Appendix II: Basic Source Geometries and Attenuation Relationships

Abstract

The evaluation of radiation levels from a source is a fundamental problem in health physics. Common source confi gurations include the point, line, disk, and slab geometries. Although numerous computer codes are available for the assessment of these and more complicated geometries, it is often advantageous to have the capability to make rapid assessments using hand calculations before performing more detailed calculations. Knowledge of these basic geometries, as well as the capability to utilize them in dose estimates, will be valuable to the practicing health physicist. Source Confi gurations-No Shielding Point Source Geometry The point source approximation is applicable whenever the dose is calculated at a distance that is at least three times the largest source dimension. The student should test this statement by performing sample calculations for various geo-metries. A comparison of point and disk geometries will be provided later in this appendix. The exposure, absorbed dose, or dose rate () X r at a location a distance r from a point source is given by = 2 0 () / X r A G r (II.1) where 0 () X r = unattenuated exposure rate (R/hr), absorbed dose rate (rad/hr), or dose rate (rem/hr) at point r. Although () X r is normally expressed in terms of the Roentgen, the conversion to either rem or rad units is straightforward and these units are used rather loosely in external dose assessments. A = source activity (Ci) G = gamma constant (R-m 2 /hr-Ci) r = distance from the point source (m)