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.
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,
is now largely done by accelerated mass spectroscopy.