699 research outputs found
Nucleon-deuteron elastic scattering as a tool to probe properties of three-nucleon forces
Faddeev equations for elastic Nd scattering have been solved using modern NN
forces combined with the Tucson-Melbourne two-pion exchange three-nucleon
force, with a modification thereof closer to chiral symmetry and the Urbana IX
three-nucleon force. Theoretical predictions for the differential cross section
and several spin observables using NN forces only and NN forces combined with
three-nucleon force models are compared to each other and to the existing data.
A wide range of energies from 3 to 200 MeV is covered. Especially at the higher
energies striking three-nucleon force effects are found, some of which are
supported by the still rare set of data, some are in conflict with data and
thus very likely point to defects in those three-nucleon force models.Comment: 30 pages, 14 Postscript figures; now minor changes in figures and
reference
Three-Nucleon Force Effects in Nucleon Induced Deuteron Breakup: Predictions of Current Models (I)
An extensive study of three-nucleon force effects in the entire phase space
of the nucleon-deuteron breakup process, for energies from above the deuteron
breakup threshold up to 200 MeV, has been performed. 3N Faddeev equations have
been solved rigorously using the modern high precision nucleon-nucleon
potentials AV18, CD Bonn, Nijm I, II and Nijm 93, and also adding 3N forces. We
compare predictions for cross sections and various polarization observables
when NN forces are used alone or when the two pion-exchange Tucson-Melbourne
3NF was combined with each of them. In addition AV18 was combined with the
Urbana IX 3NF and CD Bonn with the TM' 3NF, which is a modified version of the
TM 3NF, more consistent with chiral symmetry. Large but generally model
dependent 3NF effects have been found in certain breakup configurations,
especially at the higher energies, both for cross sections and spin
observables. These results demonstrate the usefulness of the kinematically
complete breakup reaction in testing the proper structure of 3N forces.Comment: 42 pages, 20 ps figures, 2 gif figure
Visualizing brain inflammation with a shingled-leg radio-frequency head probe for (19)F/(1)H MRI
Magnetic resonance imaging (MRI) provides the opportunity of tracking cells in vivo. Major challenges in dissecting cells from the recipient tissue and signal sensitivity constraints albeit exist. In this study, we aimed to tackle these limitations in order to study inflammation in autoimmune encephalomyelitis. We constructed a very small dual-tunable radio frequency (RF) birdcage probe tailored for (19)F (fluorine) and (1)H (proton) MR mouse neuroimaging. The novel design eliminated the need for extra electrical components on the probe structure and afforded a uniform -field as well as good SNR. We employed fluorescently-tagged (19)F nanoparticles and could study the dynamics of inflammatory cells between CNS and lymphatic system during development of encephalomyelitis, even within regions of the brain that are otherwise not easily visualized by conventional probes. (19)F/(1)H MR Neuroimaging will allow us to study the nature of immune cell infiltration during brain inflammation over an extensive period of time
Generalized stacking fault energy surfaces and dislocation properties of aluminum
We have employed the semidiscrete variational generalized Peierls-Nabarro
model to study the dislocation core properties of aluminum. The generalized
stacking fault energy surfaces entering the model are calculated by using
first-principles Density Functional Theory (DFT) with pseudopotentials and the
embedded atom method (EAM). Various core properties, including the core width,
splitting behavior, energetics and Peierls stress for different dislocations
have been investigated. The correlation between the core energetics and
dislocation character has been explored. Our results reveal a simple
relationship between the Peierls stress and the ratio between the core width
and atomic spacing. The dependence of the core properties on the two methods
for calculating the total energy (DFT vs. EAM) has been examined. The EAM can
give gross trends for various dislocation properties but fails to predict the
finer core structures, which in turn can affect the Peierls stress
significantly (about one order of magnitude).Comment: 25 pages, 12 figure
A new form of three-body Faddeev equations in the continuum
We propose a novel approach to solve the three-nucleon (3N) Faddeev equation
which avoids the complicated singularity pattern going with the moving
logarithmic singularities of the standard approach. In this new approach the
treatment of the 3N Faddeev equation becomes essentially as simple as the
treatment of the two-body Lippmann-Schwinger equation. Very good agreement of
the new and old approaches in the application to nucleon-deuteron elastic
scattering and the breakup reaction is found.Comment: 20 pages, 3 eps figure
Signatures of three-nucleon interactions in few-nucleon systems
Recent experimental results in three-body systems have unambiguously shown
that calculations based only on nucleon-nucleon forces fail to accurately
describe many experimental observables and one needs to include effects which
are beyond the realm of the two-body potentials. This conclusion owes its
significance to the fact that experiments and calculations can both be
performed with a high accuracy. In this review, both theoretical and
experimental achievements of the past decade will be underlined. Selected
results will be presented. The discussion on the effects of the three-nucleon
forces is, however, limited to the hadronic sector. It will be shown that
despite the major successes in describing these seemingly simple systems, there
are still clear discrepancies between data and the state-of-the-art
calculations.Comment: accepted for publication in Rep. Prog. Phy
Dynamically coupling full Stokes and shallow shelf approximation for marine ice sheet flow using Elmer/Ice (v8.3)
Ice flow forced by gravity is governed by the full Stokes (FS) equations,
which are computationally expensive to solve due to the nonlinearity
introduced by the rheology. Therefore, approximations to the FS equations are
commonly used, especially when modeling a marine ice sheet (ice sheet, ice
shelf, and/or ice stream) for 103 years or longer. The shallow ice
approximation (SIA) and shallow shelf approximation (SSA) are commonly used
but are accurate only for certain parts of an ice sheet. Here, we report a
novel way of iteratively coupling FS and SSA that has been implemented in
Elmer/Ice and applied to conceptual marine ice sheets. The FS–SSA coupling
appears to be very accurate; the relative error in velocity compared to FS is
below 0.5 % for diagnostic runs and below 5 % for prognostic runs.
Results for grounding line dynamics obtained with the FS–SSA coupling are
similar to those obtained from an FS model in an experiment with a periodical
temperature forcing over 3000 years that induces grounding line advance and
retreat. The rapid convergence of the FS–SSA coupling shows a large
potential for reducing computation time, such that modeling a marine ice
sheet for thousands of years should become feasible in the near future.
Despite inefficient matrix assembly in the current implementation,
computation time is reduced by 32 %, when the coupling is applied to a
3-D ice shelf.</p
A Note on the Carlson-Parkin Method of Quantifying Qualitative Data
Qualitative surveys enjoy huge popularity among business cycle analysts and research institutes since they provide fast information on the stance of the economy. However, in order to derive quantitative statements researchers have to rely on assumptions about the relation between quantitative and qualitative information. This paper introduces a micro data set that combines individual quantitative and qualitative information and presents first tests of common assumptions. It suggests a modifcation of the Carlson and Parkin (1975) method and a solution to the zero response problem
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