A transistorized radiation monitor

This paper describes the investigation of a number of possible semiconductor devices which can be used for the detection of radioactive particles. The sensitivities of two types of transistor (a p-n-p alloy junction and an n-p-n grown junction) and a p-n alloy junction diode operated up to its "avalanche" condition were determined. The possible uses of such detectors are discussed and they are shown to be • particularly suitable where high rates of counting, with good efficiency and small detecting area (or low geometry) are required. A completely transistorized radiation monitor has been developed using a p-n-p transistor as the detector head. This is specifically designed for a-particle detection but the detector head with slight modification should be suitable for proton or neutron detection

Critical phenomena: 150 years since Cagniard de la Tour

Critical phenomena were discovered by Cagniard de la Tour in 1822, who died 150 years ago. In order to mark this anniversary, the context and the early history of his discovery is reviewed. We then follow with a brief sketch of the history of critical phenomena, indicating the main lines of development until the present date. Os fen\'omenos cr\'{\i}ticos foram descobertos pelo Cagniard de la Tour em Paris em 1822. Para comemorar os 150 anos da sua morte, o contexto e a hist\'oria initial da sua descoberta \'e contada. Conseguimos com uma descri\c{c}\~ao breve da hist\'oria dos fen\'emenos cr\'{\i}ticos, indicando as linhas principais do desenvolvimento at\'e o presente.Comment: Latex2e, 8 pp, 3 eps figures include

Theory based design and evaluation of multimedia presentation interfaces

Multimedia (MM) Applications currently suffer from an ad hoc development process. This places the usability and effectiveness of many MM products in doubt. This thesis develops a theoretically motivated design method and tools to address these problems. The thesis is based on an analysis of the cognitive processes of attending to and comprehending an MM presentation. A design method is then developed based on these cognitive processes. The method addressesth e problem of selecting media to presenting information requirements,h ow to design the media to effectively deliver the desired content, how to combine verbal and visual media successfully, and how to direct the user's attention to particular part of the presentation. A number of studies are then presented which provide validation for the method's claims. These include eye tracking to analyse the user's reading / viewing sequence, and tests of expert and novice recall of MM and conventional text / speech presentations. A set of re-authoring studies show that application of guidelines improves retention of the content. The method is supported by a design advisor authoring tool. The tool applies the guidelines using a combination of a critiquer and expert system. The tool demonstrates that the guidelines are tractable for implementation, and provides a novel approach to providing authoring advice. Both the method and the tool are also validated in case studies with novice users. These demonstrate that the method and tool are both usable and effective

Introduction to Magnetic Monopoles

One of the most basic properties of magnetism is that a magnet always has two poles, north and south, which cannot be separated into isolated poles, i.e., magnetic monopoles. However, there are strong theoretical arguments why magnetic monopoles should exist. In spite of extensive searches they have not been found, but they have nevertheless played a central role in our understanding of physics at the most fundamental level.Comment: 22 pages, 7 figures. To be published in Contemporary Physic

A phenomenological model of weakly damped Faraday waves and the associated mean flow

A phenomenological model of parametric surface waves (Faraday waves) is introduced in the limit of small viscous dissipation that accounts for the coupling between surface motion and slowly varying streaming and large scale flows (mean flow). The primary bifurcation of the model is to a set of standing waves (stripes, given the functional form of the model nonlinearities chosen here). Our results for the secondary instabilities of the primary wave show that the mean flow leads to a weak destabilization of the base state against Eckhaus and Transverse Amplitude Modulation instabilities, and introduces a new longitudinal oscillatory instability which is absent without the coupling. We compare our results with recent one dimensional amplitude equations for this system systematically derived from the governing hydrodynamic equations.Comment: Complete paper with embedded figures (PostScript, 3 Mb) http://www.csit.fsu.edu/~vinals/mss/jmv1.p

Heap Formation in Granular Media

Using molecular dynamics (MD) simulations, we find the formation of heaps in a system of granular particles contained in a box with oscillating bottom and fixed sidewalls. The simulation includes the effect of static friction, which is found to be crucial in maintaining a stable heap. We also find another mechanism for heap formation in systems under constant vertical shear. In both systems, heaps are formed due to a net downward shear by the sidewalls. We discuss the origin of net downward shear for the vibration induced heap.Comment: 11 pages, 4 figures available upon request, Plain TeX, HLRZ-101/9

Fingering Instability in Combustion

A thin solid (e.g., paper), burning against an oxidizing wind, develops a fingering instability with two decoupled length scales. The spacing between fingers is determined by the P\'eclet number (ratio between advection and diffusion). The finger width is determined by the degree two dimensionality. Dense fingers develop by recurrent tip splitting. The effect is observed when vertical mass transport (due to gravity) is suppressed. The experimental results quantitatively verify a model based on diffusion limited transport

Faraday Instability in a Surface-Frozen Liquid

Faraday surface instability measurements of the critical acceleration, a_c, and wavenumber, k_c, for standing surface waves on a tetracosanol (C_24H_50) melt exhibit abrupt changes at T_s=54degC above the bulk freezing temperature. The measured variations of a_c and k_c vs. temperature and driving frequency are accounted for quantitatively by a hydrodynamic model, revealing a change from a free-slip surface flow, generic for a free liquid surface (T>T_s), to a surface-pinned, no-slip flow, characteristic of a flow near a wetted solid wall (T < T_s). The change at T_s is traced to the onset of surface freezing, where the steep velocity gradient in the surface-pinned flow significantly increases the viscous dissipation near the surface.Comment: 4 pages, 3 figures. Physical Review Letters (in press

Kink-induced transport and segregation in oscillated granular layers

We use experiments and molecular dynamics simulations of vertically oscillated granular layers to study horizontal particle segregation induced by a kink (a boundary between domains oscillating out of phase). Counter-rotating convection rolls carry the larger particles in a bidisperse layer along the granular surface to a kink, where they become trapped. The convection originates from avalanches that occur inside the layer, along the interface between solidified and fluidized grains. The position of a kink can be controlled by modulation of the container frequency, making possible systematic harvesting of the larger particles.Comment: 4 pages, 5 figures. to appear in Phys. Rev. Let

Periodically kicked turbulence

Periodically kicked turbulence is theoretically analyzed within a mean field theory. For large enough kicking strength A and kicking frequency f the Reynolds number grows exponentially and then runs into some saturation. The saturation level can be calculated analytically; different regimes can be observed. For large enough Re we find the saturation level to be proportional to A*f, but intermittency can modify this scaling law. We suggest an experimental realization of periodically kicked turbulence to study the different regimes we theoretically predict and thus to better understand the effect of forcing on fully developed turbulence.Comment: 4 pages, 3 figures. Phys. Rev. E., in pres