1. 1
THE GEIGER-MULLER TUBE
EXPERIMENT .
INTRODUCTION :
The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the
detection of ionizing radiation. It was named after Hans Geiger, who invented the principle in 1908 and
Walther Müller, who collaborated with Geiger in developing the technique further in 1928 to produce a
practical tube that could detect a number of different radiation types.
It is a gaseous ionization detector and uses the Townsend avalanche phenomenon to produce an easily
detectable electronic pulse from as little as a single ionizing event due to a radiation particle. It is used for
the detection of gamma radiation, X-rays, and alpha and beta particles. It can also be adapted to detect
neutrons. The tube operates in the "Geiger" region of ion pair generation. This is shown on the
accompanying plot for gaseous detectors showing ion current against applied voltage.
While it is a robust and inexpensive detector, the G–M is unable to measure high radiation rates
efficiently, has a finite life in high radiation areas and cannot measure incident radiation energy, so no
spectral information can be generated and there is no discrimination between radiation types; such as
between alpha and beta particles.
DISCUSSION OF APPARATUS :
Scaler
Holder for GM tube
2. 2
Thin window GM tube
Gamma source, as pure as possible e.g. Co-60 with a filter to stop β, or Ra-226 with a thick filter
Beta source, pure (Strontium 90)
Holder for radioactive sources
Gamma GM tube if available
Box of matches
High voltage power supply
Oscilloscope
THEORY AND BACKGROUND OF EXPERIMENT :
The ionizing effect of radiation is used in the Geiger-Muller (GM) tube as a means of detecting the
radiation. The GM tube is a hollow cylinder filled with a gas at low pressure. The tube has a thin window
made of mica at one end. There is a central electrode inside the GM tube. A high voltage supply is
connected across the casing of the tube and the central electrode as shown in the following diagram.
When alpha, beta or gamma radiation enters the tube it produces ions in the gas. The ions created in the
gas enable the tube to conduct. A current is produced in the tube for a short time. The current produces a
voltage pulse. Each voltage pulse corresponds to one ionising radiation entering the GM tube. The voltage
pulse is amplified and counted. The greater the level of radiation, the more ionisation in the tube so the
greater the number of counts. The GM tube counting the number of ionizations may not provide a
completely accurate reading, as the number of counts will simply keep increasing. The quantity Activity
gives an indication of how radioactive a substance is.
PROCEDURE :
3. 3
Put a radioactive source in a holder. Fix this in a clamp on a retort stand.
Put the Geiger-Müller tube in a stand. Adjust it so that it is pointing at the source, and is about 5
cm away from it.
Plug the Geiger-Müller tube into the scaler (counter) and switch on.
Start the voltage at about 200 volts. Make a note of the number of counts in, say, a 15 second
interval.
Increase the voltage in steps of 25 volts.
You will find that the counts vary with voltage and then reach a plateau.
After the threshold voltage, the count will reach a plateau. It will stay constant over a range of
voltages. Set the voltage at a value of between 50 to 100 V above the threshold.
If the clicking increases when you increase the voltage, then you have moved off the plateau.
Turn the voltage back down.
Put the source back in a safe place until you carry out the demonstration.
Switch on the Geiger-Müller tube counting system.
Highlight the fact that there is a background count.
Bring a radioactive source up to the Geiger-Müller tube and draw attention to the increase in
counts.
You could measure the background count and the count with the source nearby. Do this over a
period of 30 seconds. Draw attention to the difference.
OBSERVATIONS :
Activity is the number of radioactive atoms which disintegrate and emit radioactivity per second. Activity
is measured in units called Becquerels (Bq) named after Henri Becquerel, the French scientist who shared
the Nobel Prize for Physics in 1903 with his students - Marie and Pierre Curie.
Where time is in seconds (s) and activity is measured in Becquerels (Bq). The number of disintegrations
has no units. The number of disintegrations cannot be determined easily in practical work, but the count
of radioactive particles detected by a Geiger Muller counter is a useful approximation at this level, and
can give an indication of the rate of change of activity.
RESULT ANALYSIS :
Geiger-Müller tubes are set up to operate at a voltage within their ‘plateau’. In self-contained
systems, this is set automatically. The voltage across a Geiger-Müller tube is generally kept low
enough so as not to produce a roaring spark when an energetic particle enters it.
Geiger-Müller tubes are very delicate, especially if they are designed to measure alpha particles.
The thin, mica window allows alpha particles to enter the chamber. It needs a protective cover to
prevent it from being accidentally damaged by being touched. A good alpha detecting Geiger-
4. 4
Müller tube will also count photons. If you light a match in front of it, a few ultra violet photons
will be detected.
The actual phenomena inside a tube are much more complicated than the simple story of
ionization producing an avalanche of electrons. Inside the tube ultra violet photons probably play
an important part, as well as colliding electrons and ions, and the detailed picture is extremely
complex.
