Determining the Effective Dose Equivalent for External Photon Radiation: Calculational Results for Beam and Point Source Geometries

In 1994, US nuclear plants must implement a new Code of Federal Regulations which adopts the methodology of ICRP 26 for effective dose equivalent (HE) from internally deposited radionuclides. For exposure to photons from sources outside the body, the new regulations propose an ad hoc extension of the ICRP 26 methodology by defining a new tissue - 'the whole body' - and assigning it a weighting factor of 1. This study provides basic data to build a methodology for assessing HE based on the original ICRP 26 organ dose methods. Specifically, Monte Carlo photon transport calculations were done with anthropomorphic male and female phantoms and a hermaphroditic phantom to calculate organ doses, and thereby HE for each gender, for selected photon energies, for photon beams at any three-dimensional angle, and for point sources anywhere external to the torso. It was found that anterior-posterior beam exposures created the highest HE per unit fluence among all azimuthal and polar beam angles. It was also found that point sources in contact with the gonads yield the highest HE per photon emitted for males and point sources on the sternum yield the highest HE per photon emitted for females.