What is radiation?
A particle ray consisting of two protons and two neutrons (namely, a nucleus of helium). Alpha rays are produced following spontaneous decay of certain radioactive atoms, such as radium, plutonium, uranium, and radon. Because of its large mass and positive charge, an alpha ray can usually pass only a short distance--less than 1 mm--in water. A single piece of paper can stop an alpha ray effectively. Therefore, health effects of alpha-ray exposures appear only when alpha-emitting materials are ingested (ie, internal exposure).
A particle ray consisting of a fast electron whose mass is nearly 1/2000 of the mass of a proton or neutron. Beta rays are produced following spontaneous decay of certain radioactive materials, such as tritium (an isotope of hydrogen), carbon-14, phosphorus-32, and strontium-90. Depending on its energy (ie, speed), a beta ray can traverse different distances in water--less than 1 mm for tritium to nearly 1 cm for phosphorus-32. As with alpha rays, the major concern for health effects is after their ingestion (ie, internal exposure).
An electromagnetic wave, a gamma ray is similar to ordinary visible light but differs in energy or wavelength. Sunlight consists of a mixture of electromagnetic rays of various wavelengths, from the longest, infrared, through red, orange, yellow, green, blue, indigo, and violet, to the shortest in wavelength, ultraviolet. A gamma ray's wavelength is far shorter than ultraviolet (ie, it is far higher in energy). Gamma rays are produced following spontaneous decay of radioactive materials, such as cobalt-60 and cesium-137. A cobalt-60 gamma ray can penetrate deeply into the human body, so it has been widely used for cancer radiotherapy.
X rays have the same characteristics as gamma rays, although they are produced differently. When high-speed electrons hit metals, electrons are stopped and release energy in the form of an electromagnetic wave. This was first observed by Wilhelm Roentgen in 1895, who considered it a mysterious ray, and thus called it an X ray. X rays consist of a mixture of different wavelengths, whereas gamma-ray energy has a fixed value (or two) characteristic to the radioactive material.
Neutron particles are released following nuclear fission (splitting of an atomic nucleus producing large amounts of energy) of uranium or plutonium. In fact, it is neutrons that trigger the nuclear chain reaction to explode an atomic bomb. Neutrons hardly damage cells because they do not carry any electrical charge. However, the human body contains a large amount of hydrogen (a constituent of water molecules that occupy 70% of the human body), and when neutrons hit the nucleus of hydrogen, ie, a proton that is positively charged, the proton causes ionizations in the body, leading to various types of damage. At equivalent absorbed doses, neutrons can cause more severe damage to the body than gamma rays.