Study of the influence of phantom Material and size on the calibration of ionization chambers in terms of absorbed dose to water

Abstract

In the IAEA?s code of practice (TRS 398) and AAPM?s dosimetry protocol (TG 51), full scatter water phantoms are recommended for the determination of the absorbed dose for both photon and electron beams and consequently for the calibration of the user?s ionisation chambers. This procedure is applied in secondary Standard Dosimetry Laboratories where the calibration is performed in a 60Co gamma beam, by comparison with reference chambers whose absorbed dose to water calibration coefficients, ND,w, are known. In this work, we present the results of the calibration of nine Farmer like ionisation chambers calibrated in three water phantoms (with sizes 20 x 20 x 15 cm3 , 30 x 30 x 30 cm3 and 40 x 40 x 47 cm3 ) and two 20 x 20 x 20 cm3 plastic phantoms (PMMA and polystyrene). Calibrations are performed by the substitution method using an ionization chamber whose ND,w has been supplied by the IAEA?s reference laboratory. It is shown that the results, expressed as the percentage ratio of the calibration coefficient in a given phantom to that of the standard IAEA phantom, is less than 0.3% for all the investigated chambers, and that the standard deviation of the mean of the ND,w calibration coefficients determined in all the 5 phantoms is less than 0.06%. Furthermore, a chamber-to-chamber dependence of the calibration coefficient has been shown to vary by up to 2.8%. These results emphasize that the phantom dimensions and its material are not sensitive criteria for the calibration of cylindrical ionisation chambers in terms of absorbed dose to water, and show that generic calibration coefficients could not be considered for a given type of chamber.