SCINTILLATION COUNTER

PRINCIPLE When light radiations strike fluorescent material it produces flashes of light called scintillations. These are detected with the help of photomultiplier tube.

BLOCK DIAGRAM :

PHOTO MULTIPLIER TUBE:

TYPES OF SCINTILLATION MATERIALS: Intermettalic compound of cesium and antimony Zinc sulphide for alpha particles. Anthracene for beta particles. Sodium iodide and cesium iodide for gamma rays.

CONSTRUCTION AND WORKING: 1)T is a tube made of glass or quartz coated with some photosensitive material like Cs 3 Sb,this is called photo cathode. when photons fall on cathode photo-electrons are emitted. 2)Facing photo cathode C there is a series of secondary electrons emitting surfaces D1,D2,D3called dynodes. First dynode is at positive potential of about 80 to 100 volts with respect to photo cathode C. successive dynodes are kept at similar positive potential with respect to dynodes immediately preceding them. 3)When an energetic photo electron emitted by photo cathode is accelerated by potential difference of about 100 v to first dynode then this single electron having kinetic energy of 100eV collides with surface of dynode which emit 2 or more electron. 4)these secondary electron s accelerated to second dynode which is maintained at a potential of v w.r.t first at the surface electron multiplication takes place by secondary emission.

6)In photomultiplier there are 10 dynodes and there 10 stages of electron multiplications. The secondary electrons emitted by last dynode are collected at anode A. 7)charge multiplication built by dynodes makes a suitable voltage pulse which is amplified by the amplifier,detected,counnted. 5 )single photo electron emitted by photo cathode approximately produces 10 6 electrons.

ADVANTAGES: It can operate in air or vacuum. electron pulse generated is proportional to energy of incident particles. Counting rate is very high.

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