Answer to Question #3077 Submitted to "Ask the Experts"

First, I would recommend reviewing the text Radiation Detection and Measurement by G. Knoll cited in this topical area of ATE for an in-depth discussion of Geiger-Müller (G-M) counters and radiation response.

But to address your question, as a source moves away or towards the detector, there is a geometric variation in flux of particles (for example, alpha, beta, gamma, or x rays). Assuming no attenuation in air (which is only valid for medium- to high-energy photons), the particle flux (number per square centimeter) will vary by the inverse square of the distance. Thus, as one doubles the distance between the source and G-M counter, the flux will geometrically drop to one-fourth the value in the initial position. If you go four times the distance, the flux will decrease to one-sixteenth, and so on. G-M counters are basically particle counters, with varying efficiency for alpha, beta, gamma, or x-rays.

Depending on the active volume of the G-M counter, terrestrial gamma rays and ground-level cosmic rays will produce a background count rate. This needs to be subtracted from a gross count with a source present, to obtain net counts resulting from the source being measured.

G-M counters produce a count by an initial ionization causing the counting gas to avalanche and produce an electrical pulse. The period of time after an avalanche and full amplitude pulse, during which the G-M counter is insensitive to other ionizing events, is the dead time.

Another good technical reference for G-M counters is the IEEE Standard 309-1999 (ANSI N42.3-1999) on Test Procedures for G-M Counters.

David J. Allard, CHP