Now here is how the detection system works. The vial is lowered into a dark chamber with photoelectric detectors on each side. No detection system perfect, of course. In liquid scintillation counting, cosmic rays, beta particles from decaying potassium in the glass vial, spontaneous discharges from the very sensitive photodetectors, and chemicals dissolved in the scintillation fluid all can contribute to spontaneous flashes of light that are recorded as counts. The CPM attributable to such sources are called background.
Background counts are often so low relative to the activity being measured that they are ignored.
It is a good practice to include a vial containing everything except added radioactivity as a control to determine the background level. Background CPM are then subtracted directly from the CPM for the experimental samples. Unfortunately, there are two more little problems. No instrument is capable of recording all of the atomic disintegrations within a scintillation vial. Because of the geometry of the vial and photoelectric detectors some events go undetected.