Charged Particles and y Rays

The efficiency of the GM counter for the detection of y radiation is low (about 1 %), owing to the indirect method of detection. A more efficient device is the scintillation counter, where the ionization and excitation due to the electrons produced by one or more of the processes outlined above is converted into fluorescent radiation from a suitable scintillating material. The commonest scintillator is a single crystal of sodium iodide with a small amount of thallium added to it. The scintillator is optically coupled to a 

A variety of special circuits can be used to improve the performance of 

One way of improving the discrimination against background is the coincidence method, which can be used when the radionuclide being measured emits two or more y rays essentially simultaneously (as, for example, when an excited level decays to ground in a cascade process involving several intermediate levels). A typical coincidence arrangement is shown in Fig. 

The sample is placed between two separate scintillation detectors, and the pulses from each are amplified and fed to single-channel analyzers. The output pulses from one detector are transmitted to a multichannel analyzer, but on the way have to pass through a gate which is normally closed and can only be opened by receiving simultaneous pulses from the SCAs of both detectors. If several coincident y rays are present, the SC A of detector 1 can be set to cover only the limits of the photopeak corresponding to one of the 

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