Optimisation of Monitor Weighting Factors for the Estimation of Effective Dose Equivalent from External Photon Exposures

A procedure is proposed for using front and back personnel monitors for the estimation of effective dose equivalent HE from external photon irradiation. The optimised model calls for weighing the front monitor derived estimate of HE by 0.7 and the back monitor HE estimate by 0.3. In order to arrive at these weights, published coefficients for the conversion of physical and operational quantities to HE were used in simulations of HE, and published conversion coefficients for the conversion of phantom calibrated dosemeters were used to simulate idealised dosemeter based quantities. Radiation exposures were modelled as mixtures of antero-posterior (AP) and postero-anterior (PA) photon irradiations for which the AP variable was drawn randomly from normal to uniform distributions having mean values of 0 to 100% AP exposure. The results show that for a wide range of photon energy and randomly varying (i.e. 'uncertain') irradiation geometries, both the accuracy improves and uncertainty in HE estimation is minimised using the two-dosemeter approach compared to a single monitor. The described approach is applicable for a wide variety of exposure scenarios in which the energy and geometry of irradiation are uncertain. In addition, the approach provides a method for testing the effects of external constraints, such as regulatory limits on the uncertainty and accuracy of monitor based estimates of HE.