Answer to Question #382 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — External dose calculations

The following question was answered by an expert in the appropriate field:

Hello. I have a technical question that I hope someone here can help me with. I'm having a hard time trying to understand how to calculate dose equivalent rates (for gamma radiation), in mSv/h, for example. Specifically: 1. Do "specific gamma ray dose constants" obey the inverse-square law? I have a standard table for values derived/calculated (at a distance of 1 m) for nuclides such as ¹¹C, ¹²⁵I and so forth. At a distance of 2 m, for example, are the values of these gamma constants, 1/4 of their original values (i.e. at a distance of 1 m)? 2. What role does surface area (upon which the gamma rays are incident) play in the dose equivalent rate? Is there any relationship between (incident surface area and final dose equivalent *rate*)? Thank you!

The gamma ray constants do obey the inverse-square law when used for points sources. Corrections will need to be made for line or area sources.

With regard to your second question, I'm assuming the question refers to small or localized areas of the body being subjected to gamma radiation.

Typical whole body dose calculations are based on the fact that the entire body is exposed to the same gamma fluence. If a small area of the body is exposed, such as in a directional source, then a "weighting factor" can be used to determine the committed effective dose equivalent. The weighting factor will take into account the area of the body and what internal organs are exposed.

Jerry Hensley
Battelle Pacific Northwest National Laboratory

Answer posted on August 11, 2000. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.