Answer to Question #1405 Submitted to "Ask the Experts"

This is a very good Geiger-Mueller (GM) tube question. With low count rates and no dead-time loss, a monoenergetic beam, and a uniformly irradiated detector, a particular GM tube's response in cpm per mR h^-1 (or μGy h^-1 to air) is a function of its active cross-sectional area. Manufacturers publish such response data for the various style GM tubes (for example, a "pancake" GM tube is about 3,500 cpm per mR h^-1 for ¹³⁷Cs). Thus, to correct for under response, you need to know the beam area and cross-sectional area of the "active length or window" of the GM tube in question. Specifically, the narrow gamma-ray beam is only producing secondary electrons from that area of the GM tube wall and resulting in a count rate lower than if the GM tube was uniformly irradiated. This is a common problem with the measurement of x rays streaming through a small housing gap or shielding void with analytical x-ray equipment. The Health Physics Society has just published ANSI Standard N43.2-2001 on the safety of analytical x-ray equipment. Section A.4.2 of the N43.2-2001 discusses using the ratio of the detector to beam areas as a multiplicative scaling factor to correct this underresponse. This ANSI standard is on the Health Physics Society Web site, free to members.

David J. Allard, CHP