The Interdepartmental Equipment Facility
The γ-Counter
Different Isotopes of the same element have the same number of protons but different numbers of neutrons. Some isotopes are stable, but radioisotopes disintegrate spontaneously emitting one or more of the three main types of radiation.
α emitters release a particle with 2 protons and 2 neutrons.
β radiation involves emission of an electron and a neutrino.
γ rays consist of electromagnetic radiation similar to X-rays.
Gamma radiation may accompany either alpha or beta particle emission. An example of a gamma emitter which is in common use is iodine-131
Gamma counting differs from Liquid scintillation counting in that no scintillation media is used in the sample vial. The gamma ray is detected after it has passed out of the sample vial and impinges on a crystal of NaI(Tl) which is the scintillator. A photomultiplier tube "sees" this light, amplifies it and sends the electrical signal to be further processed. The energy of the gamma ray emitted from the decaying isotope varies but is more energetic than the alpha or beta particles normally detected by LSC. Polypropylene tubes are recommended for maximum counting efficiency.