Geiger counter is also called as G.M. Counter which stands for Geiger Muller Counter is a type of particle detector that measures ionizing radiation.It detects the emission of nuclear radiation- alpha particles , beta particles or gamma rays by the ionization produced in low pressure gas in Geiger Muller tube, which is the instrument name.It is the best known radiation instruments.The principle was discovered in 1908 in early radiation research.
G.M. Counter were invented by H.Geiger and E.W.Muller in 1928 , and used to detect radioactive particles.
A typical G.M. Counter consists of a G.M.tube having a thin edge window , a high voltage supply for the tube , a scalar to record the number of particles for the tube , and the time which will stop the action of the scalar at the time of preset interval .
Some of the Objectives are-
1) To verify the inverse square relationship between the distance and intensity of radiation.
2) To determine the maximum energy of decay of beta particle.
3) To investigate the relationship between absorber material , absorption thickness and back scattering.
PRINCIPLE USED :
G.M. Counter works on main principle which consists of two major elements in it : the Geiger Muller tube and the processing and display electronics.
The inert gas is a radiation sensing elements which briefly conducts electric charge when a particle or photon of radiation makes the gas conductive by ionization.
It produces an easily measured current pulse which is passed to the processing electronics.
When an ionization radiation enters the Geiger tube some of the energy of the radiation may get transferred to gas molecules within the tube.
This absorption of energy results in ionization , producing electron-ion pair .
1) Set-up for ST-350 counter.
2) G.M. tube and stand
3)Shelf stand , serial cable and source holder
4) Radioactive source.
The main limitations of G.M. Counter is that the inability to measure high radiation rates due to the dead time of the tube.
This is the insensitive period after each ionization of the gas during which any further incident radiation will not result in count , and the indicated rate is therefore lower than actual.
Typically the dead time will reduce indicated count rate above about 10^4 to 10^5 per seconds depending on the characteristics of the tube.
The output pulse of the G.M. Counter is always the same magnitude regardless of the energy of the incident radiation, the tube cannot differentiate between radiation types.
The HSE in the United Kingdom has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned.
This covers all radiation instrument technologies, and is a useful comparative guide to the use of G.M. detectors.
QUESTIONS AND ANSWERS BASED ON G.M. Counters.
Q 1) WHY IS ARGON GAS USED IN A G.M. COUNTER ?
Ans 1) Argon is electrically complete in the outer electron shell, making it Electro – chemically stable.However in the presence of ionizing radiation ,such as gamma or beta , electrons can be “knocked off” the inner wall of the GM tube by the radiation.
Q 2) WHAT IS THE DEAD TIME OF GM TUBE ?
Ans 2)When the beta from the decay product enters the Geiger Muller tube ,it collides with an atom ionizing it which in turn releases more electron.This causes the chain reaction of cascading electrons until it hits the electrodes which is measured as one count.After this event, the tube is filled with slow moving positive ions that advance towards the outer walls.
Q 3) A G.M. COUNTER IS USED FOR ?
Ans 3) Devices used for detecting and counting individual particles of radiation. The device is a gas filled metal tube with a wire through its axis and a high voltage applied to the wire. As a particle enter the tube , they create a large avalanche of ionization in the gas which than discharges , creating a brief electrical pulse
Q 4) What is the difference between dosimeter and GM tube ?
Ans 4) G.M. Counter measures the rate of radiation falling on it like some unit in rontgens per hour . A dosimeter indicates the total amount of radiation it has received so far.G.M. tube detects the presence and intensity of radiations while dosimeter indicated how much it has received.