Evaluation of mean conversion coefficients from air-kerma to H (10) using secondary and transmitted x-ray spectra in the diagnostic radiology energy range.

Ambient dose equivalent H (10) is an operational quantity recommended by the IAEA to establish dose constraints in area monitoring for external radiation. The direct measurement of H (10) is not common due to the complexity in the calibration procedures of radiation monitors involving the use of expanded and aligned radiation fields. Therefore, conversion coefficients are used to assess H (10) from the physical quantity air-kerma. Conversion coefficients published by international commissions, ICRU and ICRP, present a correlation with the radiation beam quality. However, Brazilian regulation establishes 1.14 Sv Gy as unique conversion coefficient to convert air-kerma into H (10), disregarding its beam quality dependence. The present study computed mean conversion coefficients from secondary and transmitted x-ray beams in order to improve the current assessment of H (10). The weighting of conversion coefficients corresponding to monoenergetic beams with the spectrum energy distribution in terms of air-kerma was used to compute the mean conversion coefficients. In order to represent dedicated chest radiographic facilities, an anthropomorphic phantom was used as scatter object of the primary beam. Secondary x-ray spectra were measured in the diagnostic energy range at scattering angles of 30°, 60°, 90° 120° and 150° degrees. Barite mortar plates were used as attenuator of the secondary beam to produce the corresponding transmitted x-ray spectra. Results show that the mean conversion coefficients are about 43% higher than the recommended value accepted by Brazilian regulation. For secondary radiation measured at 100 kV the mean coefficient should be 1.46 Sv Gy, which represent the higher value in the mean coefficient set corresponding to secondary beams. Moreover, for transmitted x-ray beams at 100 kV, the recommended mean conversion coefficient is 1.65 Sv Gy for all barite mortar plate thickness and all scattering angles. An example of application shows the discrepancy in the evaluation of secondary shielding barriers in a controlled area when the shielding goals is evaluated. The conclusion based on these results is that a unique coefficient may not be adequate for deriving the H (10).