A well counter is a device used for measuring radioactivity in small samples. It usually employs a sodium iodide

Sodium iodide

Sodium iodide is a white, crystalline salt with chemical formula NaI used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction.-Uses:Sodium iodide is commonly used to treat and prevent iodine deficiency....

 crystal detector

Scintillator

A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...

.
It was invented in 1951 by Hal Anger

Hal Anger

Hal Oscar Anger was an American electrical engineer and biophysicist at Donner Laboratory, University of California, Berkeley, known for his invention of the Anger camera....

, who is also well known for inventing the scintillation camera.

Construction and functioning

The well counter is so called because the samples are inserted into a well within the crystal in order to maximize sensitivity by collecting as many of the emitted gamma rays as possible.

Modern well counters can automatically record activity in different samples sequentially. Samples in test tubes are inserted into the well, one sample at a time, and counted

Scintillation counter

A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...

 for a predetermined time. Results are presented as a graphic

Information graphics

Information graphics or infographics are graphic visual representations of information, data or knowledge. These graphics present complex information quickly and clearly, such as in signs, maps, journalism, technical writing, and education...

, and corrected for the decay

Radioactive decay

Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

of the sample.