Correct voltage for a Geiger-Muller tube

 

I was showing my class how to detect background radiation. The tube works by setting up an electric field between an anode needle in the middle and a cylindrical cathode round the outside. There is a low pressure gas between. When the gas is ionised by incoming radiation, it triggers and avalanche of ionisations that result in a strong electrical pusle that can be counted. A particular voltage has to be maintained between the anode and the cathode to make this happen. I had it in my head that the correct voltage is 415V. Looking it up, I found this https://spark.iop.org/geiger-muller-tube#gref It suggests an experiment that I have never tried. The amount of ionisation depends on the voltage but there is a plateau section where a range of voltages will give a steady reading. So one particular voltage isn't necessarily the right answer and there is an experimental method to find the optimum voltage for my tube.