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Decay Counting

Decay counting methods are used to determine the activity or concentration of radioactive isotopes. Decay counting works by measuring the number of decays that occur in a given period of time. Then, with knowledge of the decay rate, one can calculate the abundance of the radioactive isotope. Methods include gas proportional counting and liquid scintillation counting.

Gas proportional counting (GPC) systems are used to determine alpha-ray or beta-ray emission rates of radionuclides. GPC uses P-10 gas (10% methane and 90% argon) to detect radiation as it enters the counting chamber. The gas is ionized and the applied chamber voltage causes the ionization to cascade, resulting in a current pulse that can be detected. GPC is used for gaseous radioisotopes such as 14C, 39Ar, and 85Kr, although 14C dating is now largely done by accelerated mass spectroscopy.

Liquid scintillation counting (LSC) may also be used to detect alpha and beta emitters, and is more effective than gas proportional counting systems in detecting beta particles, which have limited penetrating ability. Scintillation counters also allow high speed counting of x-rays and gamma rays.

LSC involves dissolving a sample in a solvent or sequence of solvents (usually benzene or toluene), which act as scintillators, or materials that flouresce when struck by a charged particle or high-energy photon. The decay products produced in the sample excite the pi electrons in the scintillator. This is followed by loss of the excitation energy through fluorescence (light).

Using a photomultiplier tube (PMT) this light is converted into electric pulses, amplified, and counted. Samples are quantified using calibration standards of a known activity.








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