Since the development of the photoelectric tube and the photomultiplier
tube (a combination of photoelectric cell and amplifier), the
scintillation counter has become the most popular instrument for most
purposes described in this booklet. The flash of light produced when an
individual ionizing particle or ray strikes a sodium-iodide crystal is
noted by a photoelectric cell. The intensity of the flash is a measure
of the energy of the radiation, so the voltage of the output of the
photomultiplier tube is a measure of the wavelength of the original
gamma ray. The scintillation counter can observe up to a million counts
per minute and discriminate sharply between gamma rays of different
energies. With proper windows it can be used for alpha or beta counts as
well.