6.1. PHYSICAL BASIS OF RADIONUCLIDE METHOD OF DIAGNOSIS
The basis of the radionuclide method of diagnosis is in the natural radioactivity. Antoine Henri Becquerel discovered this phenomenon at the end of the XIX century. He was the first to demonstrate that some chemical elements are able to emit «invisible rays» that illuminate the X-ray plate as well as the X-rays. In 1903, Henri Becquerel, Pierre Curie and Maria Sklodowska-Curie awarded the Nobel prize in physics «in recognition of the extraordinary services they have rendered to science by joint researches on the radiation phenomena, discovered by Professor Henri Bec-querel».
The discovery of X-ray emission and natural radioactivity laid the foundation for modern nuclear physics and medical radiology. At first, the discovered by Becquerel radiation was named after him, as X-rays after Roentgen. Then it turned out that the radiation is not homogeneous and consists of three parts that were called according to the first three letters of the Greek alphabet: α-, β- and γ-radiation.
α-Radiation (α, α2+) is a flux of helium stripped atoms. The α-particle is dipositive (two protons and two neutrons), and its mass is equal to 4 atomic units. The path of α-particles in the human body is several tens of microns.
The β-radiation is the flux of β-particles - electrons (e-) or positrons (e+). Each particle has one elementary positive or negative electric charge. The mass of an electron is only about 1/1840 of the mass of a hydrogen atom. Electrons that are formed during the decay of radionuclides penetrate several millimetres into human tissues.
The γ-radiation (or gamma radiation) is electromagnetic radiation emitted during radioactive decay. Unlike continuous X-ray radiation, the gamma-ray spectrum is discrete, as the transition of the nucleus of an atom from one energy state to another is carried out abruptly. The properties of γ-radiation are determined by the wavelength (λ) and the quantum energy (E).