O processamento por radiação gama para beneficiamento de produtos tem se tornado uma tecnologia aceita mundialmente. Tem como aplicações a esterilização de produtos médicos, a irradiação de alimentos e produtos agrícolas, a modificação de materiais poliméricos e biomateriais, a preservação de objetos de patrimônio cultural, a esterilização de tecidos humanos para transplante, a mudança de cor em gemas, dentre outros. Desde 2004, o Irradiador Multipropósito de Cobalto-60 do Centro de Tecnologia das Radiações do IPEN tem sido utilizado no estudo e aperfeiçoamento dessas aplicações com objetivos de pesquisa e no estabelecimento de aplicações rotineiras. Este, embora concebido para trabalhar no modo contínuo de irradiação, é utilizado no modo estacionário mais indicado para irradiações de materiais delicados ou de pesquisa. Neste trabalho foram validadas e verificadas experimentalmente as adequações dos métodos à irradiação estacionária; bem como as interrupções para medição de doses em dosímetros de polimetilmetacrilato em processos fracionados, onde seu uso se mostrou adequado para taxas de dose maiores que 2kGy·h-1. A descrição do processo de carga das fontes mostra a importância do seu planejamento e seu impacto na distribuição de taxas de dose. Os mapeamentos das doses e taxas de dose permitiram localizar regiões de valores homogêneos e máximos. Foi estabelecido um procedimento de irradiação de tecidos para aloenxertia utilizando monitoração dosimétrica externa. Foram levantados os custos e despesas de irradiação, aplicáveis a instalações governamentais. O balanço energético da piscina de armazenamento das fontes permitiu calcular o fluxo de calor do sistema e definir as condições máximas de operação sem trocador de calor. / The gamma radiation processing is a well-accepted technology around the world. The mean applications are sterilization of medical products, irradiation of food and agriculture products, polymeric and biomaterials modification, Cultural Heritage objects preservation, humans tissues sterilization, gemstone colorization. Since 2004, the Multipurpose Gamma Irradiation Facility at the Radiation Technology Center of the Nuclear and Energy Research Institute has been used for improvement of research applications and for establishment of routine processes. Moreover, this equipment was developed to work in continuous irradiation mode; it has been used in stationary irradiation mode most of times - recommended to the irradiation of sensible materials. In this work, the applicability of the stationary irradiation methods were experimentally validated and verified. Results shown that absorbed dose measurements carried out with polymethyl methacrylate dosimeters in irradiation processes with interruption can be performed at dose rates upper 2kGy·h-1. Radioactive sources loading and installation processes have been documented showing the importance of planning, as well the impact in the dose rates distribution at the source racks. The dose and dose rate mapping allowed finding homogeneous and higher dose rates zones. A procedure for human tissues irradiation using only external dosimetry was established. The irradiation service costs and expenses related have been estimated, applicable particularly for government facilities. The energetic balance of the sources storage pool was carry out to evaluate the heat transfer conditions and limitations of the system.

PROPERTIES OF LIGHT EMITTING DIODES FOLLOWING COBALT-60 IRRADIATION &Ouml / zcan, Safak M.S., Department of Physics Supervisor: Prof. Dr. ibrahim G&uuml / nal September 2004, 71 pages The main purpose of this study is to investigate the effects of gamma radiation on the properties of the light emitting diodes. GaP and GaAsP LEDs are used in the study. It is observed that the exposure of a light emitting diode affects its various properties. A cobalt-60 gamma-cell is used to irradiate the selected light emitting diodes. For the different total doses of gamma pre-irradiation and post-irradiation I-V characteristics and spectral responses are recorded. The capacitance characteristics are measured at 1MHz at room temperature. Gamma ray bombardment of these LEDs results in reduction of electroluminescent intensity and increase in forward current up to levels tested. In GaP diodes dominant current transport mechanism has found to be effected by irradiation. No noticeable change is observed in the series resistances. The impurity density remains same in the green LED and increases in the red one due to the irradiation, which is deduced from the C-V characteristics. Both the circuit designers and the users should be aware of these effects in order to reach a reliable application for these components in a radiation environment.

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