LIQUID SCINTILLATION COUNTER

Typical Number in Hospital: 2

Cost Bands: 6

References: 3

Gamma rays of less than 20 keV and beta rays of less than 500 keV cannot be counted by the usual sodium iodide type of scintillation counter. This problem is overcome in the liquid scintillation counter (also known as a beta counter) by mixing the scintillator with the sample and using photomultiplier (PM) tubes to count the scintillations. The scintillators used are complex organic molecules, and secondary scintillators may be used to convert the wavelength to match the PM tube. The sample and PM tubes must be housed in a light-tight box.

The PM tube itself produces emission of electrons which may affect the results and so the sample is often cooled (e.g. to 5[d]C) and only scintillations detected simultaneously by two tubes are counted.

Normally several hundred samples are counted in turn by use of an automatic sample changer, and calibration and background samples are included, to enable computer correction factors to be included in the results.

These devices are intended for counting low-energy isotopes such as tritium and carbon 14. They may be found in chemical pathology, nuclear medicine, and immunology departments, used for radioimmunoassay (RIA), or competitive binding analysis.