3.6. Geiger-Mueller Counters 197
However since at lower voltages the avalanche multiplication can not be achieved
unless the pressure is lowered therefore in such situations the gas pressure is lowered
to less than atmospheric pressure. The drawback to this approach as compared
to the tubes that operate on atmospheric pressure is that they require specially
designed containers and walls to be able to withstand the pressure gradients. In any
case it must be ensured that the Paschen condition remains fulfilled.
We saw earlier that even an small amount of electronegative contaminant in the
filling gas can drastically decrease the electron population due to parasitic absorp-
tion. In proportional counters this results in a decrease in the output signal strength.
For GM counters the problem is not of signal strength, rather the spread and sus-
tainment of the avalanche. With too much absorption of the electrons this process
may die out soon enough and the resulting pulse may not be high enough to pass
the discriminator threshold. Therefore a necessary condition for proper operation of
a GM counter is that its filling gas should be as free from electronegative impurities
as possible.
3.6.D Quenching.............................
The positive ions moving towards the cathode attract electrons from the cathode
wall. The impact of these electrons on the ions may for them to emit more elec-
trons. These new electrons can initiate another breakdown process. The result is
a pulsating response of the detector after the main signal pulse has died. The two
most commonly used quenching methods to reduce the dead time of a GM counter
can be classified asinternalandexternalquenching methods.
D.1 InternalQuenching.......................
In this type of quenching a small amount of a polyatomic gas is added to the main
fill gas in a concentration of around 5-10%. The main idea is to transfer the positive
charge of the ion produced in the avalanche process to a molecule that, unlike the
original ion, does not subsequently emit an electron thus suppressing the probability
of another avalanche. This implies that there are two conditions that must be met
by the added gas:
It should have low enough ionization potential to ensure that the charge is
efficiently transferred.It should not de-excite by emitting an electrons.These two conditions are fulfilled by many polyatomic molecules. They have
low ionization potentials and a number of closely spaced vibrational energy levels.
Furthermore they generally de-excite by dissociating into simpler molecules.
D.2 ExternalQuenching
The externally applied electric field helps the detector in multiple pulsing and is
therefore the main cause of the dead time. An obvious method to solve this problem
is then to decrease the high voltage rapidly so that the subsequent avalanches do
not occur.