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Electron transport in vacuum deposited a-Se films has been investigated by Interrupted-Field Time-of-Flight (IFTOF) transient photoconductivity experiments to examine the effect of sample temperature (T) and applied electric field (F) on X-ray induced changes in the electron lifetime. Upon exposure to x-rays, the electron lifetime decreases. The decrease in normalized lifetime is almost linearly proportional to the absorbed dose, and is more significant at higher temperatures. Upon the cessation of x-ray irradiation, the lifetime recovers towards its equilibrium value through a structural relaxation process, and is characterized by a structural relaxation time. The structural relaxation time decreases with temperature in an Arrhenius fashion, and exhibits an activation energy that is roughly 1.4 eV. The structural relaxation time at room temperature (21 C) is 2 – 4 hrs whereas at 35 C, 6 – 10 mins. These measurements are important in characterizing the charge collection efficiency of a-Se based x-ray detectors, and its dependence on x-ray exposure and temperature. The results indicate that the rate of change of electron lifetime per unit exposure is less than 2%/Gy.

The utilization of Computed Tomography in paediatric examinations constantly increases. During the procedure, a high amount of dose is delivered to children, which could be avoided. This study examined whether the selection of scanning parameters – tube voltage and tube current-time product- could be based on patient size instead of patient age or weight aiming, for dose reduction. The SRS78 spectrum processor software was employed to generate spectra with tube voltage between 80kVp and 120kVp and with tube current-time product between 50mAs and 165mAs. These spectra were attenuated by different thicknesses of polymethylmethacrylate (PMMA) phantoms. The simulation technique was validated with experimental measurements acquired on CTDI phantoms on a Siemens Somatom plus 4 scanner. The image quality was assessed in terms of noise, contrast and contrast-to-noise ratio (CNR). Furthermore the contrast of iodine, adipose tissue and cortical-bone relative to muscle were calculated in order to examine how the contrast of different materials was influenced when tube voltage changed. The data analysis shows that there is a definite relationship between image quality and the size of a patient. When exposure settings are kept constant, the level of noise, contrast and Contrast-to-Noise Ratio (CNR) depends on the size of the phantom. Noise is increased exponentially and contrast is reduced linearly as the size of the phantom is increased. CNR is markedly higher in small size phantoms. Moreover, when tube voltage was reduced the noise level was increased less in the small size phantoms and the contrast of high atomic number materials is reduced more when tube voltage is reduced. The CNR for high atomic number materials presents modest improvements when tube voltage is increased therefore examinations with contrast agents could be performed at lower tube voltages. Furthermore the high CNR in small size phantoms could be traded off with lower mAs. In particularly the mAs could be reduced by up to 95% while maintaining the same CNR as for adults resulting in dramatic dose reductions for children. Moreover, since Computed Tomography stands out from all the other X-ray techniques due to its ability to detect structures of similar densities the detect ability of low contrast details was investigated. The Catphan phantom and particularly the CTP515 module was employed. The phantom was scanned with the Siemens Somatom plus 4 scanner at 80kVp, 120kVp and 140kVp and with tube current-time product between 43mAs and 165mAs. The image quality was assessed subjectively and objectively. It is observed that when 120kVp and 140kVp are applied there are not sufficient differences on image quality which justify the selection of 140kVp in paediatric protocols. When 80kVp is applied structures with contrast lower than 10HU are not detected. Concerning mAs does not contribute to the detection of low contrast details except if it is combined with high tube voltages. However, mAs contribute to the visualisation of smaller in size details but above a threshold value, higher mAs does not serve any purpose and the value of 300mAs employ in many protocols is not justified. In conclusion, the reduction of dose during paediatric Computed Tomography examinations is more than probable since scanning parameters could be reduced without degradation of image quality. However in order to assure the reduction of dose without side effects, protocols must be constructed which will individualize the Computed Tomography examinations. That is, the optimum spectrum must be selected relative to the diagnostic task and the size of the patient. / -

Total ionizing dose (TID) radiation effects modeling and simulation on digital, analog and mixed signal systems remains a significant bottle neck in the development of radiation-hardened electronics. Unverified modeling techniques and the very high computational cost with today's commercial simulation tools are among the primary hindrances to the timely hardened IC design, particularly to the design in commercially available processes. SPICE-based methods have been used for total dose radiation degradation simulations. While SPICE is effective in predicting the circuit behavior under circumstances when the electrical parameters stay constant during operation, it's not effective predicting aging behavior with gradual change with time. Behavioral modeling language, such as VHDL-AMS is needed to effectively capture the time-dependent degradation in these parameters in response to environmental stresses, such as TID radiation.This dissertation describes a method for accurate and rapid TID effect simulation of complex mixed-signal circuits. The method uses a hierarchical structure where small sub-circuits, such as voltage comparators, references, etc. are simulated using SPICE. These SPICE simulations of small circuits for multiple radiation doses are used to tune behavioral VHDL-AMS models for the sub-circuits. The created behavioral models therefore contain the electrical circuit behavior combined with the radiation response. The entire combined system is then simulated using VHDL-AMS.In a simulation experiment that was used to validate the speed and accuracy of the new method, a commercial 8-bit sub-ranging analog to digital converter netlist containing more than 2000 MOS transistors was simulated with TID models using a contemporary SPICE-based method and the new method. The new method shortened the simulation time by three orders of magnitude, while accuracy remained within reasonable limits compared to the SPICE-based method. Moreover, the automated procedures for circuit node bias monitoring, TID model replacement and result collection that are included in the simulation code of the new method decreased the "hands-on" engineering work significantly. Results from an experiment where the new TID effect simulation method was used as a hardness assurance test procedure for integrated circuits designed to be operated in radiation-harsh environments are also included in this dissertation.

This dissertation explores beta dose profile of microspheres packed in arteries, various source geometries of 142Pr that can be used for therapeutic purpose, and dose backscatter factors for selected beta sources. A novel treatment method by injecting microspheres into feeding arteries of arteriovenous malformation (AVM) is under pre-clinical investigation. To optimize radiation dose to the clinically important area, i.e. arterial wall, preliminary dosimetric studies were needed. Monte Carlo calculations were performed for several geometries simulating arteries filled with microspheres packed by random packing methods. Arterial radii used in the simulation varied from 50 mm to 3 mm; microsphere radii varied from 10 mm to 0.7 mm. Dose varied significantly as a function of microsphere size, for constant arterial sizes. For the same sizes of arteries, significant dose increase was observed because of inter-artery exposure for large arteries (> 0.1 cm rad.) filled with large microspheres (> 0.03 cm rad.). Dose increase between small arteries (0.03 cm rad.) was less significant. The dose profiles of prototype 142Pr beta brachytherapy sources were calculated using MCNP 4C Monte Carlo code as well as dose point kernel (DPK) for selected cases. Dose profiles were similar to beta sources currently used indicating that 142Pr can substitute for current sources for certain cases and the DPK was closely matched with MCNP result. Backscattering of electrons is a prominent secondary effect in beta dosimetry. The backscattering is closely correlated with factors such as geometry of source and scattering material, and composition of scattering material. The backscattering factors were calculated for selected beta sources that are currently used as well as potentially useful sources for therapeutic purpose. The factors were calculated as a function of distance from the interface between water and scatterers. These factors were fit by a simple function for future incorporation into a DPK code. Backscattering effect was significant for short distance from the surface of interface between water and scattering material.

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The complex dose distributions delivered by modern, conformal radiation therapy techniques present a considerable challenge in dose verification. Traditional measurement tools are difficult and laborious to use, since complete verification requires that the doses be determined in three dimensions (3D). The difficulty is further complicated by a required target accuracy of ± 5% for the dose delivery. Gel dosimetry is an attractive option for realizing a tissue-equivalent, 3D dose verification tool with high resolution readout capabilities. However, much important work remains to be completed prior to its acceptance in the clinic. The careful development of easily accessible, fast optical readout tools such as cone beam optical computed tomography (CT) in combination with stable and reliable low-toxicity gel dosimeters is one key step in this process. In this thesis, the performance capabilities and limitations of the two main classes of cone beam optical CT-based absorbing and scattering gel dosimetry are characterized, and their measurement improved through careful matching of dosimeter and scanner performance. These systems are then applied to the evaluation of clinically relevant complex dose distributions. Three-dimensional quality assurance assessments of complex treatment plan dose distributions are shown to be feasible using an optically absorbing Fricke-xylenol-orange-gelatin-based gel dosimeter. Better than 95% voxel agreement is achieved between the plan and the delivery, using 3% dose difference and 3 mm spatial distance-to-agreement gamma function comparison criteria. Small field dose delivery evaluations are demonstrated to be viable using an optically scattering N-isopropylacrylamide (NIPAM)-based polymer gel, with the same comparison criteria. Full treatment process quality assurance is also possible using a NIPAM dosimeter in-phantom, but is limited in its accuracy due to the inherent difficulty of managing the effects of stray light pertubation in the optical attenuation-to-dose calibration. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2010-09-02 15:01:48.501

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A wealth of studies have examined the impact of alcohol on violence; however, only a small number have addressed differences elicited by different doses of alcohol. Such studies are seriously limited by mixed findings, small sample sizes, inconsistent alcohol doses and control conditions, a bias toward studying only male participants, and the predominant use of only one particular measure to assess aggression. The present laboratory investigation was designed to elucidate and advance this literature by improving upon these limitations. Participants were 187 (95 men and 92 women) social drinkers. Following the consumption of one of 6 alcohol doses (i.e., 0.0g/kg; 0.125g/kg; 0.25g/kg; 0.5g/kg; 0.75g/kg; and 1.0g/kg), participants were tested on a laboratory task in which electric shocks were received from, and administered to, a fictitious opponent under the guise of a competitive reaction-time task. Aggression was operationalized as the intensity of shocks administered to their opponent. Analyses revealed that higher alcohol doses clearly elicited greater aggression in both genders consistent with a linear non-threshold dose-response model. Our data help to clarify a body of literature that has been afflicted with numerous limitations and will also help in the selection of alcohol doses for researchers conducting future laboratory-based aggression studies.

Thrombosis is the pathological formation of platelet aggregates that cause stroke and heart attack\textemdash the leading causes of death in developed nations. Determining effective dosages for platelet therapies (e.g. aspirin, Integrilin, and Plavix) to prevent thrombosis is a persistent medical challenge (studies estimate up to 45% of patients exhibit insufficient responses to these drugs) and recent studies have implicated pathological flow conditions of high shear rates and stenosis morphology as primary factors. However, there are currently no diagnostic instruments able to recapitulate a range of such pathological flow conditions for evaluating thrombosis with and without these drugs. In this work, a microfluidic device and associated optical system were designed and fabricated for simultaneous measurement of platelet aggregation at multiple initial wall shear rates within multiple stenotic channels in label-free whole blood and used to characterize thrombosis at varying dosages of two platelet therapies: acetyl-salicylic acid (aspirin) and eptifibatide (Integrilin). Results from our studies show the effects of pathologically high shear rates on enhancing platelet thrombosis and demonstrate the widely varied, shear-dependent efficacy of each therapy. This study lays the foundation for the future development of a medical diagnostic for optimizing the type and dosage of patient platelet therapy and to better understand their mechanisms of action.