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.

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.

Radiotherapy is used in clinics to treat cancer with highly energetic ionizing particles. The radiation dose can be measured indirectly by means of radiation detectors or dosimeters. The dose deposited in a detector can be related to dose deposited in a point within the patient. In theory, however, this is only possible under charged particle equilibrium (CPE). The motivation behind the dissertation was driven by the difficult, yet crucial, dosimetry in non-CPE regions. Inaccurate dose assessment performed with standard dosimetry using ionization chambers may significantly impact the outcomes of radiotherapy treatments. Therefore, advanced dosimetry methods tailored specifically to suit non-CPE conditions must be used. This work aims to improve dosimetry in two types of non-CPE conditions that pose dosimetric challenges: regions near interfaces of tissues with low- and high- density media and in small photon fields. To achieve the main dissertation objectives, an enhanced film dosimetry protocol with a novel film calibration approach was implemented. This calibration method is based on the percent depth dose (PDD) tables and was shown to be efficient and accurate. As a result, the PDD calibration method was used for the film dosimetry process throughout the dissertation work. Monte Carlo (MC) calculations for the small field dosimetry were performed using phase-space files (PSFs) provided by Varian for TrueBeam linac. The MC statistical uncertainty in these types of calculations is limited by the number of particles (due to latent variance) in the used PSFs. This study investigated the behaviour of the latent variances (LV) with beam energy, depth in phantom, and calculation resolution (voxel size). LV was evaluated for standard 10x10 cm2 fields as well as small fields (down to 1.3 mm diameter). The results showed that in order to achieve sub-percent LV in open 10x10 cm2 field MC simulations a single PSF can be used, whereas for small SRS fields (1.3—10 mm) more PSFs (66—8 PSFs) would have to be summed. The first study in this dissertation compared the performance of several dosimetric methods in three multi-layer heterogeneous phantoms with water/air, water/lung, and water/steel interfaces irradiated with 6 and 18 MV photon beams. MC calculations were used, along with Acuros XB, anisotropic analytical algorithm (AAA), GafChromic EBT2 film, and MOSkin dosimeters. PDDs were calculated and measured in these heterogeneous phantoms. The result of this study showed that Acuros XB, AAA, and MC calculations were within 1% in the regions with CPE. At media interfaces and buildup regions, differences between Acuros XB and MC were in the range of +4.4% to -12.8%. MOSkin and EBT2 measurements agreed to MC calculations within ~ 2.5%-4.5%. AAA did not predict the backscatter dose from the high-density heterogeneity. For the third, multilayer lung phantom, 6 MV beam PDDs calculated by all treatment planning system (TPS) algorithms were within 2% of MC. 18 MV PDDs calculated by Acuros XB and AAA differed from MC by up to 3.2 and 6.8%, respectively. MOSkin and EBT2 each differed from MC by up to 3%. All dosimetric techniques, except AAA, agreed within 3% in the regions with particle equilibrium. Differences between the dosimetric techniques were larger for the 18 MV than the 6 MV beam. This study provided a comparative performance evaluation of several advanced dosimeters in heterogeneous phantoms. This combination of experimental and calculation dosimetry techniques was used for the first time to evaluate the dose near these interfaces. The second study in the dissertation aims to improve dose measurement accuracy in small radiotherapy fields. Field output factors of 6 MV beams from TrueBeam linear accelerator (linac) collimated with 1.27-40 mm diameter cones were calculated and measured using MC and EBT3 films. A set of detector specific correction factors for two widely used dosimeters (EFD-3G diode and PTW-60019 microDiamond detectors) were determined based on GafChromic EBT3 film measurements and calculated using MC methods. MC calculations were performed for microDiamond detector in parallel and perpendicular orientations relative to the beam axis. The result of this study showed that the measured OFs agreed within 2.4% for fields ≥10 mm. For the cones of 1.27, 2.46, and 3.77 mm diameter maximum differences were 17.9%, 1.8% and 9.0%, respectively. MC calculated OF in water agreed with those obtained using EBT3 film within 2.2% for all fields. MC calculated output correction factors for microDiamond detector in fields ≥10 mm ranged within 0.975-1.020 for perpendicular and parallel orientations. MicroDiamond detector correction factors calculated for the 1.27, 2.46 and 3.77 mm fields were 1.974, 1.139 and 0.982 with detector in parallel orientation, and these factors were 1.150, 0.925 and 0.914 in perpendicular orientation. EBT3 and MC obtained correction factors agreed within 3.7% for fields of ≥3.77 mm and within 5.9% for smaller cones. This work provided output correction factors for microDiamond and EFD-3G detectors in very small fields of 1.27 – 3.77 mm diameter and demonstrated over and under-response of these detectors in such fields. These correction factors allow improve the accuracy of dose measurements in small photon fields using these detectors. / Graduate / 2019-08-30