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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Bjørn wrote ...
I have tried this and did not manage to trigger it with an Am-241 source. The only thing I found that would trigger it was a white LED.
Even non-window GM tubes may have a problem detecting 241Am except over long counting periods. The γ detection efficiency of GM tubes is very low - typically <0.05% - 0.1%. - The γ spectrum of 241Am is, in any case, distinctly feeble - 59.5 keV 35.9 %, 26.3 keV 2.4 %, and 13.9 keV 42 % - with only the 59.5 keV having any hope of making their presence known - except in extended time domain experiments when the 26.3keV might be detected in a statistical fashion with a little patience.
While alpha and betas can interact directly with the counting gas in a GM tube, and so be detected with high efficiency, gammas must knock Compton and photoelectric effect electrons out of the tube wall to initiate the Townsend avalanche that will register as a click. But with the neon bulb, we have no metal tube with which the incoming gammas can interact, but only glass, which will produce far fewer interactions.
Now given that the sensitivity of a GM tube is dependent on gas volume, what shall we call the effective (potentially active) gas volume of a neon bulb? Presumably, the volume of the space between the two electrode wires - a tiny number compared with even the smallest and least sensitive steel cased dosimetry type GM tubes.
My point: it is possible that the neon bulb might be functioning as a detector, but with such poor efficiency that only very long counting periods - or high dose rates - could hope to show it. The neon bulb might be brilliant - and glow brilliantly - at 1000 Gy/hr!
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neon lamp as GM tube
