Sources of Background in a Scintillation Counter

One of the major sources of background in a scintillation counter is the dark current of the phototube (see Sec. 6.6.1). Other background sources are naturally occurring radioisotopes, cosmic rays, and phosphorescing substances. 

The holder of a liquid scintillator may contain small amounts of naturally occurring isotopes. In particular, is always present (isotopic abundance of is 0.01 percent). Another isotope, C, is a constituent of contemporary organic materials. Solvents, however, may be obtained from petroleum, consisting of hydrocarbons without C. 

The term phosphorescence refers to delayed emission of light as a result of deexcitation of atoms or molecules. Phosphorescent half-lives may extend to hours. This source of background may originate in phosphorescent substances contained in the glass of the phototube, the walls of the sample holder, or the sample itself. 

Cosmic rays, which are highly energetic charged particles, produce background in all types of detectors, and scintillators are no exception. The effect of cosmic-ray background, as well as that of the other sources mentioned earlier, will be reduced if two counters are used in coincidence or anticoincidence. 

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