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.