CAN HALF VALUE LAYER MEASUREMENTS BE USED TOGETHER WITH THE EFFECTIVE ENERGY TO OBTAIN CONVERSION COEFFICIENTS FOR X-RAY SPECTRA?

Conversion coefficients are a substantial vehicle in practical radiation protection to determine the dose (rate) of a given radiation field. According to ICRU report 57, their values shall be obtained by means of spectrometry. This is, however, a time-consuming complicated procedure that cannot be performed by all dosimetry laboratories. Therefore, it is desired to find acceptable alternative methods to replace spectrometry. One possibility is to set up the X-ray facility in accordance with international standard ISO 4037-1:1997 and use the tabulated values from that standard. However, this needs to be considered during the construction phase of the X-ray facility. In this work, the combined usage of half-value layer measurements and the effective energy (both with respect to air kerma) to determine the conversion coefficients is investigated and compared with the values obtained by spectrometry. The investigations utilise all combinations of the H-, W-, N- and L-series, reference distances of 1 and 2.5 m and aluminium and copper as attenuation materials. We find that for most of the radiation qualities, the investigated method results in conversion coefficients that show an unacceptable deviation from the conventionally true values. However, the values of conversion coefficients of selected N- and L-qualities could be reproduced with high accuracy (within ±1 %).