Describe the principle construction and working of a scintillation counter. With suitable diagram explain various components of a scintillation counting system.
Principle Scintillation counters
: when a charged particle passes through phosphors some of its atoms are excited. These excited atoms return to their ground states after emitting tiny flashes of light called scintillations. Similarly when rays fall on a crystal of sodium iodide with a little of Thallium impurity they emit visible light due to fluorescence. The fluorescent material is called phosphors. The essential units of a modern scintillation counter are the scintillate a material which emits fluorescent light when traversed by a charged particle and the photomultiplier tube which converts the light output of the scintillate units into a readable electrical pulse. Inorganic materials like sodium iodide or potassium iodide activated with thallium crystalline form of Csl non alkali materials like Bismuth Germinated crystalline organic materials like naphthalene anthracites and solution of organic compounds like terphenyl dissolved in xylem of toluene are the various phosphors mostly used. The phosphor used should have the sufficient thickness to absorb the entire ionizing particle passing through it. A schematic diagram of a scintillation counter is shown A crystal of a phosphor, surrounded by thin aluminum foil, acts as a light shield reflecting the light flashes produced in the scintilla or onto the photo sensitive cathode of the photomultiplier tube which is at its top end. This cathode is usually made of antimony and cesium. A nucleate particle entering the phosphor produces photons in it which when fall on the cathode produce electrons and as such the cathode is also known as photo cathode. The electron from the photo cathode then falls on the electrodes of the photomultiplier tube. These electrodes are called dynodes. The photo electrons are accelerated in the electrostatic field between the first dynode and the cathode. These accelerated electrons then produce secondary electrons from the first dynode. The multiplication is usually 5-10 per one primary electron. The secondary electrons are accelerated again across the gap to the second dynode where the number is further multiplied. This process is repeated and the electron current is amplified as electrons are accelerated from dynode to dynode. A photomultiplier tube usually contains 10-18 dynodes. Photons striking the photo cathode thus cease and avalanche of electrons which eventually hit the electronic counting system. Each pulse is proportional to the energy of the particle incident on the phosphor. The scintillation counter along with electronic system capable of measuring energy of the incident particle is thus used as a scintillation spectrometer. The amplitude of the pulse appearing at the output of a photomultiplier tube is proportional to the energy of the incident unclear radiation. The pulse height V at the output will be relation to the amount of charge collected at the anode through the relation.
Advantages (1) +with large size and highly transparent phosphor it displays very high frequency. (2) The pulse height is proportional to the energy dissipated in the phosphor by the incident radiation. Hence it is possible to determine the energies of individual incoming particles. (3) The time of pulse being very short so that resolving power is high. It can detect particles whose time of arrival is separated considerably by less than 10-8-sec. (4) Because of very small dead time. Scintillation counter is capable for fast counting rate. (5) It is more efficient for ray counting with a large scintilla or the scattered rays also counted and get a increased photo peak efficiency.
Disadvantage (1) poor energy resolution. In spots of its high detection efficiency the recovering energy in the process of f converting it into light flashes and into photoelectrons. Such detectors are capable of handling high counting rates in spectroscopy work also because of (1) Time resolution: The time resolution is dependent on the spread in the transit time of the electrums in the photomultiplier tube. The spreading time is 2-5 ns. As the electrons are collected in the anode we get negative pulse from the anode. (2) The decay time of the anode pulse is around 250 ns. Hence such detectors are capable of handling high counting rates in nuclear spectroscopy work.