Stephen Bosi

The effective size of the solar cone for solar concentrating systems

In this paper we define a virtual solar cone, whose principle axis is aligned with the solar vector, having a radial angular displacement containing a pre-defined proportion of the terrestrial solar radiation. By simulating various sunshape profiles, the angular extent of the energy distribution is established to give the effective size of the solar cone for a range of atmospheric conditions. Then, by simulating the reflection of these solar distributions off a set of non-ideal mirrored surfaces, accounting for non-specular reflection and mirror shape errors, the combined effect of sunshape and mirror properties on the solar image is obtained. Clear trends are presented that show the dependence of the effective size of the solar image on the accuracy of a mirrored surface for different sunshapes. We then identify the effective size of the solar image at the absorber plane that must be accommodated in the design and optimisation of solar concentrating systems.

Investigation of radiological properties and water equivalency of PRESAGE ® dosimeters

PURPOSE PRESAGE is a dosimeter made of polyurethane, which is suitable for 3D dosimetry in modern radiation treatment techniques. Since an ideal dosimeter is radiologically water equivalent, the authors investigated water equivalency and the radiological properties of three different PRESAGE formulations that differ primarily in their elemental compositions. Two of the formulations are new and have lower halogen content than the original formulation. METHODS The radiological water equivalence was assessed by comparing the densities, interaction probabilities, and radiation dosimetry properties of the three different PRESAGE formulations to the corresponding values for water. The relative depth doses were calculated using Monte Carlo methods for 50, 100, 200, and 350 kVp and 6 MV x-ray beams. RESULTS The mass densities of the three PRESAGE formulations varied from 5.3% higher than that of water to as much as 10% higher than that of water for the original formulation. The probability of photoelectric absorption in the three different PRESAGE formulations varied from 2.2 times greater than that of water for the new formulations to 3.5 times greater than that of water for the original formulation. The mass attenuation coefficient for the three formulations is 12%-50% higher than the value for water. These differences occur over an energy range (10-100 keV) in which the photoelectric effect is the dominant interaction. The collision mass stopping powers of the relatively lower halogen-containing PRESAGE formulations also exhibit marginally better water equivalency than the original higher halogen-containing PRESAGE formulation. Furthermore, the depth dose curves for the lower halogen-containing PRESAGE formulations are slightly closer to that of water for a 6 MV beam. In the kilovoltage energy range, the depth dose curves for the lower halogen-containing PRESAGE formulations are in better agreement with water than the original PRESAGE formulation. CONCLUSIONS Based on the results of this study, the new PRESAGE formulations with lower halogen content are more radiologically water equivalent overall than the original formulation. This indicates that the new PRESAGE formulations are better suited to clinical applications and are more accurate dosimeters and phantoms than the original PRESAGE formulation. While correction factors are still needed to convert the dose measured by the dosimeter to an absorbed dose in water in the kilovoltage energy range, these correction factors are considerably smaller for the new PRESAGE formulations compared to the original PRESAGE and the existing polymer gel dosimeters.

Initial investigation of a novel light-scattering gel phantom for evaluation of optical CT scanners for radiotherapy gel dosimetry

There is a need for stable gel materials for phantoms used to validate optical computerized tomography (CT) scanners used in conjunction with radiation-induced polymerizing gel dosimeters. Phantoms based on addition of light-absorbing dyes to gelatine to simulate gel dosimeters have been employed. However, to more accurately simulate polymerizing gels one requires phantoms that employ light-scattering colloidal suspensions added to the gel. In this paper, we present the initial results of using an optical CT scanner to evaluate a novel phantom in which radiation-exposed polymer gels are simulated by the addition of colloidal suspensions of varying turbidity. The phantom may be useful as a calibration transfer standard for polymer gel dosimeters. The tests reveal some phenomena peculiar to light-scattering gels that need to be taken into account when calibrating polymer gel dosimeters.

Diverging ion motion in an inertial electrostatic confinement discharge

An inertial electrostatic confinement device operating in the gaseous discharge pressure regime (units to tens of mTorr) is shown to consist of a substantial flux of neutrals diverging from the cathode center. Using Doppler shift spectroscopy, it is shown that directional ion beams, originating from the center, increase in energy as they move away from the center. Moreover, through charge exchange, these ions become energetic neutrals and travel out of the cathode to the anode. Although naturally there are converging ions, it is shown that this is a lesser component of the energetic particle beams in this pressure range.

Radiological characterization and water equivalency of genipin gel for x-ray and electron beam dosimetry.

The genipin radiochromic gel offers enormous potential as a three-dimensional dosimeter in advanced radiotherapy techniques. We have used several methods (including Monte Carlo simulation), to investigate the water equivalency of genipin gel by characterizing its radiological properties, including mass and electron densities, photon interaction cross sections, mass energy absorption coefficient, effective atomic number, collisional, radiative and total mass stopping powers and electron mass scattering power. Depth doses were also calculated for clinical kilovoltage and megavoltage x-ray beams as well as megavoltage electron beams. The mass density, electron density and effective atomic number of genipin were found to differ from water by less than 2%. For energies below 150 keV, photoelectric absorption cross sections are more than 3% higher than water due to the strong dependence on atomic number. Compton scattering and pair production interaction cross sections for genipin gel differ from water by less than 1%. The mass energy absorption coefficient is approximately 3% higher than water for energies <60 keV due to the dominance of photoelectric absorption in this energy range. The electron mass stopping power and mass scattering power differ from water by approximately 0.3%. X-ray depth dose curves for genipin gel agree to within 1% with those for water. Our results demonstrate that genipin gel can be considered water equivalent for kilovoltage and megavoltage x-ray beam dosimetry. For megavoltage electron beam dosimetry, however, our results suggest that a correction factor may be needed to convert measured dose in genipin gel to that of water, since differences in some radiological properties of up to 3% compared to water are observed. Our results indicate that genipin gel exhibits greater water equivalency than polymer gels and PRESAGE formulations.

Spatial distribution of ion energies in an inertial electrostatic confinement device

The spatial distribution of ion energies was measured in an inertial electrostatic confinement device, which can be used as a small nuclear fusion source for the generation of neutrons. Using Doppler shift spectroscopy and a single ended Langmuir probe it was found that a virtual anode was established at the center of the electrostatic potential well resulting in much reduced ion energies at this point. Moreover, there was direct correlation between the ion energies and the spatial plasma potential showing the validity of the probe measurements. It was shown that the ion energy is a maximum at the point where the potential is the deepest, which occurs about halfway between the center and the cathode radius.

Color, iridescence, and thermoregulation in Lepidoptera

This paper examines evidence for the hypothesized connection between solar thermal properties of butterfly and moth (Lepidoptera) wings, iridescence/structural color, and thermoregulation. Specimens of 64 species of Lepidoptera were measured spectrophotometrically, their solar absorptances calculated, and their habitat temperatures determined. No correlation was found between habitat temperature and the solar absorptance of the wings. It was found, however, that the iridescent specimens exhibited, on average, substantially higher solar absorptance than noniridescent ones.

The mechanism of light reflectance in silverfish

The reflectance of the common silverfish was first studied by Robert Hooke and is reported in his book Micrographia. Here we report a new study of the mechanism of the reflectance in the common silverfish (Ctenolepisma sp.). The reflectance from the body of the animal was measured with and without scales using optical spectrophotometry. The structure of the surface region of the body of the animal was studied in cross‐section using transmission electron microscopy, revealing a doubly chirped multilayer stack in the upper layers of the exoskeleton. By calculating the reflectance of this multilayer, it is shown that it accounts for most of the visible reflectance but with a large deficit in the infrared compared with observation. The scales of the silverfish give rise to some absorption in the visible and the periodically spaced ribs give rise to some reflectance in the infrared. Modelling the ribs as an array of chitin cylinders accounts for the reflectance of the scales. The remainder of the observed reflectance in the infrared is attributed to reflectance from the body of the animal underneath the multilayer stack.

A preliminary study of the measurement of slice-width dose profiles (SWDP) on diagnostic x-ray CT scanners using PAGAT polymer gel dosimeters with optical CT read-out

The slice-width dose profile (SWDP) is a measurement undertaken during acceptance testing and subsequent quality assurance measurements of diagnostic x-ray CT scanners for the determination, of patient dose. In a previous study (Hill B, Venning A J and Baldock C 2005 Med. Phys. 32 1589-1597) normoxic polymer gel dosimeters were used to measure dose, the SWDP and subsequently calculate computer tomography dose index (CTDI) during acceptance testing of a CT scanner. In the current study, a preliminary investigation was undertaken to determine the SWDP of a diagnostic x-ray CT scanner using the PAGAT polymer gel dosimeters with optical CT read-out.

The characterisation of a genipin-gelatin gel dosimeter

Genipin cross links gelatin to slowly form a blue colour that bleaches upon irradiation. Spectrophotometric measurements of the absorbance change following irradiation to doses up to 100 Gy gives a linear dose response for certain concentrations of the gel ingredients; genipin, gelatin and sulphuric acid. Dose sensitivity increases with increasing concentrations of sulphuric acid and genipin and is also strongly dependent on the time allowed for the genipin-gelatin cross linking reaction (referred to here as blending) to take place. The optimum formulation of this gel was found for genipin concentration between 0.3 – 0.5 mM and blending time of at least 4 h.