Evaluation of ionization chambers in X-ray beams used for instruments calibration

Abstract

The periodically evaluation of ionization chambers and quality control of standard X-ray beams used for purpose to calibration is important to guarantee the calibration procedures of dosimeters and instruments used in radiation protection, diagnostic radiology and radiotherapy routines to ensure that procedures are following the national and international recommendations. Based on that, a quality control routine must be made periodically, to guarantee the good working of these reference ionization chambers and X-ray beams. In this work was compared the performance of different kind of ionization chambers in X-ray beams, level mammography. It has been used as reference a 6 cm³ ionization chamber, from RadcalTM, with MylarTM window, specific for use in mammography beams. The performance of that ionization chamber was compared with other two 180 cm³ (parallel plate chambers) and two 6 cm³ (cylindrical type). The ionization chambers’ stability has been made using a 90Sr+90Y radioactive source (20 MBq, 2007). After the stability control, the ionization chambers were placed on X-ray beams, mammography level (25, 27.5 and 35 kV, and current of 30 mA), tungsten target with additional filtration of 0.06 mm of molybdenum for direct beams, and plus 2 mm of aluminum for attenuated beams. The results show that the 6 cm³ cylindrical ionization chambers have a maximum variation of 5.08% in the energies range studied, the results show that they could be used as working standard in the calibration laboratory. The 180 cm³ ionization chambers have an average variation of about 20% to direct beams, and about 7,8% to attenuated beams. It is possible conclude, that the 180 cm³ ionization chambers are not very efficient to low energies measurements, although they also can be used in diagnostic radiology X-ray qualities, but to higher energies, and can also be used in radiation protection X-ray qualities.