School of Earth and Environment

Scintillation Counter

Background to the technique

Liquid scintillation counting is widely used to detect and measure radioactivity. It is the standard technique to quantify the radioactivity of low energy radioisotopes, mainly beta and alpha-emitting isotopes. The method requires specific cocktails to absorb energy, this energy is transformed into detectable light pulses which are counted by the instrument. This count can be converted into becquerel by creating correlation curves of standards with known activity.

Instrument capabilities

Andy checking on the progress of the Scintillation Counter.
Andy checking on the progress of the Scintillation Counter.

We have a Packard Tricarb 2100 TR liquid scintillation analyser. It is a computer-controlled, benchtop liquid scintillation analyser for detecting amounts of alpha, beta and gamma radioactivity. It has pre-set counting protocols for common radionuclides such as carbon-14 and it also provides the ability to construct protocols with count times, energy spectrum and statistics all tailored to your specific samples and radionuclide.

Samples are added to the scintillation cocktail and then placed in the machine for counting. For aqueous samples the cocktail is made up of three components, firstly a surfactant is needed to enable the aqueous sample to come into contact with the aromatic solvent, creating a stable microemulsion for the counting period. The aromatic, organic solvent absorbs the energy emitted during a decay event. This energy is then transferred to a scintillator which converts the energy to photons. Photomultiplier tubes are then used to detect and convert the photons into electrical pulses. Detection limits vary depending on the energy associated with the decay of the radionuclide. For soft beta emitters such as carbon-14, levels can be measured of ~ 2 Bq mL-1, which is the background level of carbon-14 in the environment.

Practical considerations

Inside the counter.
Inside the counter.

There is a variety of cocktails available to optimise the counting of your sample. The total volume of sample and cocktail is usually around 10 mL. The ratio of sample tot cocktail varies, but for aqueous samples it is usually 1:9 mL. The choice of scintillation cocktail is very important as it must be suited to the radionuclide and sample.

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    Written by Aislinn Boylan (21/08/2014)

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