Characteristics of a tandem system of ionization chambers in X-ray beams, mammography level

Full text: Performance check tests of standard equipment are very important in dosimetric procedures. It is essential to verify if the dose measurements are accurate. It is very important to assure the good performance not only of the dosimetric equipment (such as ionization chambers), but also of the characteristics of the X-ray beams utilized in calibration laboratories. In this work, two parallel plate ionization chambers were studied in relation to their operational characteristics in order to be used for the quality control of X-ray beams. A quality control program of X-ray equipment is required for the periodic verification of its beams constancy. The two parallel plate ionization chamber were recently assembled at IPEN, and they have only one difference: one has an inner collecting electrode made of graphite and the other, of aluminium. These ionization chambers make up a tandem system, which consists of two dosimetric systems with different energy dependence that allows the determination of characteristics as the effective energy and air kerma rates of X-ray beams. The tandem system also allows the confirmation of the half-value layers or effective energies in X radiation beams, previously determined by the conventional method using well known absorber layers. The tandem system is a very simple measurement system: it is only necessary to study the ratio of the responses of the dosimeters obtained in X-ray beams to verify their energy constancy. These studies have been carried out at the Calibration Laboratory of IPEN in standard X radiation qualities of mammography level. The ionization chambers presented good results for the operational tests (saturation, ionic recombination, polarity effects, etc.) as recommended by the IEC 60731 [1] and IEC 61674 [2] standards. (author)