Isotopes naturally found on earth are usually those that are stable or have a decay time larger than, or at least a significant fraction of the age of the Earth (about 5 x 10 years). Isotopes with shorter halflifes mainly exist as decay products from longer lived isotopes, and, as in C14, from irradiation of the upper atmosphere.
Radioisotopes with a sufficiently long halflife, and whose decay produces an appreciable amount of gamma rays are:
* Potassium K with half-life of 1.3 x 10 years, which emits 0 α, 1 β, and 1 γ-ray
* Thorium Th with half-life of 1.4 x 10 years, which emits 7 α, 5 β, and numerous γ-ray with different energies
* Uranium U with half-life of 4.4 x 10 years, which emits 8 α, 6 β, and numerous γ-ray with different energies
Each of these elements emits gamma-rays with distinctive energy. Figure 1 shows the energies of emitted gamma-ray from the three main isotopes. Potassium 40 decays directly to stable argon 40 with the emission of 1.46 MeV gamma-ray. Uranium 238 and thorium 232 decay sequentially through a long sequence of various isotopes until a final stable isotope. The spectrum of the gamma-rays emitted by these two isotopes consists of gamma-ray of many different energies and form a complete spectra. The peak of thorium series can be found at 2.62 MeV and the Uranium series at 1.76 MeV.