Com o objetivo de contribuir para o entendimento de recentes resultados experimentais relativos à transições óticas entre estados eletrônicos Localizados, observados em estruturas de múltiplos poços quânticos , realizamos um estudo teórico investigando os níveis de energias de um elétron ligado a uma impureza hidrogenóide num poço quântico simples. Utilizando a equação de massa efetiva calculamos através do método variacional as energias de um elétron Ligado a uma impureza doadora em um poço quântico de barreiras de potencial finitas . Neste cálculo a blindagem eletrônica do potencial da impureza é tratada no formalismo RPR, podendo tanto o elétron Ligado quanto os elétrons livres ocuparem as duas primeiras subbandas do sistema. Representamos nossos resultados para poços quânticos de GaAs-Ga1-xAlxAs variando a densidade eletrônica e a posição da impureza. Também a densidade de estados de impureza é calculada, uma vez que não restringimos a posição da impureza no poço. O efeito da não-parabolicidade da banda de condução do GaAs e a presença do campo magnético são levados em conta para melhor interpretação dos dados experimentais, que em geral estão em boa concordância com nossos resultados. / The mass-effective equation is used in order to calculate the intra and inter-subband transition energies in the Quantum Wett structures. The electronic screening of the shallow donor impurity is inclued in the RPR method and the energies are found through variational formalism. The binding energies are calcutated as function of the electronic density and impurity position inside the welt. We have calculated the density of states due to impurity position and also the non-parabolicity effect of the GaAs conduction band is analised. For better fitting between our results and the experimental data, we have inclued the magnetic field in the system. Our results seem in a good agreement to those experimentally obtained.

O estudo da distribuição de dose de radiação na região de separação entre dois meios de composições químicas diferentes, irradiados por feixes de fótons de alta energia empregados em radioterapia, tem sido restrito a situação em que o feixe de fótons incide perpendicularmente a superfície plana que separa os dois meios. Este trabalho apresenta um estudo bidimensional da distribuição de dose em um meio heterogêneo formado por água e alumínio, que simulam a interface entre tecido muscular e osso, irradiado por feixes de fótons de energias de 1,25 MeV e 10 MeV. A técnica de medida consiste em irradiar filmes radiográficos posicionados paralelamente a direção dos feixes, passando entre as bases de dois cilindros iguais de alumínio, com os eixos coincidentes, de raio menor que a largura do feixe. A distribuição de dose absorvida nos dois meios, no plano do filme, é medida em função da densidade btica apresentada pelos filmes, explorados por um microdensitômetro. Com isso pretendemos explorar a propriedade do filme de medir a dose de radiação com uma resolução espacial dificilmente igualada por outro tipo de dosímetro. Os resultados obtidos revelaram a existência de regiões onde ocorrem diferenças grandes entre a dose medida e a dose calculada sem considerar a heterogeneidade, coro é feito normalmente no cálculo de doses prescritas nos tratamentos. A comparação dos resultados obtidos no feixe de 1,25 MeV com os resultados de outros autores mostrou-se satisfatória, indicando que a técnica de medida, pouco utilizada nesse tipo de estudo, é bastante viável, principalmente se as informações contidas nos filmes forem totalmente processadas por um programa de computador desenvolvido para essa finalidade. / An experimental study of the perturbation caused by a cylindrical inhomogeneity of aluminium on dose distribution in a water phantom, irradiated by high energy photons used in radiation therapy, was performed, employing radiographic film as dosimeter. With these materials we intended to simulate the interface between bone and muscular tissue. The film was placed parallel to the beam axis, passing between the bases of two aluminium rods, with their axis being perpendicular to the film plane. This system was immersed in a cubic water phantom and irradiated by two photon beams: a gamma ray beam of a cobalt 60 source, and a 10 MeV bremsstrahlung X-ray, both produced by radiation therapy machines. By scanning the films in a computer controlled microdensitometer, the dose distributions were measured in the plane of the films. A previous study of the film response, showed that the optical density is proporcional to dose and film sensitivity is nearly the same in water and aluminium, under charged particle equilibrium condition. It was found that large errors occur when the inhomogeneity is ignored in dose calculation. A dose enhancement of about 9% and 11% in aluminium was observed, respectively, for the gamma and X rays beams. It was concluded that the film is good for this kind of measurements and may be employed in high energy photon beams, in particular, when high spatial resolution is needed. The method may be improved by the development of a software to fully exploit the microdensitometer-computer interface capability.

No description available.

Call number: LD2668 .T4 1979 H364 / Master of Science / Mechanical and Nuclear Engineering

Call number: LD2668 .T4 1984 G33 / Master of Science

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

No description available.

This thesis details the measurement and simulation of patient skin doses arising from X-ray exposure during interventional cardiology procedures. Interventional cardiology procedures can be long and complex resulting in high skin doses, to the extent that radiation burns may be produced. Twenty patients were used in the study consisting of 10 coronary angiogram and 10 coronary angioplasty procedures. Radiochromic films were used to measure skin dose directly. The Gafchromic® XR-RV2 film was chosen for its suitability for this project. The key characteristics of this film were experimentally determined including: dose response, energy dependence, polarisation and post-exposure growth. The dose range was found to be ideally suited for the doses encountered in this study. Energy dependence was found to be ~14% between 60 and 125 kVp at 1 Gy and introduced an unavoidable uncertainty into dose calculations from unknown beam energies. Document scanner characteristics were also been investigated and a scanning protocol is determined. A mathematical model was created to use the geometry and exposure information encoded into acquisition files to reconstruct dose and dose distributions. The model requires a set of study files encoded according to the DICOM format, as well as user input for fluoroscopic estimations. The output is a dose map and dose summary. Simulation parameters were varied and results compared with film measurements to provide the most accurate model. From the data collected the relation between dose area product, maximum skin dose and fluoroscopic time were also investigated. The results demonstrated that a model based on acquisition information can accurately predict maximum skin dose and provide useful geometrical information. The model is currently being developed into a standalone program for use by the Medical Physics and Bioengineering department.

In radiation treatment, a sufficiently high radiation dose must be delivered to the tissue volumes containing the tumor cells while the lowest possible dose should be deposited in surrounding healthy tissue. We developed an original approach that is fast and easy to implement for the early assessment of the efficiency of radiation sensitizers and protectors. In addition, we characterized a new femtosecond laser pulse irradiation technique. We are able to deposit a considerable dose with a very high dose rate inside a well-controlled macroscopic volume without deposition of energy in front or behind the target volume. The radioprotective efficiency was measured by irradiation of the Fricke solution incorporating a compound under study and measuring the corresponding production of ferric ions G (Fe3+ ). The production of ferric ions is most sensitive to the radical species produced in the radiolysis of water. We studied experimentally and simulated with a full Monte-Carlo computer code the radiation-induced chemistry of Fricke/cystamine solutions. Results clearly indicate that the protective effect of cystamine originates from its radical-capturing ability, which allows this compound to compete with the ferrous ions for the various fre radicals - especially · OH radicals and H· atoms - formed during irradiation of the surrounding water. The sensitizing capacity of radiation sensitizers was measured by irradiation of a polyacrylamide gel (PAG) dosimeter incorporating a compound under study and measuring the corresponding increase in the gradient between spin-spin relaxation rate (R2 ) and absorbed dose. We measured an irradiation energy-dependent increase in R 2 -dose sensitivity for halogenated compounds or a decrease for radioprotectors. Finally, we studied a novel laser irradiation method called "filamentation". We showed that this phenomenon results in an unprecedented deposition of energy and the dose rate thus achieved exceeds by orders of magnitude values previously reported for the most intense clinical radiotherapy systems. Moreover, the length of the dose-fre entrance region was adjusted by selecting the duration of femtosecond laser pulses. In addition, we provided evidence that the biological damage caused by this irradiation was similar to other ionizing radiation sources. [symboles non conformes]

This study investigated the photo-excitation and thermoluminescence properties of diamonds which were synthesised primarily for thermoluminescence dosimetry. For investigations of thermally stimulated processes occurring in these crystals an analysis of the thermoluminiscence & low spectra and the temperature dependent isothermal decay spectra was undertaken [Abbreviated abstract. Open document to view full version] / GR2016