Scintillation vials. Gloved hand arranges scintillation vials. Each vial contains a radioactive biological sample which is mixed with scintillation fluid. The fluid contains molecules which emit light when they are hit by radiation in a radioactivity counter. The radioactivity counter uses sensitive photomultipliers (which convert light into an electrical impulse) to detect the light emitted. The amount of light emitted is proportional to the amount of radioactivity in the biological sample. This allows the accurate measurement of radiation levels in the sample.