By Sunil Bhardwaj

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When the radioactive radiations are incident, on phosphor. They produce flash of light, this phenomenon is known as scintillation. ZnS and Ba[Pt(CN)6] are phosphors for $$\alpha$$-rays, The scintillation can be measured in terms of the pulse of current, with the help of photomultiplier tube (PMT) and electrometer.

In 1947, H. Kallman devised the scintillation counters. It is consist of following three components:

1. Phosphor: It may be organic or inorganic compound. Organic solid phosphors are Naphthalene, Anthracene, Stilbene etc. and Organic Liquid phosphors are terphenyl in toluene OR terphenyl in xylene.
Inorganic solid phosphors are sodium iodide crystals activated with 0.5 mole thallium (Tl). As the NaI is hygroscopic, it must be kept in air-tight metal box and front side is made up of thin Al film.
When ionising radiations like alpha and beta particles are incident on it. It produce scintillation i.e. flash of light. This light is scattered in all the possible directions. As the light is proportional to the no. of alpha beta particles incident, so the light is very important. It may strike with the walls of container and may be wasted to avoid this, the interior part of metallic box is coated with light reflector. So all the light which is produced due to scintillation phenomenon is allowed to incident on photo cathode of PMT.

2. PMT is Photomultiplier tube. It works on the phenomenon of secondary emission. It is long metallic tube. A sensitive photo anode is placed at one end of the tube and series of electrodes called Dynodes are arranged in the tube. The surface of photocathode is coated by Caesium and Antimony alloy. The Dynodes are connected in such a way that each dynode is at a more positive potential than its predecessor. Thus the electron beam travelling from one dynode to the other is continuously accelerated.
When light strikes a dynode the electrons are ejected out from the surface of photocathode due to photoelectric effect phenomenon and gets attracted towards next dynode where they can eject even more electrons. Each incident electron on the surface of Dynode is capable to eject the four electrons. Thus the electrons go on multiplying. Finally all these accelerated and multiplied electrons are incident on photo anode, which is fixed at the other end of P.M.T.

3) Electrometer:The photo anode of P.M.T. is connected with electrometer which gives the current. From the magnitude of current, we can find out the amount of radiations which have entered the metallic box in the given time interval.

1. The scintillation counter can be used for the measurement of $$\alpha$$, $$\beta$$ and $$\gamma$$ -rays only we have
2. to change phosphor.
3. The amplification is very high is about 106 to 108.

Limitations:
1. Large number of radiations are wasted as the source is kept outside the metal box.
2. The instrument is costly.

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