Answer to Question #2997 Submitted to "Ask the Experts"

For mica window G-M tubes, the typical specification limit is 50 degrees Celsius (C); for nonwindow steel-walled tubes 75 degrees C, and for specialized nonwindow industrial gauging steel-walled tubes 200 degrees C. All of these G-M tubes are sealed and, as the temperature rises excessively on a mica window tube, I'd expect the gas volume to increase, perhaps to the point where the mica bursts. This will not happen with a windowless steel-walled tube (though it may fail because of a leak). However, with a temperature rise in this case, the internal pressure will also rise because it is a fixed volume. If you're in the temperature specification limits, you may see the starting voltage and plateau shift somewhat, but the plateau should be long enough where sensitivity doesn't change more than a few percent. Thus, for a fixed background (cosmic ray and terrestrial gamma particle fluence) you should not see a significant increase in background. Lastly, the metals used in G-M tube construction are well "passivated" too, so I wouldn't expect any reduction in quench gas at higher temperatures, or spurious counts.

With ion chambers (which are normally air filled and open to the atmosphere), having an increasing temperature, I'd expect the gas to expand out of the collection volume, thus your air mass and sensitivity would be reduced. This would cause a reduction in the current observed with a fixed background fluence, or some higher-level field one may be trying to measure. Calibrations for ion chambers are normalized to standard temperature and pressure conditions, thus measurements need to be corrected if made at a temperature and/or pressure that is significantly different.

With a scintillation detector (for example, an NaI coupled to a photomultiplier tube [PMT]), I would expect the background to increase with temperature. This is because the PMT's photocathode will have increased thermionic emission of electrons with increasing temperature. This is why a liquid scintillation counter will have a refrigeration system, to reduce background noise with low-level counting of tritium and other beta emitters.

David J. Allard, CHP