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.
"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.