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Contribuição para a realização do sistema MiniPET: projecto do sistema digital de controlo e da electrónica de vanguarda
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Nesta Tese são focados o projecto e a implementação, em VHDL, de um sistema digital de controlo, realizados com o auxílio de uma placa de desenvolvimento que inclui uma FPGA, e o projecto de eletrónica analógica rápida de "front-end", tarefas estas realizadas no âmbito da concepção de um instrumento de MiniPET.
O instrumento foi pensado para servir essencialmente como ferramenta educacional. O MiniPet é caracterizado por possuir dois detectores, montados cara-a-cara, que operam fazendo o varrimento circular em torno do objecto de estudo. A electrónica analógica de frontend trata os sinais provenientes dos detectores. Para fazer uma estimativa da energia dos fotões que chegam aos detectores como resultado da aniquilação dos positrões, são integradas as cargas ou os impulsos de corrente debitados pelos detectores, são convertidos os valores dessa integração numa palavra digital através de um ADC, e é gerado um sinal de "trigger" por cada evento detectado. O controlador digital faz a gestão do sistema de detecção, valida as coincidências entre os dois detectores e pré-processa os dados referentes aos valores de integração.
O MiniPET divide-se em quatro blocos estruturais principais: um sistema de detecção, que inclui duas matrizes de cristais cintilantes do tipo LYSO e dois foto-multiplicadores com 4_4 canais; uma placa que contém a eletrónica analógica rápida de "front-end"; uma placa de desenvolvimento, dotada de uma FPGA, que serve de plataforma de implementação da eletrónica digital de controlo; e, por último, um programa em C++ que faz uso de uma API proveniente do fabricante da placa da FPGA, que permite a comunicação entre um computador e a FPGA para, assim, ser possível registar os dados da detecção para posterior análise e configurar alguns parâmetros da electrónica analógica de "front-end".
The purpose of this thesis is to report on the design of a digital control system using VHDL, on its implementation into a development board that includes a FPGA, and on the design and implementation of the front-end's fast analogue electronics. All these tasks are pursued in the context of building a MiniPet instrument. The instrument's main purpose is to be useful as an educational tool. The PETs are characterized by having two detectors, face-to-face, that operate by doing a synchronized circular scanning around the analysis subject. The front-end's analogue electronics is responsible for processing and conditioning the signals that come out from the detectors. This electronic block gets a measure of the energy of the incoming photons, generated in the positron annihilation, by integrating the charge or current pulses spit out by the detectors, converts the integration values into a digital word with an ADC, and also generates a trigger for each detected signal. The digital control block manages the detection system, validates the coincidences between events occurring in both detectors and pre-processes all the data related with the integration values. The MiniPet is divided into four main structural parts: a detection system composed of two scintillating crystal LYSO matrices and two photo-multipliers with 4 _ 4 channels each; a front-end board that includes the fast analogue electronic components; a development board equipped with a FPGA for the implementation of the digital control blocks; and, finally, a computer program, written in C++, which makes use of an API library from the FPGA board's company, for establishing the communication between the computer and the FPGA, in order to be possible to record the detector data for posterior analysis and to configure some parameters in the front-end electronics.
The purpose of this thesis is to report on the design of a digital control system using VHDL, on its implementation into a development board that includes a FPGA, and on the design and implementation of the front-end's fast analogue electronics. All these tasks are pursued in the context of building a MiniPet instrument. The instrument's main purpose is to be useful as an educational tool. The PETs are characterized by having two detectors, face-to-face, that operate by doing a synchronized circular scanning around the analysis subject. The front-end's analogue electronics is responsible for processing and conditioning the signals that come out from the detectors. This electronic block gets a measure of the energy of the incoming photons, generated in the positron annihilation, by integrating the charge or current pulses spit out by the detectors, converts the integration values into a digital word with an ADC, and also generates a trigger for each detected signal. The digital control block manages the detection system, validates the coincidences between events occurring in both detectors and pre-processes all the data related with the integration values. The MiniPet is divided into four main structural parts: a detection system composed of two scintillating crystal LYSO matrices and two photo-multipliers with 4 _ 4 channels each; a front-end board that includes the fast analogue electronic components; a development board equipped with a FPGA for the implementation of the digital control blocks; and, finally, a computer program, written in C++, which makes use of an API library from the FPGA board's company, for establishing the communication between the computer and the FPGA, in order to be possible to record the detector data for posterior analysis and to configure some parameters in the front-end electronics.
Descrição
Tese de mestrado em Engenharia Física, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2010
Palavras-chave
MiniPet Electrónica analógica rápida Electrónica digital de controlo FPGA VHDL Teses de mestrado - 2010