Radiotherapy => Radiation
Radiation, the process of transmitting waves or particles through space, or some medium; or such waves or particles themselves. Waves and particles have many characteristics in common; usually, however, the radiation is predominantly in one form or the other. Mechanical radiation consists of waves, such as sound waves, that are transmitted only through matter.
Electromagnetic radiation is independent of matter for its propagation; the speed, amount, and direction of the energy flow, however, are influenced by the presence of matter. This radiation occurs with a wide variety of energies. Electromagnetic radiation carrying sufficient energy to bring about changes in atoms that it strikes is called ionizing radiation . Particle radiation can also be ionizing if it carries enough energy. Examples of particle radiation are cosmic rays, alpha rays, and beta rays. Cosmic rays are streams of positively charged nuclei, mainly hydrogen nuclei (protons). Cosmic rays may also consist of electrons, gamma rays, pions, and muons. Alpha rays are streams of positively charged helium nuclei, normally from radioactive materials. Beta rays are streams of electrons, also from radioactive sources. .
The spectrum of electromagnetic radiations ranges from the extremely short waves of cosmic rays to waves hundreds of kilometres in length, with no definite limits at either end. The spectrum includes gamma rays and "hard"X-rays ranging in length from 0.005 to 0.5 nanometres (a five-billionth to a 50-millionth of an inch). (One nanometre, or 1 nm, is a millionth of a millimetre.) "Softer" X-rays merge into ultraviolet radiation as the wavelength increases to about 50 nm (about two millionths of an inch); and ultraviolet, in turn, merges into visible light, with a range of 400 to 800 nm (about 16 to 32 millionths of an inch). Infrared radiation ("heat radiation") is next in the spectrum and merges into microwave radio frequencies between 100,000 and 400,000 nm (between about 4 thousandths and 16 thousandths of an inch). From the latter figure to about 15,000 m (about 49,200 ft), the spectrum consists of the various lengths of radio waves; beyond the radio range it extends into low frequencies with wavelengths measured in tens of thousands of kilometres.
Ionizing radiation has penetrating properties that are important in the study and use of radioactive materials. Naturally occurring alpha rays are stopped by the thickness of a few sheets of paper or a rubber glove. Beta rays are stopped by a few centimetres of wood. Gamma rays and X-rays, depending on their energies, require thick shielding, made of a heavy material such as iron, lead, or concrete.