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A very important part of the P-10 gas is the “10”, which is methane. Methane is a quench gas which needs to be replenished.
“Pure noble gases can be used for alpha counting at low voltages where the multiplication factor is below 100. As a rule however, a quench gas is added to prevent the proportional counter from acting like a geiger muller detector. During the formation of an avalanche, some gas molecules/atoms are excited rather than ionized. In other words, the energy absorbed by these proportional gas atoms/molecules promotes electrons to higher energy levels rather than frees them completely from the atoms/molecules. When the electrons deexcite and return to their original energy levels, they emit photons of visible light or UV. The problem with this is that these photons can interact with the proportional gas and cause the avalanche to spread along the anode. This can result in a non-linear relationship between the energy deposited in the detector gas and the size of the resulting pulse. These photons, particularly if they interact with the cathode wall, can also lead to the production of spurious pulses. The solution is to add a small amount of a polyatomic quench gas such as methane. The quench gas preferentially absorbs the photons, but unlike the fill gas (e.g., argon), it does so without becoming ionized.” From http://www.orau.org/ptp/collection/proportional%20counters/introprops.htm
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