Types of radiation
Radiation is categorized in several different ways. One is on the basis of energy levels: ionizing radiation is sufficiently energetic that it can cause an atom or molecule to be stripped of an electron, turning it into an ion. This depends on the energy level of the individual particles or waves and has nothing to do with the total number of them. Non-ionizing radiation is simply that which doesn’t have enough energy to liberate an electron.
Another way to classify radiation is in terms of whether it is electromagnetic (consisting of photons) or particle radiation. There are three types of particle radiation: alpha decay, based on the emission of two protons and neutrons bound together in a helium nucleus, beta decay, wherein the particle emitted is an electron, and neutron radiation, where atoms release neutrons. Alpha particles are not generally very dangerous, because they are unable to penetrate much of substance. Even a few centimetres of air can have a strong protective effect. That said, ingestion can still be highly dangerous. The Polonium-210 that killed Alexander Litvinenko is an alpha emitter. Beta particles can usually be shielded from using a few milimetres of lead. Neutron radiation is unusual insofar as it is capable of producing radioactivity in the atoms it encounters. Shielding consists of a large mass of hydrogen rich materials.
Electromagnetic radiation with sufficient energy to be ionizing cosists of x-rays and gamma rays. Both consist of high-energy photons (those with short wavelengths), with gamma rays having shorter wavelengths than x-rays (10^(-12)m rather than 10^(-10)m). Shielding, especially for gamma rays, must be dense and fairly extensive.
Radiation is also measured in a variety of ways: important ones being Roentgens, rads, rems (Roentgen equivalent in man), Curies, Becquerels, and Sieverts.
Becquerels are a unit of radioactive decay based only on the number of decays per second. A Curie is equal to 3.7 x 10^10 Becquerels, and is approximately equivalent to the activity of 1 gram of Radium isotope. These units reflect the number of emissions only – not their physical or biological effects.
A Roentgen is a measure of ionizing radiation based on the ratio between charge and unit mass. Rads are a largely obselete unit of radiation dose, equal to 100 ergs of energy being absorbed by one gram of matter. Rems are the product of the number of Roentgens absorbed, multiplied by the biological efficiency of the radiation. Rems are also considered highly dated as a measure of radiation. 450 rems is an approximate lethal dose (LD50), for those who do not receive prompt treatment.
Sieverts are the recommended replacemend, “found by multiplying the absorbed dose, in grays, by a dimensionless “quality factor” Q, dependent upon radiation type, and by another dimensionless factor N, dependent on all other pertinent factors.” The LD50 for ionizing radiation is about 5 grays or about 3-5 Sieverts. If the biological efficiency used to calculate rems equals one, one Sievert is 100 rems.