Using Monte Carlo techniques, the 18 MV accelerators CL2300 and CL21EX, 8 physical wedges and the enhanced dynamic wedge have been modeled and validated. The simulated PDDs, profiles, output factors and wedge factors are in good agreement with measured values. These models have been applied in the dose calculation of lung cancer patients to investigate the effect of tissue heterogeneity by comparing with the CADplan treatment planning system with and without heterogeneity correction. Plan I and plan II for 8 patients have been re-calculated with the Monte Carlo method. On average, the mean PTV dose calculated with the Monte Carlo method is 2.4% higher than that calculated with CADplan without correction but 2.5% lower than that with EqTAR correction. The Monte Carlo method predicts 13% higher mean lung dose than CADplan without heterogeneity correction. Because of this difference, the risk of radiation pneumonitis based on available NTCP models will be underestimated. A relationship of mean lung dose between two types of dose algorithms can be used to correct this underestimation.

This study involves the measurement of the neutron equivalent dose ( NED) and the induced activity produced from medical linear accelerators. For the NED, various parameters such as the profile, field effects and energy responses were studied. The NED in a Solid Water(TM) phantom was measured and a new quantity, the neutron equivalent dose tissue-air ratio (NTAR), was defined and determined. Neutron production for electron beams was also measured. For the induced activity, comparisons were carried out between different linacs, fields and dose rates. The half life and activation saturation were also studied. A mathematical model of induced activity was developed to explain the experimental results. Room surveys of NED and induced activity were performed in and around a high energy linear accelerator room. Unwanted doses from photoneutrons and induced activity to the high energy linear accelerator radiotherapy staff and patient were estimated.

Electron arc therapy is the treatment of choice for tumours involving large curved surfaces. At the Montreal General Hospital a unique approach to the electron arc therapy was developed in 1986 and has been used clinically ever since. The approach is based on the concept of the characteristic angle beta. / We measured radial percentage depth doses in a polystyrene cylindrical phantom irradiated with electron arc beams having angles beta in the range from 5$ sp circ$ to 100$ sp circ,$ for 9 MeV, 12 MeV, 15 MeV, and 18 MeV electron beam energies. We showed that the characteristic angle-$ beta$ concept can be extended to the beams with nominal energy of 18 MeV. The validity of the empirical relationship, relating the doses in two beams with different energies, was confirmed. A linear relationship between the angle $ beta$ and the depth of dose maximum, the depth of the 85% depth dose, and the depth of the 50% depth dose, was established. The surface dose dependence on the angle $ beta$ was also determined and the bremsstrahlung contamination in the electron arc therapy studied.

An algorithm for optimization of numerous modulated beam weights has been developed. This algorithm employs a penalty function theorem and a simulated annealing (SA) routine to model a large-scale constrained optimization problem incorporating dose and dose volume constraints in reflecting the goal of inverse treatment planning by sparing sufficient healthy tissues while delivering a necessary tumorcidal dose. The convergence property of the dose-volume SA algorithm is investigated for validation. Its performance is also evaluated by comparing the algorithm with a gradient technique minimizing the same dose-volume objective function that incorporates the target dose objectives and organ dose-volume constraints by the penalty functions. The comparison shows that the objective function exhibits a global valley in which multiple local minima with similar outcomes in terms of the function values, the dose-volume histograms, and the dose distributions exist. Thus, the gradient algorithm is preferred for this optimization approach due to its fast efficiency.

A prototype electrostatic imager has been developed for megavoltage portal imaging in radiation therapy. The imager utilizes amorphous selenium (a-Se) with a front metal plate as the imager receptor and a high resolution voltmeter probe for image readout. Imaging characteristics of a-Se have been investigated theoretically through Monte Carlo simulations, and experimentally by measuring radiation discharging curves and phantom tests. The results of this study have shown that the prototype imager has high sensitivity, good spatial resolution and low noise level. Our study also reveals the potential of electrostatic imaging with metal/a-Se in megavoltage imaging. / Two computer algorithms have also been developed for automatic segmentation and contrast-enhancement of digital portal images, and for radiation field shape verification. Based on a priori knowledge of the properties of portal images, the segmentation and contrast-enhancement algorithm employs multiple criteria and dynamic reasoning to achieve optimal segmentations of individual images, and has been proved to be accurate, robust and fast. The algorithm for radiation field shape verification is an adaptation of the chamfer matching technique to a specific application: matching closed contours. By incorporating geometric features of the radiation field and using a simple minimization method which is more specific to this task, the algorithm appears to be able to improve the matching results of the standard method.

Radiosurgery is a radiation treatment modality in which a high radiation dose (few 1000 cGy) is given to a small volume (few cm$ sp3$) within the patient's brain during a single treatment. The main physical characteristics of radiosurgery are narrow circular radiation beams, stringent requirements on the numerical ($ pm$2%) and spatial ($ pm$1 mm) accuracy of dose delivery to the target and the need for sharp dose fall-offs outside the target volume. / Physical aspects of radiosurgery based on isocentric linear accelerators (linacs) are presented. The equipment and techniques used in the measurement of various radiosurgical beam parameters are discussed. Also discussed is the accuracy of radiation beam delivery to the target, the calculation and measurement of 3-dimensional isodose distributions obtained from circular beams, and the production of cylindrical dose distributions with rectangular beams. It is shown from the physics point-of-view that linac-based radiosurgery is a viable alternative to radiosurgery with the commercially available Gamma unit.

The dynamic wedge is created by moving one of the primary beam limiting collimators in a continuous fashion to create a continuously decreasing field width while the radiation beam is on. Because of this collimator motion, different segments of the treatment field will be exposed to the primary beam for different intervals of time. This process of shrinking the field width while modulating the collimator jaw velocity and dose rate creates the desired wedge-shaped isodose gradient across the treatment field. The amount of dose that is delivered to the patient as the collimator moves across the treatment field is controlled by pre-specified computer selected look-up tables. / Before a dynamic wedge option can be implemented clinically, it must be thoroughly tested by measuring pertinent beam characteristics, thereby determining the reliability of this technology. Unlike beam measurements in static field radiotherapy, dynamic beam radiatiotherapy requires the use of an integrating dosimetry technique. In this thesis data was accumulated with the use of film dosimetry and direct integration ionization chambers. Using such systems the dosimetric properties of the dynamic wedge installed on a dual energy Clinac 2300 C/D linear accelerator were investigated. (Abstract shortened by UMI.)

<p> Silicon nitride passivation on AlGaN\GaN heterojunction devices can improve performance by reducing electron traps at the surface. This research studies the effect of displacement damage caused by 1 MeV electron irradiation as a function of the variation of passivation layer thickness and heterostructure layer variation on AlGaN/GaN HEMTs. The effects of passivation layer thickness are investigated at thicknesses of 0, 20, 50 and 120 nanometers on AlGaN\GaN test structures with either an AlN nucleation layer or a GaN cap structures which are then measured before and immediately after 1.0 MeV electron irradiation at fluences of 10¹⁶cm^-2. Hall system measurements are used to observe changes in mobility, carrier concentration and conductivity as a function of Si₃N₄ thickness. Models are developed that relate the device structure and passivation layer under 1 MeV radiation to the observed changes to the measured photoluminescence and deep level transient spectroscopy. A software model is developed to determine the production rate of defects from primary 1 MeV electrons that can be used for other energies and materials. The presence of either a 50 or 120 nm Si₃N₄ passivation layer preserves the channel current for both and appears to be optimal for radiation hardness.</p>

<p>Recent results at RHIC seem to confirm T.D.Lee's hypothesis that a new form of matter, the quark-gluon plasma (QGP), could be formed in heavy-ion collisions at high energies. Heavy quarks, being formed in the early stages of heavy-ion collisions, form a good probe for the properties of the QGP. The energy loss of heavy quarks as they traverse the medium is predicted to be less than that of the lighter quarks. However; previous measurements of the nuclear modification factor at RIHC indicate that the energy loss of heavy and light quarks is comparable. Thus measurements of the in-medium energy loss of heavy-quarks are of particular interest. In this thesis, a measurement of the differential production cross-section of electrons from the semi-leptonic decay of heavy-flavor quarks in [special characters omitted] = 2.76 TeV <i>pp</i> is presented. This provides a stringent test of perturbative QCD in a new energy regime, and forms a crucial baseline for Pb-Pb collisions where the in-medium energy loss mechanism can be studied. </p>

<p> The electric (<i>G_E</i>) and magnetic (<i> G_M</i>) form factors of the proton are fundamental observables which characterize its charge and magnetization distributions. There are two methods to measure the proton form factors: the Rosenbluth separation method and the polarization transfer technique. However, the ratio of the electric and magnetic form factors measured by those methods significantly disagree at momentum transfer <i>Q</i>² > 1 GeV². The most likely explanation of this discrepancy is the inclusion of two-photon exchange (TPE) amplitude contributions to the elastic electron-proton cross section which significantly changes the extraction of <i>G_E</i> from the Rosenbluth separation measurement. The Jefferson Lab CLAS TPE experiment determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections. The primary electron beam was used to create an intense bremsstrahlung photon beam. Some of the photons were then converted to a mixed <i>e</i>⁺/<i> e</i>^- beam which then interacted with a liquid hydrogen target. The <i>e</i>⁺<i>p</i> and <i> e</i>^-<i>p</i> events were detected by the CLAS (CEBAF Large Acceptance Spectrometer). The elastic cross section ratios ((&sigma;(<i> e</i>⁺<i>p</i>)/(&sigma;(<i>e</i>^-<i>p</i>)) were measured over a wide range of virtual photon polarization <i>&epsiv;</i> and <i>Q</i>². The cross section ratios displayed a strong <i>&epsiv;</i> dependence at <i>Q</i>² = 1.45 GeV². There is no significant <i>Q</i>² dependence observed at <i> &epsiv;</i> = 0.45. The results are consistent with a recent measurement at the VEPP-3 lepton storage ring in Novosibirsk and with the hadronic calculation by Blunders, Melnitchouk and Tjon. The hadronic calculation resolves the disagreement between the Rosenbluth separation and polarization transfer extractions of <i> G_E/G_M</i> at <i>Q</i>² up to 2 &ndash; 3 GeV². Applying the GLAS TPE correction to the Rosenbluth cross section measurements significantly decreases the extracted value of <i>G_E</i> and brings it into good agreement with the polarization transfer measurement at <i>Q</i>²&sim;1.75 GeV². Thus, these measurements appear to resolve the proton electric form factor discrepancy for <i>Q</i>² &lt; 2 GeV².</p>