Why isn the coincidence technique used for gamma counting?
With gamma emitters being monoenergetic so that all gamma rays have the same one or two energies rather than the broad distribution of betas which begins at zero energy, and with photon production being five times greater for equal energies, the amount of light produced in the sodium iodide crystal for a typical gamma decay event is much more than for beta events. Thus, the output pulse of the photomultiplier tends to be above the noise level for a gamma event whereas it is likely to overlap the noise level for low-energy beta events.
