Some elements are naturally radioactive, releasing alpha particles, beta particles, or gamma radiation as they decay to more stable elements. The decay in a fixed period of time is proportional to the number of unstable atoms, with exponential decay dictating a half-life for radioactive elements.
Alpha particles (α) consist of two neutrons and two protons, resulting in a positively-charged relatively large mass equivalent to a stable helium nucleus. Emission of an alpha particle causes the radioactive element to decrease in atomic mass by four, and in atomic number by two. The alpha particle is relatively large with large kinetic energy, but loses that energy quickly to collisions.
At thermal energies, the alpha particle will quickly acquire two electrons, evolving into a regular helium atom over an average distance of less fractions of a millimetre in solid rocks. This means that alpha particles have extremely weak penetration, and can be blocked by a piece of paper. They are easily blocked by any overburden, even a thin layer of sand or gravel, so may only be detected from bare rock exposed at the surface.
Beta particles (β) consist of electrons ejected from the nuclei, resulting in a negatively-charged small mass equivalent with higher kinetic energy than a normal electron. Emission of a beta particle causes the radioactive element to become an isotope of the original element. The beta particle loses kinetic energy quickly to collisions, particularly by colliding with other electrons.
The beta particle will quickly lose its excess kinetic energy, becoming indistinguishable from surrounding electrons within centimes of travel within solids or liquids. This means that beta particles have weak penetration, and can be blocked by a thin sheet of aluminium. Like alpha particles, beta particles are also easily blocked by any overburden.
Gamma rays (γ) are electrically neutral photos with energy (≥ 0.1 MeV) proportional to frequency (≥ 0.25 x 1020 Hz ) and a very short wavelength (≈ 10 m). Emission of a gamma ray results in the original element reducing from an excited state to an unexcited state. Not all decay stages produce gamma rays, and not all gamma rays have equal energy.
Because gamma rays are electrically neutral, they have greater penetrating power than electrically charged alpha and beta particles and are more likely to be detected through overburden, giving them greater utility during geological surveys. Even so, approximately 90% of detected particles originate in the top 20 to 30 centimetres of rock, or 50 centimetres of soil. Water increases the attenuation of gamma rays, so measuring dry rock is preferred to avoid absorption by moisture. Attenuation is frequency dependent, with higher frequency rays penetrating better, although all frequencies can be blocked by centimetres of lead.
Natural gamma rays can be proceed by terrestrial or cosmic sources. The terrestrial sources are related to stages in the decay chains, while those from cosmic sources are typically much higher energy. The background noise of gamma radiation consists of very high energy gamma rays produced by cosmic sources, gamma rays that have lost energy to electron-ejecting collisions (Compton scatter- ing), and gamma rays that are absorbed in electron-ejecting collision (photoelectric effect). This background radiation curve must be accounted for when processing gamma ray spectra.
Additionally, the particular gamma ray frequency produced by one decay series might overlap with the frequency of gamma rays produced by a different decay chain. This must be corrected by stripping the measured gamma ray windows, an automated instrument-dependent process. Geologically important peaks are at 2.62 MeV (for thorium decay into 208Th), 1.76 MeV (for uranium decay into 214Bi), and 1.46 MeV (for potassium decay into 40K). If the primeval elements are at equilibrium with the daughter products, the corrected gamma rays measured as a window around can be used in a spectrum analysis to calculate concentrations of the three parent elements.