Answer to Question #4405 Submitted to "Ask the Experts"

Q

Line vs Point Source Calculations At what point do I use a point source calculation,inverse square law vs inverse law, as I back away from a line source. I have always used L/2 as this point but some of my study material is using L (being the length of the line source) for ¹³⁷Cs. And does it make a difference what isotope is the source?

A

The general rules of thumb to answer the question: "when may an extended source be treated as a point source" are as follows: Any source for which the source extent is small compared to the distance to the detector can be treated as a point source:

1. When the distance is three times the largest source dimension this approximation only introduces a few percent error.
2. When the distance between the point of interest and a line source is great, such that the angle between the point of interest and the top and bottom of the source is less than 20 degrees, then the line source can be treated as a point source with an error of only a few percent.

The radionuclide being modeled does not impact the appropriateness of this type of assumption. If the reader is interested in learning more about the point kernel technique he/she is referred to the Engineering Compendium on Radiation Shielding edited by R.G. Jaeger et al., Springer-Verlag: New York; 1968, ISBN (Library of Congress Catalog number) 68-19816. (It is suggested that the reader consider Volume One of the three-volume set for such basic types of questions).

Another useful reference could be the Reactor Shielding Design Manual by Theodore Rockwell, D. Van Nostrand Company Inc., New York; 1956.

Richard R. Brey, PhD, CHP