expansion describes experiments very well to about 60 degrees in the
center-of-mass frame, which coincides roughly with the validity of the NNLO
chiral interaction used to calculate both the NN amplitudes and the one-body
nuclear density.Comment: 10 pages, 14 figures, 1 tabl
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