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Home > Articles >Build Your Own Geiger Counter, Page 2
Geiger Mueller Tube
Geiger
Mueller tubes are simple devices that detect and measure radioactivity.
The original design by H. Geiger and E.W. Mueller in 1928 hasn’t change
very much. The basic sensor functioning remain the same.
A cut away drawing of a typical Geiger Mueller (GM) tube is shown in Figure 2.
The wall of the GM tube is a thin metal (cathode) cylinder surrounding
a center electrode (anode). The metal wall of the GM tube serves as the
cathode of the GM Tube. The front of the tube is a thin Mica window
sealed to the metal cylinder. The thin mica window allows the passage
and detection of the weak penetrating alpha particles. The GM tube is
first evacuated then filled with Neon, Argon plus Halogen gas.
Figure 2. Cross-section and function of typical Geiger Mueller tube
Our
GM tube is put into an initial state (ready to detect a radioactive
particle), by applying + 500-volt potential to the anode (center
electrode) through a ten mega ohm current limiting resistor. A 470K-ohm
resistor is connected to the metal wall cathode of the tube and to
ground. The top of the 470K resistor is where we see our pulse signal
whenever a radioactive particle is detected.
In
this initial state the GM tube has a very high resistance. However,
when a radioactive particle passes through the GM tube, it ionizes the
gas molecules in its path and creates a momentary conductive path in
the gas. This is analogous to the vapor trail left in a cloud chamber
by a particle. In the GM tube, the electron liberated from the atom by
the particle, and the positive ionized atom both move rapidly towards
the high potential electrodes of the GM tube. In doing so they collide
with and ionize other gas atoms, creating a momentary avalanche of
ionized gas molecules. And these ionized molecules create a small
conduction path allowing a momentary pulse of electric current to pass
through the tube allowing us to detect the particle.
This
momentary pulse of current appears as a small voltage pulse across the
470 K ohm resistor. The halogen gas quickly quenches the ionization and
the GM tube returns to its high resistance state ready to detect more
radioactivity.
Build Your Own Geiger Counter pg 3 "Dead Time"
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