The Scintillation Counters 

     One of the earliest methods of radiation detection was the spinthariscope (Fig-1). 

Fig - 1

      It consists of a small wire, the tip of which is dipped in Radium bromide(R) or any other radioactive salt. It is placed in front of a zinc sulphide screen  S and viewed through a microscope. When and α or β-particle falls on the zinc sulphide  screen. they produce light flashes which can by seen by a microscope (M) in a dark room. The visible luminescence excited in zinc sulphide by α-particles was used by Rutherford for counting the particles. The process of counting these scintillations through a low power  microscope is a tedious one and the limitations of observation with the eye restrict the counting rate to about 100 per minute. This process, whereby the energy of the particle is converted to light, is the basis of scintillation counter.

 

Fig - 2

       The main parts of a scintillation counter are shown in Fig. 2, the atoms of the phosphor are excited or ionised by the energy loss of an impinging  α,β or γ ray. When the  atoms return to their ground states, photons are emitted, in the blue and ultraviolet regions of the optical spectrum. The phosphors optically coupled to the envelope of a photomultiplier tube. The photons strike the photocathode, causing the ejection of photo-electrons Fig - 3. As these photo-electrons leave the photocathode, they are directed by a focusing electrode to the first multiplier electrode or dynode. The electrode has the property of emitting three, four or five electrons for every single electron which strikes its surface.

Fig - 3

    There may be from 10 to 14 such multiplier stages in a given tube. Hence, from the emission of one single electron from the cathode, a burst of one million electrons may impinge on the final stage in the tube ( the anode.) The output pulse from the photomultiplier is fed to a pulse amplifier followed by a scaler circuit.