Applicability of Hp(10) for assessment of effective dose in anisotropic photon fields

A readout of an appropriately calibrated individual dosemeter placed on the surface of the anthropomorphic phantom could be considered as a good approximation of the personal dose equivalent Hp(10) that is recommended by ICRP and ICRU as a conservative estimation of effective dose in radiation protection. The response of such a dosemeter as well as the effective dose are modelled by Monte Carlo in order to study angular dependencies of the E/ Hp(10) conversion coefficient for the phantom irradiated by parallel photon beam within the energy range from 50 keV to 1 MeV. Beam directions varied in 4π geometry with an angle increment 30o. It is demonstrated that application of Hp(10) is inadequate for estimation of effective dose in strongly anisotropic radiation fields when irradiation occurs from certain “unfavourable” directions when significant under estimation of effective dose occurs. With regard to application of Hp(10) for approximation of E, typical workplace conditions were classified as isotropic, quasi-isotropic and anisotropic. For isotropic and quasi-isotropic radiation fields application of integrated conversion coefficients was proposed. Stochastic simulation was employed for assessment of variations of these group conversion coefficients as a function of conditions of exposure. From the analysis of resulting distributions of random realizations of conversion coefficients it is possible to find some effective value of the conversion coefficient that gives reasonably conservative estimation of the effective dose. These parameters were derived under the assumption of energy spectrum typical for Chernobyl “Object “Shelter” conditions.