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

摘要

The periodically evaluation of ionization chambers and quality control of standard X-ray beams usedfor purpose to calibration is important to guarantee the calibration procedures of dosimeters and instruments usedin radiation protection, diagnostic radiology and radiotherapy routines to ensure that procedures are following thenational and international recommendations. Based on that, a quality control routine must be made periodically, toguarantee the good working of these reference ionization chambers and X-ray beams. In this work was comparedthe performance of different kind of ionization chambers in X-ray beams, level mammography. It has been used asreference a 6 cm~3 ionization chamber, from Radcal?, with Mylar? window, specific for use in mammographybeams. The performance of that ionization chamber was compared with other two 180 cm~3 (parallel platechambers) and two 6 cm~3 (cylindrical type). The ionization chambers’ stability has been made using a ~(90)Sr+~(90)Yradioactive source (20 MBq, 2007). After the stability control, the ionization chambers were placed on X-raybeams, mammography level (25, 27.5 and 35 kV, and current of 30 mA), tungsten target with additional filtrationof 0.06 mm of molybdenum for direct beams, and plus 2 mm of aluminum for attenuated beams. The results showthat the 6 cm~3 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 cm~3 ionizationchambers have an average variation of about 20% to direct beams, and about 7,8% to attenuated beams. It ispossible conclude, that the 180 cm~3 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 usedin radiation protection X-ray qualities.