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In my class, we talked about the GM-counter and we were shown a graph with the voltage on the x-axis and intensity on the y-axis. But I do not understand why the graph has the characteristics it has, such as under a certain voltage it does not work, then it hits a plateau and suddenly it goes up again. I asked my teacher for clarification, but he was rather vague. He said that the particles need a certain voltage to get ionized (I understand this partly), but he also said that there was a continuous discharge above a certain voltage (the part I do not completely understand). I could not find the same graph as in my text book but I found one similar enough, so I have enclosed that picture. I hope someone can help me understand this concept. Thank you in advance.

Graph

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"Intensity" is the count rate, the number of voltage pulses with a height over a certain threshold value.

There is always a spread in pulse heights. A larger bias voltage will make all pulses higher. When all events due to radioactivity are higher than the threshold value, there is a plateau in count rate. This is the bias voltage that one wants to have. The count rate is stable then.

When the bias voltage gets too high, there will be spontaneous discharges that are not due to radioactivity. This must be avoided.

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  • $\begingroup$ Thank you for your reply! I understand why the intensity goes up again now, but this part 'There is always a spread in pulse heights. A larger bias voltage will make all pulses higher. When all events due to radioactivity are higher than the threshold value, there is a plateau in count rate.' did not make much sense to me. Could you clarify please? $\endgroup$
    – Yassin B.
    Commented Mar 13, 2018 at 22:56
  • $\begingroup$ @YassinB. When a gamma photon or a beta electron ionizes an atom in the GM tube, it depends on where in the tube this happens how large the charge pulse on the central wire will be. One wants all of those pulses to be counted, so one wants them all to be large enough to be higher than the discriminator threshold. When the bias voltage is too low, one only gets maybe half of them. Or maybe 40 % when there is a little bit of change in voltage or in temperature. So one wants the bias voltage to be on the plateau. $\endgroup$
    – user137289
    Commented Mar 13, 2018 at 23:06
  • $\begingroup$ Thanks again for taking the effort! If I understand correctly you want to hit that plateau so that the particle will be ionized wherever it is in the tube? So the counting is most optimal. That makes sense, I think that I understand it now. $\endgroup$
    – Yassin B.
    Commented Mar 13, 2018 at 23:10
  • $\begingroup$ @YassinB. Yes, one wants the ionization events to be multiplied (avalanche effect) so that the charge pulse on the central wire is large enough for the discriminator threshold. $\endgroup$
    – user137289
    Commented Mar 13, 2018 at 23:13

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