Applicability of Hp(10) for assessment of effective dose in anisotropic photon fields V. Chumak, E. Bakhanova Radiation Protection Institute ATS Ukraine, Melnikova 53, Kiev 04050, Ukraine, E-mail: chumak@leed1.kiev.ua Abstract. 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 H p (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/ H p (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 H p (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 H p (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. 1. Introduction The effective dose E, according to ICRP recommendations [1], is included into majority of international and national safety regulations as a measure of individual radiation exposure that needs to be monitored. Since this quantity cannot be measured directly; the personal dose equivalent, H p (10), was recommended by ICRU [2] for individual monitoring as an operational quantity that can be moderately conservative estimation of effective dose in case of photon irradiation. Unfortunately, little attention was given to establishing the quantitative relationships between H p (10) and E until recently. The most comprehensive ICRP publication 74 [3] gives the tables of conversion coefficients from air kerma K a to effective dose organ doses for several standard irradiation geometries. However, it does not contain sufficient data for evaluation of conversion coefficients valid for other geometries of exposure. Kim [4] provided detailed angular dependencies of E for several photon energy lines demonstrating that effective dose has pronounced anisotropy regarding both polar and azimuth angles. Another serious drawback of ICRP 74 [3] is the lack of information on the angular dependence of H p (10). This task was approached by Zankl [5] by using a voxel phantom and simulation of a personal dosemeter as a sliding aggregate of several sub-surface voxels. It was demonstrated that under some unfavourable conditions of exposure, H p (10) may significantly underestimate effective dose. However, the author [5] had limited her consideration to five standard irradiation geometries. Trying to fill a gap in the published data we have modelled H p (10) to E conversion coefficients for variety of energies and directions of exposure in a broad parallel beam as well as for several placements of personal dosemeters Since practical application of the results was envisaged and since majority of modern personal dosemeters are designed in such a way that they are capable of measurement of H p (10) under broad variety of radiation fields, in this paper by personal dose equivalents H p (10) we understand the approximation of strictly defined H p (10), which is measured by properly designed and calibrated individual dosemeter. Therefore, as in our previous work [6], H p (10) symbol used throughout the text, represents the measurable approximation of this canonical operational quantity. 1