4,338 research outputs found
The influence of television on vocabulary, concepts and preferences of children in grades one and two.
Thesis (M.A.)--Boston Universit
Quality assurance in stereotactic radiosurgery/radiotherapy according to DIN 6875-1
The new DIN (' Deutsche Industrie- Norm') 6875- 1, which is currently being finalised, deals with quality assurance ( QA) criteria and tests methods for linear accelerator and Gamma Knife stereotactic radiosurgery/ radiotherapy including treatment planning, stereotactic frame and stereotactic imaging and a system test to check the whole chain of uncertainties. Our existing QA program, based on dedicated phantoms and test procedures, has been refined to fulfill the demands of this new DIN. The radiological and mechanical isocentre corresponded within 0.2 mm and the measured 50% isodose lines were in agreement with the calculated ones within less than 0.5 mm. The measured absorbed dose was within 3%. The resultant output factors measured for the 14-, 8- and 4- mm collimator helmet were 0.9870 +/- 0.0086, 0.9578 +/- 0.0057 and 0.8741 +/- 0.0202, respectively. For 170 consecutive tests, the mean geometrical accuracy was 0.48 +/- 0.23 mm. Besides QA phantoms and analysis software developed in- house, the use of commercially available tools facilitated the QA according to the DIN 6875- 1 with which our results complied. Copyright (C) 2004 S. Karger AG, Basel
Scaling in the Positive Plaquette Model and Universality in SU(2) Lattice Gauge Theory
We investigate universality, scaling, the beta-function and the topological
charge in the positive plaquette model for SU(2) lattice gauge theory.
Comparing physical quantities, like the critical temperature, the string
tension, glueball masses, and their ratios, we explore the effect of a complete
suppression of a certain lattice artifact, namely the negative plaquettes, for
SU(2) lattice gauge theory. Our result is that this modification does not
change the continuum limit, i.e., the universality class. The positive
plaquette model and the standard Wilson formulation describe the same physical
situation. The approach to the continuum limit given by the beta-function in
terms of the bare lattice coupling, however, is rather different: the
beta-function of the positive plaquette model does not show a dip like the
model with standard Wilson action.Comment: 35 pages, preprint numbers FSU-SCRI-94-71 and HU Berlin-IEP-94/1
Four Dimensional CFT Models with Rational Correlation Functions
Recently established rationality of correlation functions in a globally
conformal invariant quantum field theory satisfying Wightman axioms is used to
construct a family of soluble models in 4-dimensional Minkowski space-time. We
consider in detail a model of a neutral scalar field of dimension 2. It
depends on a positive real parameter c, an analogue of the Virasoro central
charge, and admits for all (finite) c an infinite number of conserved symmetric
tensor currents. The operator product algebra of is shown to coincide
with a simpler one, generated by a bilocal scalar field of
dimension (1,1). The modes of V together with the unit operator span an
infinite dimensional Lie algebra whose vacuum (i.e. zero energy lowest
weight) representations only depend on the central charge c. Wightman
positivity (i.e. unitarity of the representations of ) is proven to be
equivalent to .Comment: 28 pages, LATEX, amsfonts, latexsym. Proposition 2.3, and Conjecture
in Sec. 6 are revised. Minor errors are correcte
Assessing Rural Coalitions That Address Safety and Health Issues
Community coalitions can help national organizations meet their objectives. Farm Safety 4 Just Kids depends on coalitions of local people to deliver farm safety and health educational programs to children and their families. These coalitions are called chapters. An evaluation was developed to identify individual coalition\u27s strengths and weaknesses. Ten FS4JK chapter locations conducted a SWOT analysis (strengths, weaknesses, opportunities, and threats) and community focus groups to identify what strategies could be incorporated to improve each coalition\u27s functionality. The findings will help strengthen program delivery, which will guide the national organization toward a more effective support system
Real-Time Maps of Fluid Flow Fields in Porous Biomaterials
Mechanical forces such as fluid shear have been shown to enhance cell growth
and differentiation, but knowledge of their mechanistic effect on cells is
limited because the local flow patterns and associated metrics are not
precisely known. Here we present real-time, noninvasive measures of local
hydrodynamics in 3D biomaterials based on nuclear magnetic resonance. Microflow
maps were further used to derive pressure, shear and fluid permeability fields.
Finally, remodeling of collagen gels in response to precise fluid flow
parameters was correlated with structural changes. It is anticipated that
accurate flow maps within 3D matrices will be a critical step towards
understanding cell behavior in response to controlled flow dynamics.Comment: 23 pages, 4 figure
Construction of Field Algebras with Quantum Symmetry from Local Observables
It has been discussed earlier that ( weak quasi-) quantum groups allow for
conventional interpretation as internal symmetries in local quantum theory.
From general arguments and explicit examples their consistency with (braid-)
statistics and locality was established. This work addresses to the
reconstruction of quantum symmetries and algebras of field operators. For every
algebra \A of observables satisfying certain standard assumptions, an
appropriate quantum symmetry is found. Field operators are obtained which act
on a positive definite Hilbert space of states and transform covariantly under
the quantum symmetry. As a substitute for Bose/Fermi (anti-) commutation
relations, these fields are demonstrated to obey local braid relation.Comment: 50 pages, HUTMP 93-B33
- …