why does the pulse height from a GM tube continue to increase with applied voltage even after a full Geier dischgr obtained ?
Answers
Are you referring to the curve of charge collected vs. the voltage applied (as in the link)? It shows the collected charge (i.e., size of the pulsed response) as a function of the applied voltage after a particle has triggered an avalanche. The higher the voltage, the more easily the discharge occurs and the bigger the avalanche. The G-M counter is typically operated in the voltage region where the response is insensitive to the exact voltage applied but where the voltage does not lead to a continuous discharge. Increasing the voltage beyond this region means that a particle irradiation would trigger a continuous discharge of the gas, and instead of a pulse there would be just continuous current. Hence the curve will essentially climb to infinity at this point, and the tube would of course be useless for detecting particles. So, if you are referring to the charge vs. voltage curve, I do not see any reason why it should not be a monotonically increasing function. (Although when the voltage is so high that there is a discharge current without any apparent triggering event at all, the curve is not really very meaningful.)