285 research outputs found
Lattice Distortions Around a Tl+ Impurity in NaI:Tl+ and CsI:Tl+ Scintillators. An Ab Initio Study Involving Large Active Clusters
Ab initio Perturbed Ion cluster-in-the-lattice calculations of the impurity
centers NaI:Tl+ and CsI:Tl+ are pressented. We study several active clusters of
increasing complexity and show that the lattice relaxation around the Tl+
impurity implies the concerted movement of several shells of neighbors. The
results also reveal the importance of considering a set of ions that can
respond to the geometrical displacements of the inner shells by adapting
selfconsistently their wave functions. Comparison with other calculations
involving comparatively small active clusters serves to assert the significance
of our conclusions. Contact with experiment is made by calculating absorption
energies. These are in excellent agreement with the experimental data for the
most realistic active clusters considered.Comment: 7 pages plus 6 postscript figures, LaTeX. Submmited to Phys, Rev.
Ab Initio Calculation of the Lattice Distortions induced by Substitutional Ag- and Cu- Impurities in Alkali Halide Crystals
An ab initio study of the doping of alkali halide crystals (AX: A = Li, Na,
K, Rb; X = F, Cl, Br, I) by ns2 anions (Ag- and Cu-) is presented. Large active
clusters with 179 ions embedded in the surrounding crystalline lattice are
considered in order to describe properly the lattice relaxation induced by the
introduction of substitutional impurities. In all the cases considered, the
lattice distortions imply the concerted movement of several shells of
neighbors. The shell displacements are smaller for the smaller anion Cu-, as
expected. The study of the family of rock-salt alkali halides (excepting CsF)
allows us to extract trends that might be useful at a predictive level in the
study of other impurity systems. Those trends are presented and discussed in
terms of simple geometric arguments.Comment: LaTeX file. 8 pages, 3 EPS pictures. New version contains
calculations of the energy of formation of the defects with model clusters of
different size
A longitudinal study on the occurrence of Cryptosporidium and Giardia in dogs during their first year of life
<p>Abstract</p> <p>Background</p> <p>The primary aim of this study was to obtain more knowledge about the occurrence of <it>Cryptosporidium </it>and <it>Giardia </it>in young dogs in Norway.</p> <p>The occurrence of these parasites was investigated in a longitudinal study by repeated faecal sampling of dogs between 1 and 12 months of age (litter samples and individual samples). The dogs were privately owned and from four large breeds. Individual faecal samples were collected from 290 dogs from 57 litters when the dogs were approximately 3, 4, 6, and 12 months old. In addition, pooled samples were collected from 43 of the litters, and from 42 of the mother bitches, when the puppies were approximately 1 and/or 2 months old.</p> <p>Methods</p> <p>The samples were purified by sucrose gradient flotation concentration and examined by immunofluorescent staining.</p> <p>Results</p> <p>128 (44.1%) of the young dogs had one or more <it>Cryptosporidium </it>positive samples, whilst 60 (20.7%) dogs had one or more <it>Giardia </it>positive samples. The prevalence of the parasites varied with age. For <it>Cryptosporidium</it>, the individual prevalence was between 5.1% and 22.5%, with the highest level in dogs < 6 months old, and declining with age. For <it>Giardia</it>, the individual prevalence was between 6.0% and 11.4%, with the highest level in dogs > 6 months old, but the differences between age groups were not statistically significant. Significant differences in prevalences were found in relation to geographic location of the dogs. Both parasites occurred at low prevalences in Northern Norway.</p> <p>Conclusion</p> <p>Both <it>Cryptosporidium </it>and <it>Giardia </it>are common in Norwegian dogs, with <it>Cryptosporidium </it>more prevalent than <it>Giardia</it>. Prevalences of the parasites were found to be influenced by age, geographical location, and infection status before weaning.</p
- …