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Páginas: 9 (2081 palavras) Publicado: 22 de janeiro de 2013
2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-04-5

Geraldo P. Araujo1, Arno H. Oliveira1, André C. Carneiro2, Clemente J. G. Carneiro2, Felix M. Milian2 e Fermin G.Velasco2.

Programa de Pós-Graduação em Ciências e Técnicas Nucleares - PCTN Universidade Federal de Minas Gerais - UFMG

geraldo.porto@hotmail.com, arnoheeren@yahoo.com.br Centro de Pesquisa em Ciências e Tecnologias das Radiações – CPqCTR Universidade Estadual de Santa Cruz – UESC Rodovia Ilhéus – Itabuna, KM 16 45662-900 Ilhéus, BA andreccarneiro@gmail.com, clementecarneiro@yahoo.com.br,felix_mas_milian@yahoo.com, fermin@uesc.br

Av. Antônio Carlos, 6627 Campus UFMG PCA 1 – Anexo Engenharia Pampulha – Belo Horizonte – MG CEP: 31.270-90


X-rays are produced by accelerating electrons with a high voltage and allowing them to collide with a metal target. This high voltage presents fluctuations that define peak, minimum, and average voltages. Different voltages areapplied to the X-ray tube depending on the radiographic applications. A rectifier circuit converts the alternating high voltage to unidirectional high voltage to accelerate electrons in this tube. The fluctuations on the energy in the electron beam depend on the mode of rectification. Both energy of the electrons and X rays intensity fluctuates. A portable probe built with a 6LiI(Eu) detector coupledto a 10 m light guide and a Hamamatsu photon counting head H9319 was used to measuring X ray intensities. This system is designed to collect up to 10000 counts in intervals of 10 ms to 1 s. Counts were accumulated in time intervals of 10 ms during 10 s. The system starts the count before activating the X-ray apparatus, which is on during a time interval of 100ms. During this period, counts mayoverflow in consequence high voltage was adjusted to be 40kV, in order to avoid such a problem . For each of these points dose was measured using an ionization chamber. The objectives of this work are to study fluctuations on the X-ray beam and to calibrate the portable probe for measuring radiation doses. Counting rates measured for each 10 ms presented strong variations due to high voltagesfluctuations. Both dose and counting rate when correlated with distances between source and detector followed the inverse square law and presented values of R2 near of unit. A calibration curve of the portable system for dose measurements showed also R2 value near of unity.

Key words: high voltage, X-ray tube, ionization chamber, light guide


Fluctuation of X-ray beam is relatedto wave form of voltage applied to the tube. However, because the transformer is connected to the network, so low-frequency (60 Hz), the waveform of voltage applied to the tube is a sine wave rectified. High voltage in an x-ray apparatus is traditionally obtained by a transformer voltage elevator whose output is rectified by a bridge of diodes and linked directly to the tube. The valve rectifierhas the same operating principle that an x-ray tube, the cathode is a tungsten filament and the anode is a metal plate or a cylinder around the filament. Rectifier systems used are the half wave, complete wave, and three-phase. Variations in voltage between anodecathode decrease the image quality obtained from an X-ray apparatus. Variation in the voltage applied to the tube produces a significantcontribution to the patient absorbed dose. Terini, R. C. P. et al, 2009 have evaluated the practical peak voltage (PPV) for different types of x-ray equipment. PPV, standard of voltage recommended by International Atomic Energy Agency (IAEA), is determined from the acquisition of the waveform of the voltage applied to the x-ray tube during the exposure. Portable radiation detection systems can be...
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