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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
-joe
Joe Geller
Geller MicroAnalytical Laboratory, Inc.
426e Boston St., Topsfield, MA 01983
tel 978 887-7000, fax 978 887-6671
www.GellerMicro.com