Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

Comparative Study of Microwave Sintering of Zinc Oxide at 2.45, 30, and 83 GHz

Abstract: Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform “volumetric” heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two-dimensional (2-D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.

Zero sound, first sound and second sound of solids

Abstract: The response of an anharmonic crystal to low-frequency and long wavelength probes is discussed. The difference between the velocities of sound in the zero sound and first sound regions is emphasized. In the zero sound region, it is shown that measured elastic constants may, unlike the macroscopic elastic constants, not have the symmetry of a second-rank tensor in addition to the point group symmetry of the crystal. The size of the deviations from the symmetry of a second-rank tensor is calculated for alkali halides and shown to be about a per cent at 300 °K. In the first sound region the elastic constants and ultrasonic attenuation are calculated. The difference between these elastic constants and those of the zero sound region is shown to be about 10% for some elastic constants of alkali halides at 300 °K. The extent to which the propagation of first sound is adiabatic is also discussed. The velocity of the second sound mode is calculated as a function of temperature, but calculations suggest it is very heavily damped in alkali halides. Finally, the thermal conductivity of KBr is computed and compared with experiment. The extension of these results to the dielectric response function is also described.

Response of superheated droplet detectors of the PICASSO dark matter search experiment

Abstract: We present results of systematic studies of the radiation response of superheated liquid droplet detectors, which are used in the PICASSO dark matter search experiment. This detection technique is based on the phase transitions of superheated liquid Freon droplets dispersed and trapped in a polymerized gel. Phase transitions can be induced by nuclear recoils following particle interactions and, in particular, interactions with Weakly Interacting Massive Particles (WIMPs). These detectors are threshold devices since a minimal energy deposition is necessary to induce a phase transition and their sensitivity to various types of radiation depends strongly on the operating temperature and pressure. The sensitivity to neutrons, α-particles and γ-rays was determined as a function of these operating parameters and the results are compared with simulations. In particular, we present a complete characterization of the response of detector modules already in use for a dark matter search at the SNO site to detect WIMPs and discuss possible background sources.

Spontaneous mutation to streptomycin resistance and dependence in escherichia coli.

Abstract: Bacteria give promise of yielding quantitative information on the process of gene mutation, which could not be obtained from the clasical genetic materials (Luria, 1947). However, detailed studies have so far been confined almost entirely to a particular group of mutations, namely, those producing phage resistance (Demerec, 1946; Witkin, 1947b; Bryson, 1948; Beale, 1948; Newcombe, 1948; Newcombe and Scott, 1949). For this reason, the present investigation has been directed toward obtaining accurate data for a different kind of mutational change, na.nely, the development of drug resistance. The genetic origin of resistance to three of the most common ahtibacterial drugs has already been studied. These are penicillin (Demerec, 1945, using Staphylococcus aureus), sodium sulfathiazole (Oakberg and Luria, 1947, using S. aureus), and streptomycin (Klein andKimmelman, 1946; Alexander and Leidy, 1947; Demerec, 1947; Scott, 1949; using, respectively, Shigella spp., Hemophilus influenzae, S. aureus, and Escherichia coli). Of these, streptomycin was chosen since it appeared to be the only one to which a high degree of resistance could be developed in a single mutational step.

Carbohydrate requirements for the development of black spruce (Picea mariana (Mill.) B.S.P.) and red spruce (P. rubens Sarg.) somatic embryos

Abstract: Different carbohydrates were investigated for somatic embryo development of black spruce and red spruce. They were tested in a basal maturation medium consisting of Litvay's salts at half-strength containing 1 g l-1 glutamine, 1 g l-1 casein hydrolysate, 7.5 μM abscisic acid, and 0.9% Difco Bacto-agar. A comparison of different sucrose concentrations showed that 6% was optimal for embryo development. Among the nine carbohydrates tested, sucrose, fructose, glucose, maltose, and cellobiose supported embryo development while arabinose, mannitol, myo-inositol, and sorbitol did not. A comparison of sucrose, glucose, and fructose at three concentrations showed that the general pattern of response for both species followed concentration expressed as a percentage, independent of the molarity of carbohydrate in the medium. Interspecific differences were observed concerning carbohydrate requirements. For red spruce, 6% fructose was found best for embryo development, while no such preference was observed for black spruce. No significant difference was observed in the number of embryos produced with 6% sucrose or 3% sucrose plus an equimolar concentration of either mannitol, sorbitol, or myo-inositol in the maturation medium, suggesting that the effect of the carbohydrate on the maturation was partly osmotic.