2,168 research outputs found
Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals
By means of NMR experiment and MD computer simulation we investigate the
dynamical properties of a chloroadamantane orientationally disordered crystal.
We find a plastic-plastic dynamical transition at T_x ~ 330 K in the
pico-nanosecond regime. It is interpreted as the rotational analogue of the
Goldstein crossing temperature between quasi-free diffusion and activated
regime predicted in liquids. Below T_x, NMR experimental data are well
described by a Frenkel model corresponding to a strongly anisotropic motion. At
higher temperatures, a drastic deviation is observed toward quasi-isotropic
rotational diffusion. Close to T_x, we observe that two-step relaxations
emerge. An interpretation which is based on the present study of a specific
heat anomaly detected by a recent calorimetric experiment is proposed.Comment: 4 pages, 4 figures; changed abstract and references; corrected figure
First detection of Paenibacillus larvae the causative agent of American Foulbrood in a Ugandan honeybee colony
Paenibacillus larvae is a highly contagious and often lethal widely distributed pathogen of honeybees, Apis mellifera but has not been reported in eastern Africa to date. We investigated the presence of P. larvae in the eastern and western highland agro-ecological zones of Uganda by collecting brood and honey samples from 67 honeybee colonies in two sampling occasions and cultivated them for P. larvae. Also, 8 honeys imported and locally retailed in Uganda were sampled and cultivated for P. larvae. Our aim was to establish the presence and distribution of P. larvae in honeybee populations in the two highland agro-ecological zones of Uganda and to determine if honeys that were locally retailed contained this lethal pathogen. One honeybee colony without clinical symptoms for P. larvae in an apiary located in a protected area of the western highlands of Uganda was found positive for P. larvae. The strain of this P. larvae was genotyped and found to be ERIC I. In order to compare its virulence with P. larvae reference strains, in vitro infection experiments were conducted with carniolan honeybee larvae from the research laboratory at Ghent University, Belgium. The results show that the virulence of the P. larvae strain found in Uganda was at least equally high. The epidemiological implication of the presence of P. larvae in a protected area is discussed
Analysis and strengthening of carpentry joints
Joints play a major role in the structural behaviour of old timber frames [1]. Current standards mainly focus on modern dowel-type joints and usually provide little guidance (with the exception of German and Swiss NAs) to designers regarding traditional joints. With few exceptions, see e.g. [2], [3], [4], most of the research undertaken today is mainly focused on the reinforcement of dowel-type connections. When considering old carpentry joints, it is neither realistic nor useful to try to describe the behaviour of each and every type of joint. The discussion here is not an extra attempt to classify or compare joint configurations [5], [6], [7]. Despite the existence of some classification rules which define different types of carpentry joints, their applicability becomes difficult. This is due to the differences in the way joints are fashioned depending, on the geographical location and their age. In view of this, it is mandatory to check the relevance of the calculations as a first step. This first step, to, is mandatory. A limited number of carpentry joints, along with some calculation rules and possible strengthening techniques are presented here
Cherenkov and Scintillation Light Separation in Organic Liquid Scintillators
The CHErenkov / Scintillation Separation experiment (CHESS) has been used to
demonstrate the separation of Cherenkov and scintillation light in both linear
alkylbenzene (LAB) and LAB with 2g/L of PPO as a fluor (LAB/PPO). This is the
first such demonstration for the more challenging LAB/PPO cocktail and improves
on previous results for LAB. A time resolution of 338 +/- 12 ps FWHM results in
an efficiency for identifying Cherenkov photons in LAB/PPO of 70 +/- 3% and 63
+/- 8% for time- and charge-based separation, respectively, with scintillation
contamination of 36 +/- 5% and 38 +/- 4%. LAB/PPO data is consistent with a
rise time of 0.75 +/- 0.25 ns
Analogy of the slow dynamics between the supercooled liquid and the supercooled plastic crystal states of difluorotetrachloroethane
Slow dynamics of difluorotetrachloroethane in both supercooled plastic
crystal and supercooled liquid states have been investigated from Molecular
Dynamics simulations. The temperature and wave-vector dependence of collective
dynamics in both states are probed using coherent dynamical scattering
functions . Our results confirm the strong analogy between molecular
liquids and plastic crystals for which -relaxation times and
non-ergodicity parameters are controlled by the non trivial static correlations
as predicted by the Mode Coupling Theory. The use of infinitely thin
needles distributed on a lattice as model of plastic crystals is discussed
Translation-Rotation Coupling in Transient Grating Experiments : Theoretical and Experimental Evidences
The results of a Transient Grating experiment in a supercooled molecular
liquid of anisotropic molecules and its theoretical interpretation are
presented. These results show the existence of two distinct dynamical
contributions in the response function of this experiment, density and
orientation dynamics. These dynamics can be experimentally disentangled by
varying the polarisation of the probe and diffracted beams and they have been
identified and measured in a Heterodyne Detected experiment performed on
m-toluidine. The results of the theory show a good qualitative agreement with
the measurements at all temperatures.Comment: PDF format, 14 pages including 4 figures, accepted for publication in
EPL. minor modification
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