Made available in DSpace on 2014-10-09T12:38:02Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:38Z (GMT). No. of bitstreams: 1 05578.pdf: 1277201 bytes, checksum: 2ef4d6038f2cce475891ed87d5a589ad (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:54:23Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:45Z (GMT). No. of bitstreams: 0 / A avaliação do desempenho dos detectores termoluminescentes de CaSO4:Dy, produzidos pelo IPEN, tem como objetivo apresentar uma alternativa aos dosímetros de LiF:Mg,Ti comerciais (TLD-100), amplamente empregados em radioterapia, na dosimetria de feixes clínicos de elétrons. Os dois tipos de detectores termoluminescentes foram caracterizados, com a utilização de objetos simuladores de PMMA, água sólida do tipo RMI-457 e água, em campos de radiação de elétrons de 4, 6, 9, 12 e 16 MeV de energias nominais, nos quais as curvas de dose-resposta foram obtidas e as doses superficial e profunda foram determinadas. A dependência da resposta termoluminescente com a energia nominal dos elétrons e com o objeto simulador empregado foram estudadas. O CaSO4:Dy apresentou o mesmo comportamento do LiF:Mg,Ti; de maneira que sua aplicação como uma alternativa às pastilhas TLD-100 na dosimetria de feixes de elétrons em radioterapia é viável e apresenta a sensibilidade à radiação de elétrons significativamente mais alta como principal vantagem. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:24:00Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:54Z (GMT). No. of bitstreams: 1 01009.pdf: 1210828 bytes, checksum: 081f4bbe930a4a873ba638585746e8b9 (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

Carbon-doped aluminium oxide, α-Al₂O₃ : C, is a highly sensitive luminescence dosimeter. The high sensitivity of α-Al₂O₃ : C has been attributed to large concentrations of oxygen vacancies, F and F⁺ centres, induced in the material during its preparation. The material is prepared in a highly reducing atmosphere in the presence of carbon. In the luminescence process, electrons are trapped in F-centre defects as a result of irradiation of the material. Thermal or optical release of trapped electrons leads to emission of light, thermoluminescence (TL) or optically stimulated light (OSL) respectively. The thermoluminescence technique is used to study point defects involved in luminescence of α-Al₂O₃ : C. A glow curve of α-Al₂O₃ : C, generally, shows three peaks; the main dosimetric peak of high intensity (peak II) and two other peaks of lower intensity called secondary glow peaks (peaks I and III). The overall aim of our work was to study the TL mechanisms responsible for secondary glow peaks in α-Al₂O₃ : C. The dynamics of charge movement between centres during the TL process was studied. The phototransferred thermoluminescence (PTTL) from secondary glow peaks was also studied. The kinetic analysis of TL from secondary peaks has shown that the activation energy of peak I is 0.7 eV and that of peak III, 1.2 eV. The frequency factor, the frequency at which an electron attempts to escape a trap, was found near the range of the Debye vibration frequency. Values of the activation energy are consistent within a variety of methods used. The two peaks follow first order kinetics as confirmed by the TM-Tstop method. A linear dependence of TL from peak I on dose is observed at various doses from 0.5 to 2.5 Gy. The peak position for peak I was also independent on dose, further confirmation that peak I is of first order kinetics. Peak I suffers from thermal fading with storage with a half-life of about 120 s. The dependence of TL intensity for peak I increased as a function of heating rate from 0.2 to 6ºCs⁻¹. In contrast to the TL intensity for peak I, the intensity of TL for peak III decreases with an increase of heating rate from 0.2 to 6ºCs⁻¹. This is evidence of thermal quenching for peak III. Parameters W = 1.48 ± 0:10 eV and C = 4 x 10¹³ of thermal quenching were calculated from peak III intensities at different heating rates. Thermal cleaning of peak III and the glow curve deconvolution methods confirmed that the main peak is actually overlapped by a small peak (labeled peak IIA). The kinetic analysis of peak IIA showed that it is of first order kinetics and that its activation energy is 1:0 eV. In addition, the peak IIA is affected by thermal quenching. Another secondary peak appears at 422ºC (peak IV). However, the kinetic analysis of TL from peak IV was not studied because its intensity is not well defined. A heating rate of 0.4ºCs⁻¹ was used after a dose of 3 Gy in kinetic analysis of peaks IIA and III. The study of the PTTL showed that peaks I and II were regenerated under PTTL but peak III was not. Various effects of the PTTL for peaks I and II for different preheating temperatures in different samples were observed. The effect of annealing at 900ºC for 15 minutes between measurements following each illumination time was studied. The effect of dose on secondary peaks was also studied in this work. The kinetic analysis of the PTTL intensity for peak I showed that its activation energy is 0.7 eV, consistent with the activation energy of the normal TL for peak I. The PTTL intensity from peak I fades rapidly with storage compared with the thermal fading from peak I of the normal TL. The PTTL intensity for peak I decreases as a function of heating rate. This decrease was attributed to thermal quenching. Thermal quenching was not observed in the case of the normal TL intensity. The cause of this contrast requires further study.

Physical cones equipped on GammaKnife, Cyberknife, and C-arm linacs have been the standard practice in Stereotactic Ablative Radiotherapy (SART) for small intracranial lesions, such as treating trigeminal or glossopharyngeal neuralgia targets. The advancement of high-definition multi-leaf collimators (HDMLC), treatment planning systems, and small field dosimetry now allows for treatment without the need for an auxiliary mounted physical cone. This treatment type uses the “virtual cone”, a permanent high-definition MLC, arrangement to deliver “very small fields” with comparable spherical dose distributions to physical cones. The virtual cone therapy, on a Varian Edge™ linac using multiple non-coplanar arcs with static HDMLCs, is a comparable technique that can be used to treat small intracranial neuralgia or other small lesions. In this investigation, two flattening filter free (FFF) photon beams, 6MV FFF and 10MV FFF, were tested for optimal delivery and safety conditions for treating intracranial lesions. The virtual cone method on a Varian Edge™ Linear accelerator using rapid arc stereotactic radiosurgery was used to treat cranial neuralgia for chronic pain for six patients. Absolute dose, relative dose measurements, and monitor units were the main characteristics that were examined to decide which energy was the best for treatment. Source-to-axis distances (SAD) of 100cm measurements were taken at depths of 10cm and 5cm, respectively. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

Radioactive sources used in brachytherapy must be dosimetrically characterized prior to clinical use as defined the TG-43 protocol. In our previous project, Gafchromic film dosimetry was used to experimentally obtain the anisotropy function for an M-19 iridium-192 brachytherapy seed being developed by Source Production Equipment Corp (St. Rose, LA). In this project, the Monte Carlo N-Particle Transport code (MCNP) was used to computationally obtain the full set of TG-43 parameters including the Dose Rate Constant, the Reference Dose Rate, the Radial Dose Function, and the Anisotropy Constant for the M-19 seed.

The phenomenon of photobleaching of a photosensitizer during photodynamic therapy (PDT) is well known. For second generation photosensitizers it may be possible to exploit this effect to enhance the volume of damaged tissue and improve the efficacy of PDT. In addition, as a consequence of photobleaching, the fluorescence emitted by the photosensitizer will decrease during PDT. Mathematical models were developed which describe fluorescence emission, photobleaching and tissue necrosis resulting from the irradiation of tissue containing photosensitizer using an appropriate light source. Diffusion theory was used to model bleaching in a semi-infinite medium caused by broad-beam irradiation, and both pencil and broad-beam fluorescence excitation of the photosensitizer. In addition, models were developed for an isotropic point source imbedded in an infinite medium. Based on the relationship between the decreasing fluorescence signal and the increasing volume of tissue damage, these models allow the extent of necrosis achieved during treatment to be monitored. By analysing spatially resolved fluorescence measurements predictions about necrosis depth that are insensitive to treatment parameters such as photosensitizer concentration, tissue optical properties and bleach rate are possible. Tissue simulating optical phantoms that allow for relatively simple and accurate alteration to optical properties were developed. Photosensitizers which still undergo fluorescence and photobleaching in the solid medium were also added. Using these phantoms, treatment was simulated and spatially resolved fluorescence was measured as a function of time for a wide range of initial treatment parameters. Photobleaching of the photosensitizers was observed to occur through a decrease in fluorescence emission. Also, spatially resolved measurements provided information about the average photosensitizer depth, which was seen to increase with time, with little knowledge of initial treatment parameters. These experimental results were then compared with predictions from the mathematical theory, illustrating the validity of the models. The value and feasibility of this technique for photodynamic therapy dosimetry are discussed, along with planned improvements. / Thesis / Master of Science (MS)

The beta dose distributions in red bone marrow equivalent material due to imbedded continuous sources were measured experimentally with ultra thin LiF thermoluminescent dosimeters near planar interfaces of cortical bone (CB) and red bone marrow (RBM), and RBM and air. It has been also investigated numerically by Cyltran, the Monte Carlo code. In the Monte Carlo approach, the dose enhancement ratio for a planar CB-RBM interface increases with electron energy and reaches a plateau at 0.50 MeV while the dose reduction ratio for a planar vacuum-RBM interface decreases to a steady value from 1.00 MeV onwards. With a semi-infinite source of 32P, dose enhancement ratios at 0-9, 79-88, and 157-166 mg/cm2 separations from a planar CB-RBM interface were measured to be 1.07±0.01, 1.03+0.03 and 0.99+0.03 respectively. The dose reduction ratios at these separations from a planar AIR-RBM interface were found to be 0.82±0.01, 0.94+0.03 and 0.97+0.03 respectively. Both the dose enhancement ratios and dose reduction ratios agree with the results calculated by the Monte Carlo approach within one standard deviation except for the dose reduction ratio at 0-9 mg/cm2 from the AIR-RBM interface. The experimental result in this case is about three standard deviations less than the Monte Carlo results. Using the same Monte Carlo code, the dose enhancement ratio at 0-20 micron separation from a planar CB-RBM interface due to a point or plane source of 0.50 MeV electrons at the interface reaches saturation at approximately 0.22 times the CSDA range of 0.5 MeV electron in CB for both plane and point source configurations. The saturation dose enhancement ratio for both configurations is 1.06±0.01. / Thesis / Master of Science (MS)

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.