UtilizaÃÃo do dosÃmetro Fricke para dosimetria de mÃquinas teleterapÃuticas, irradiadores gammacel e aparelhos de raios-X superficiais

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

The chemical dosimetry has been used as a reference in the measurement of the absorbed dose, with the Fricke dosimeter, at standardization laboratories. This dosimeter is considered an absolute dosimetric standard for measurements of radiation doses ranging from 20 to 400 Gy, covering a wide energy spectrum, from X-ray, g-ray to fast electron beam. The objectives of the present work are the implementation of the traditional Fricke dosimeter at Centro Regional de CiÃncias Nucleares (Brazilian Regional Center of Nuclear Sciences) â CRCN/CNEN - and the development of a methodology for its use with low energy X-ray, that is, X-ray with average energy ranging from 19.7 KeV to 52.2 KeV. This methodology is required so that a wider range of application of the Fricke dosimeter is viable, including low energy X-ray, used for the treatment of superficial tumors. The proposed method was acknowledged by means of intercomparison measurements with ionization chambers calibrated for Kerma measurements in air with Co-60 radiation beams and low energy (superficial) X-ray. When used in the dosimetry of high-energy gamma radiation sources, Co-60, the performance of the Fricke dosimeter was assessed by comparing its results with those obtained with a cylindrical ionization chamber, traceable to PTB â Physikalisch-Technischen Bundesanstalt â and to NPL- National Physical Laboratory. For the reproducibility analysis of the Fricke Dosimeter, a Gammacell irradiator was used, with 5 samples of Fricke solution symmetrically distributed. The irradiation was performed for 8 experiments, with irradiation times ranging from 6.4 to 160 seconds. The induced doses were evaluated and, by means of statistical assessment, a standard deviation of 1.5% was obtained. For measurements of the quantity Absorbed Dose in Water in X-ray beams, correction factors were used for the response of the Fricke Dosimeter in relation to a parallel-plate ionization chamber, traceable to PTB. The results show that the traditional Fricke Dosimeter is operating properly for high-energy (Co-60) dosimetry and, to obtain an uncertainty around 3%, absorbances with values higher than 0.15 shall be used, corresponding to a dose of approximately 40Gy. These operating conditions of the Fricke dosimeter comply with the ASTM E 1026-04 standard specifications. For superficial X-rays dosimetry, it was necessary to calibrate the response of the Fricke dosimeter with the ionization chamber, as in this energy band the Fricke dosimeter is not absolute and shall be calibrated. The introduction of this step has induced a new uncertainty, which was assessed and included in the calculation of the total uncertainty. The uncertainty in the superficial X-ray dosimetry, for doses above 62 Gy, with the Fricke dosimeter is about 10%. This is explained as, due to the accuracy correction of the Fricke dosimeter in relation to the ionization chamber, an absorbance of 0.15 corresponds to a dose of 62 Gy, in the case of the worst accuracy. As the sensitivity of the dosimetric method is limited by the measurement in the spectrophotometer of +3Fe concentration induced by radiation, this brings to the conclusion: the Fricke dosimeter is an adequate instrument to measure superficial X-rays ranging from 42 Gy to 620 Gy

ASSUNTO(S)

dosÃmetro fricke radioproteÃÃo utilizaÃÃo em raios-x de baixa energia aplicaÃÃo em medicina engenharia nuclear dosimetriaquÃmica implementaÃÃo para o crcn (centro regional de ciÃncias nucleares) medidas de dose de radiaÃÃo

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