Answer to Question #2426 Submitted to "Ask the Experts"

Q

What is the difference between dead time, resolving time, and paralysis time in GM detection?

A

A Geiger-Mueller (G-M) tube dead time is the time interval after the initiation of a normal-size pulse during which the tube is insensitive to further ionization events. (Also, see ATE Q&A number 1024.) The resolving (or resolution) time of a G-M tube, or when combined with a counting system, is the minimum time interval between two distinct ionizing events, which permit both to be counted.


I was not aware of a paralysis time G-M tube parameter per se. However, doing a key word search, it appears to be used the same as dead time. Nonetheless, Knoll's text (noted as a reference in this topical area of ATE) has an excellent discussion of models for dead time, paralysable, and nonparalysable response. In fact, G-M tubes don't have fixed-length dead times and some instruments may appear to read low in high-radiation fields. The old Civil Defense G-M counters, in particular the CDV-700, are a case in point. During a recent FEMA (Federal Emergency Management Agency) training course where we were searching for millicurie-level cesium-137 sources, I approached a source in a grass field where the source itself was not visible. The audio and rate meter continued to increase until I got right next to the source, then the instrument went silent, and the reading on the scaler went to zero. It was not until I backed off that the instrument recovered and again began to respond! That's the bad news. However, the good news is, most modern instruments have a dead time correction and a low-current circuit on the G-M tube and will maintain the audio signal and reading rather than let it fall off.


David J. Allard, CHP