A Geiger Counter is a device that is used to detect radiation. The device needs careful handling as it is a sensitive instrument. In addition anybody dealing with radiation must take precautions so that he or she does not come in contact with radiation or its source. The basic aim of the Geiger Counter is detect radiation by the ionizing effects radiation creates.
Geiger counter is a gas filled detector, which relies on the ionizing effect of radiation to produce some sort of electrical impulses that may be counted electronically.
The basic layout of any gas-filled detector system is as follows:
The operation of the above detector is basically very simple. An incoming particle or photon produces free charge in the sensitive volume of the detector via the process of ionization of the gas atoms. Since an electric field exists between the central wire (anode) and the casing (cathode), the free charge (i.e. electrons and ions) are "swept up", towards the appropriate electrodes, and a brief, small current will flow. This current pulse causes a momentary potential difference to appear across , which is detected, amplified by the amplifier and counted by the electronics of the system.
The behavior of the system is directly affected by the magnitude of the potential difference applied between the anode and the cathode. This behavior is illustrated by the following graph of output pulse magnitude variation with the applied voltage also called Extra High Tension EHT.
A word of caution is not out of place here. A Geiger device used has a very thin glass screen that is costly to repair. Under no circumstances should you allow anything to touch the screen
1. Determination of the operating voltage of a Geiger counter.
a) Turn on the EHT and allow about 5 minutes for the system to warm up.
b) Place a radioactive source in position in the holder, below the Geiger tube window. It should
be about 2/3 Cm's away from the window.
c) Slowly increase the EHT until the system starts counting. This is commonly termed the T'hreshold Frequency'
d) From this point onwards, record and plot count rate vs EHT,(Take about 30 second readings).Continue readings to an EHT approximately 60% above threshold.
e) Record all readings and plot a graph. Select a midpoint which is like a plateau. This plateau will have a slope. Your exercise will be to calculate this slope.
2. Determination of the Slope of the Geiger Plateau.
One test of a Geiger tube is to measure the slope of the plateau of the characteristic. For a good Geiger tube, this should not exceed 0.1% per volt.
3. Dead time.
With any counter, there will be a period of time during and immediately following an ionising event when the system is insensitive to any further radiation. This time is called the “Dead Time”, and will obviously lead to loss in counts particularly at high count rates.
The Geiger counter is an excellent device but needs careful handling.
Radioactivity can be harmful. Hence take the following precautions.
a) Handle the R/A sources with tweezers
b) Keep R/A sources at arms length
c) Wash tour hands after handling radioactive material
d) Do not take any source of R/A out of the area of operation.