Answer to Question #2255 Submitted to "Ask the Experts"

Q

I am a little confused by the theory of operation of the Geiger-Mueller tube. If a gamma ray passes through the tube it will hopefully generate an ion pair which will cause an avalanche that results in a count. Can a single gamma result in more than one count or does dead time preclude this? Would a single gamma produce more than one count if the dead time was zero since the volume of the tube is limited? Does the energy of the gamma affect the counting? I have the same questions for beta and alpha particles.

A

Regarding Geiger counters, with a Geiger-Mueller (G-M) tube or probe.

A G-M tube is a gas-filled device with a high voltage applied to the cathode G-M tube wall and central anode wire. When a gamma ray, beta or alpha particle enters the interior sensitive volume, an ion pair is formed most often in from the G-M tube wall with gammas, where the beta or alpha will directly ionize the counting gas in the G-M tube. A gamma interacting through a photoelectric or Compton interaction will most likely produce only a single ion pair. As you've noted, when the electron rapidly migrates toward the anode, an avalanche of other electrons is produced and we have "gas multiplication." This creates an electrical pulse each time these radiations interact with the wall or gas in the tube. However, when the avalanche arrives at the anode, the voltage drops, and the G-M tube is not active to additional radiation detection. This is the "dead time." As a G-M tube ages, quench gas may become less effective, and you can have multiple pulses with a single radiation detected—not a desirable scenario. These detected pulses are converted to a reading on the instrument meter. If the instrument has a speaker, the pulses also give an audible click. Common readout units are roentgens per hour (R/hr), milliroentgens per hour (mR/hr), rem per hour (rem/hr), millirem per hour (mrem/hr), and counts per minute (cpm). G-M probes (e.g., "pancake" type) are most often used with hand-held radiation survey instruments for contamination measurements. G-M probes are most often calibrated to ¹³⁷Cs (actually ^137mBa) 662 keV gamma rays, and when measuring cpm per mR/hr below 100 keV,depending on the G-M tube construction, one will see an over response from 2 to 10 times the ¹³⁷Cs response. However, energy-compensated G-M tubes may be employed for more flat "exposure" measurements.

David J. Allard, CHP