Conversion from dosemeter readings to effective dose in individual dosimetric monitoring: effect of anisotropy of photon fields and dosemeter design

Monte Carlo simulation (MCNP-4B code, ADAM phantom) was employed in order to estimate organ doses and doses to personal dosemeters of particular commercial design (one type of Harshaw dosemeter and two designs of RADOS dosemeters) in situations of irradiation of a person in photon radiation fields. For each photon energy and beam direction the partial conversion coefficients were calculated providing thus raw data for modeling of effective conversion coefficient in any photon radiation field. In practical situations, when specific orientation of a body in radiation field is not known or a person is changing his position and orientation, the most robust parameter representing geometry of exposure is anisotropy of radiation field. In order to assess effective conversion coefficients for variety of workplace conditions, both anisotropic field configuration and orientation of a person in radiation field were stochastically simulated and 95% percentile of stochastic distribution was taken as a value of effective conversion coefficient for given conditions of exposure. The value of effective conversion coefficient depends on anisotropy of radiation field and can be quantitatively derived for appropriate classes of anisotropy. For quasi-isotropic and moderately anisotropic fields effective conversion coefficient does not exceed unity (e.g. Hp(10) maintains conservatism) for all considered dosemeter types, while in strongly anisotropic fields Hp(10) is not conservative anymore and can underestimate effective dose. The described approach is illustrated by examples of appropriate effective conversion coefficients for several commonly used dosemeter types.