Alan J Walton

Triboluminescence of glasses and quartz

The triboluminescent spectra of a variety of glasses and of crystalline quartz were measured while specimens were cut with a rotating diamond‐impregnated saw blade. The spectra, which resemble the emission of a blackbody radiator, were recorded using an image‐intensifier spectrograph. The data were intensity‐corrected before being fitted to blackbody emission curves. Emission temperatures of around 1850 K for armor plate glass, 2100 K for Pyrex glass, 2400 K for soda lime glass, 2300 K for high‐density lead glass, and 2800 K for cut quartz were obtained. It was found that the blackbody temperatures could be accounted for by a model in which a rectangular‐shaped high‐temperature zone, uniformly heated by energy released by plastic deformation near the crack tip, is supposed to propagate with the crack velocity [R. Weichert and K. Schonert, Q. J. Mech. Appl. Math. 31, 363‐379 (1978)]. The measured blackbody temperatures imply zone widths of around 1×10−9 m. The spectrum from impact‐fractured quartz was also...

Mechanical stimulation of bioluminescence in the dinoflagellate Gonyaulax polyedra Stein

Abstract Cultures of the marine dinoflagellate Gonyaulax polyedra Stein were exposed to a variety of flow regimes in small tubes, pressure chambers, and vessels in which objects could be rotated. Bioluminescence was mechanically stimulated by changes in shear, acceleration, and pressure, not by constant values of these parameters. In a biological context, such stimuli would be associated with waves and other surface turbulence, with moving objects such as ships or some large marine organisms, or with close or direct contact as would occur if the dinoflagellate is a prey item. The effects of pressure are complicated by the observation that the luminescence response did not occur in the bulk of the fluid in a pressure chamber, but was confined to the liquid boundaries. The importance of luminescence at surfaces was also seen when objects were rotated in suspensions of G. polyedra ; light emissions were restricted to regions with sharp shear gradients. These data were obtained using an image intensifier which made it possible to visualize the spatial pattern of luminescence in the various flow regimes studied. Past results obtained with photo-multipliers are shown to be misleading.

Depth of field measurements relevant to single photon detection using image-intensified microscopy

The problems of defining a depth of field d p when individual photons emitted in a low-level luminescent process are recorded via an image-intensified microscope are discussed. Simulation studies of a self-luminous cylindrical volume source whose axis lies along the optical axis of the microscope were carried out by moving a uniformly-illuminated pinhole along the optical axis, and arranging for its in-focus image to fill exactly a circular light detector. The detector output plotted against pinhole position is approximately Gaussian in form for the objectives studied (from 10 2 /0·25 to 74 2 /0·65), and d p is defined as the full width at half maximum. These values of d p adequately fit the theoretical relation d p = 2·45 R/tan sin-1(NA/n), where NA is the numerical aperture of the objective and n is the refractive index of the immersion medium. With spherical, or near-spherical, volume sources d p is usually significantly greater than the volume of the source. The problems of defining a depth of field p...

Observations of sonoluminescence using image intensification

A high‐gain image intensifier system has been applied to the study of the light emitted when cavitation occurs in a liquid (sonoluminescence). Observations of sonoluminescence from several different magnetostrictive oscillators show that spatial and temporal distributions can be recorded in a few seconds in cases where recording on conventional film would not be possible. In the present experiments, the spectral transmission properties of the optics precluded recording of the light in ultraviolet. The spatial details of the cavitation process would not be available from photomultiplier records.

The Kelvin-Thomson atom. I. The one- to six-electron atoms

A resume is given of the models key features and it is suggested that it is (like Bohrs model) worth retaining as a didactic aid. Besides providing good examples of the application of Coulombs and Gausss laws, it serves as a good example around which to hang a discussion of modelling.

Image Intensifier Studies of Sonoluminescence from Liquids Subjected to Low Level Ultrasonic Pressure Amiplitudes

Image intensification has been used to study the light emission from bubbles in cavitating liquids (sonoluminescence), over a range of ultrasonic power inputs. The observations demonstrate that light is emitted under conditions of stable as well as transient cavitation. Since light emission indicates high temperatures and radiative recombination of free radicals in the collapsing bubbles, these results have important implications for medical applications of continuous wave ultrasound.

Spatial, Temporal and Spectral Observations of Sonoluminescence by Means of Image Intensification

Sonoluminescence (SL) is a weak light emission occurring when certain liquids are cavitated by acoustical waves. There is at present no single theory that can accommodate all of the experimental observations of this luminescence. Dark adapted eyes, photographic films, and photomultipliers have been used to observe the phenomenon when various transducers and liquids have been used, but little or no spatial information has been available. Long exposures on conventional film have yielded some evidence for standing wave patterns and some information on spectral distributions. The purpose of this paper is to provide a brief history of observations of the phenomenon and the hypotheses put forth to explain it, and to describe some initial experimental results obtained using image intensification to determine the spatial, temporal and spectral characteristics associated with sonoluminescence in water.

High‐gain image intensifier studies of electroluminescence in III–V semiconductors

A high‐gain multistage image intensifier and microscope are used to study low‐current dc electroluminescence in bulk semiconductors. A comparison is made with cathodoluminescence, and preliminary results for Ga1−xAlxAs and GaP are presented. It is shown how the technique gives information about the electrical contacts to a specimen, the electroluminescence spectra at points on a specimen, and the distribution of radiative recombination centers.

Transport processes in gases-an alternative approach

An alternative approach to transport processes occurring in gases is presented for the benefit of school teachers. The usual expressions derived for the thermal conductivity and dynamic viscosity are obtained by considering the net energy flow or momentum flow across a plane located within the body of the gas. It is considered that this approach can lead to misconceptions about the underlying mechanisms, especially for viscous flow. This alternative approach avoids these misconceptions.

The scattering of gas atoms from solid surfaces

Traditional undergraduate courses in gas kinetic theory encourage the view that in each and every collision between a gas atom and, say, the surface of a piston the angle of incidence theta 1 of the gas atom is equal to its angle of reflection theta 2. The justification which is normally given is that since there is no external force acting parallel to the surface of the piston mu1 sin theta 1=mu2sin theta 2. When the temperature of the gas is equal to that of the piston there will be no net transfer of energy from gas to piston and so, it is argued, 1/2 mu12=1/2mu22. The fallacy lies in assuming that these equations must apply to each and every collision. All that the physics demands is that on average-averaged over a great many collisions-there shall be neither a net transfer of tangential momentum nor a net transfer of energy to the piston.