A STUDY OF THE ANGULAR-DEPENDENCE PROBLEM IN EFFECTIVE DOSE-EQUIVALENT ASSESSMENT

The newly revised American National Standard N13.11 (1993) includes me asurements of angular response as part of personnel dosimeter performa nce testing. However, data on effective dose equivalent (H-E), the pri nciple limiting quantity defined in International Commission on Radiol ogical Protection (ICRP) Publication 26 and later adopted by U.S. Nucl ear Regulatory Commission (NRC), for radiation incident on the body fr om off-normal angles are little seen in the literature. The absence of scientific data has led to unnecessarily conservative approaches in r adiation protection practices. This paper presents a new set of fluenc e-to-H-E conversion factors as a function of radiation angles and sex for monoenergetic photon beams of 0.08, 0.3, and 1.0 MeV. A Monte Carl o transport code (MCNP) and sex-specific anthropomorphic phantoms were used in this study. Results indicate that Anterior-posterior (AP) exp osure produces the highest H-E per unit photon fluence in all cases. P osterior-anterior (PA) exposure produces the highest H-E among beams i ncident from the rear half-plane of the body. H-E decreases dramatical ly as one departs from the AP and PA orientations. The results also in dicate that overestimations caused by using isotropic dosimeters in as sessing effective dose equivalent from near-overhead and near-underfoo t exposures are 550%, 390%, and 254% for 0.08, 0.3, and 1.0 MeV, respe ctively. Comparisons of the angular dependence of H-E with those based on the secondary quantities defined in International Commission on Ra diation Units and Measurements (ICRU) Reports 39, 43, and 47 show sign ificant differences. This paper discusses why more accurate assessment s of H-E, are necessary and possible. An empirical equation is propose d which fan be used as the optimum dosimeter angular response function for radiation angles ranging from 0 degrees to 90 degrees for dosimet er calibration, performance testing, and design.