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3. Geiger Counter and
R adiation Basics
Radioactivity is the spontaneous emission of energy from the nucleus of certain elements, most notably
uranium. There are three forms of energy associated with radioactivity; alpha, beta and gamma radiation.
The classifications were originally made according to the penetrating power of the radiation.
Alpha r ays wer e found to be the nuclei of helium atoms, two pr otons and two neutr ons bound to-
gether. Alpha rays have a net positive charge. Alpha particles have weak penetrating ability; a couple of
inches of air or a few sheets of paper can effectively block them.
Beta r ays wer e found to be electr ons, identical to the electr ons found in atoms. Beta r ays have a net
negative charge. Beta rays have a greater penetrating power than Alpha rays and can penetrate 3mm of alu-
minum.
Gamma r ays ar e high-energy photons. This has the greatest penetrating power being able to pass through
several centimeters of lead and still be detected on the other side.
Images Digital Geiger Counters are sensitive to all three types of radioactivity.
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 cutaway drawing of the tube is shown below. The wall of the GM tube is a thin metal cylinder (cathode)
surrounding a center electrode (anode). It is constructed with a thin Mica window on the front end. The
thin mica window allows the passage and detection of alpha particles. The tube is evacuated and filled with
Neon, Argon plus Halogen gas.
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R ad ioa cti v i t y
T he G ei ger Mue l le r Tu be
Images SI, Inc.
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