The document summarizes the key components and functioning of a Geiger counter. It describes the three main parts: the transformer that amplifies the voltage to 400V needed by the Geiger tube, the Geiger tube which contains ionizable gas between electrodes and detects particles through ionization, and the signal stretcher which lengthens the short pulse detected to be visible or audible. It also provides background on radioactivity, isotopes, irradiation, contamination, and dose levels of radiation and their biological effects.

1. The Geiger Counter
Colin Hannesse and Frédéric Jacobs
With contributions from : Chloé Rousselle and Alexandre Jolly
With special thanks to : Loïc van Oldeneel and Luigi Siragusa
1 Physics notions
1.1 Radioactivity
Radioactivity is a physical natural phenomenon in which an
unstable nuclei is decaying by ejecting energy in different
ways:
• The Alpha ray (α)is the emission of an helium nucleus (2
protons, 2 neutrons), mostly observed for heavy nuclei.
• The Beta ray (β) is the emission of an electron or a
positron.
• The Gamma ray (γ) is observed in the two cases above.
It is an electromagnetic wave (same nature as light).
1.2 Isotopes
Two atoms are called isotopes if they have both the same
number of protons and a different number of neutrons.
Example : The carbon-14, written 14C, is the isotope of the
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carbon-12 (the carbon that is mostly found at the natural
state, written 12C).
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1.3 Irradiation and Contamination
• An object is irradiated when it is hit by a ionizing ray. It doesn’t make it radioactive.
• An object is contaminated when there are radioactive elements on it. The result is that the
whole is radioactive (object + radioactive dust).
Dosis in Sv (Sieverts) Symptoms
0,1 Treshold below which one never has found any effect of radiation on health.
0,2 Threshold below we do not observe any symptoms..
0,3 Small abnormalities on blood samples (less lymphocytes).
1 First clinical symptoms (nausea, vomiting).
2 Hospitalization necessary: bone marrow is affected (anemia, infections, bleeding).
3 First signs on the skin(redness), hair loss.
4,5 Lethal dose (50% mortality if untreated).
8 Bowel are reached (diarrhea) and respiratory functions too.
Death follows after several days or weeks.
10 Neurological symptoms (coma) followed by death in several days or weeks.
Sievert is a scale that marks the biological impact due to radiations.
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2. 2 The Electronic Components
The electronics of the Geiger counter is made out of 3 main parts. The transformer, the Geiger
tube and the signal stretcher.
After having passed in the chop-
per, the current coming from the bat-
tery is transformed into alternating
current. Then, the transformer am-
plifies the voltage up to 400 Volts
that is needed by the Geiger tube
to work. This high voltage on the
electrodes allows the current passing
through the tube when a particle en-
ters the tube. This event lasts only
a few microseconds. If the signal was
directly converted to a flashlight or
sound, it would be to short to be seen
or heard. For that specific reason, we
added a signal stretcher that allows
the signal to be long enough to be
seen. Given that this is an digital cir-
cuit, this means the output is either
on or off and its state doesn’t depend on the intensity and speed of the particles.
The Geiger tube is made of a metallic tube filled with ionizable gas in which are placed two
electrodes.
Above are represented the two different cases viewed from inside the Geiger tube.
• When no particle enters the tube, no current can pass through the tube because the gas is
not naturally conductive.
• When a particle enters the tube, it is likely to grab electrons from the gas. This is called
ionization and now the gas is conductive during a short moment. Therefore the current can
pass through it.
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