Radioactivity is the spontaneous disintegration of an atom of an element, and atom being the smallest unit that maintains all the qualities of the element. A stable element, one that is not radioactive, will remain intact indefinitely unless acted upon by an outside force. A radioactive element however, naturally releases energy in the process of shedding high speed charged particles in an effort to reach a stable state. All elements inherently seek to achieve equilibrium or stability if they have not already. In this process of transmutation the atom emits charged particles or electromagnetic rays depending upon the nature of its instability .An atom is made up of a dense nucleus consisting of protons and neutrons surrounded by orbiting electrons. Protons have a positive charge and occur in all atomic nuclei. Neutrons have neither a positive nor a negative charge and are only stable when a part of an atomic nucleus, otherwise they decay into a proton, an electron and an antineutrino. They occur in all atomic nuclei except hydrogen. Electrons have a negative charge and are part of all atoms. They are arranged in groups referred to as shells around the nucleus. They orbit at differing levels corresponding to their energy level. The number of electrons is sufficient to balance the number of protons so the overall charge is neutral.

Radioactive decay is the process by which a radioactive element transforms into a daughter nuclide by emitting charged particles or electromagnetic rays. The daughter element may be either radioactive or stable, but if it is radioactive it in turn will decay into a daughter nuclide, continuing to do so until it reaches the point of stability. In this way all naturally occurring radioactive elements eventually transmute in to lead. The time it takes for all of an element to be transformed is termed its mean life. Various radioactive atoms have longer or shorter mean life spans but the rate at which the transmutation occurs in a particular element is fixed. It is this feature that makes Carbon 14 dating possible. At the point of death of any carbon based life form the quantity of Carbon 14 in it becomes fixed. By determining how much of the Carbon 14 is left it is possible to determine how long ago the life form died. The decay chain of Uranium 238, the most abundant natural form, is shown in this illustration.

Half-life is a constant; all atoms of the same element have the same half-life just as they have the same mean life. The half-life of a nuclide is the time it takes for half the original number of atoms to decay. This constant rate of decay varies depending upon the particular element involved. Some such as Lithium 8 have short half lives, in this case .845 seconds, while others such as Uranium 238 have long ones, a staggering 4,510,000,000 years. The element may or may not reach a stable form in decaying. The element it becomes, the daughter nuclide, may also be radioactive, in which case it has a half-life of its own. The concept of half-life is used to illustrate the length of time until a nuclide no longer poses a risk in a rather broad sense of that term. At the end of one half-life, half of the element has decayed. At the end or two half lives half of the remainder will have decayed or one fourth of the original quantity. This is what is referred to as a logarithmic progression. Obviously, if the product of decay is itself radioactive that will lengthen the time that the nuclide poses a risk. As a rule of thumb, 10-20 half lives equals the hazardous life.

When the nucleus of an atom has too many protons it results in disequilibrium of the nucleus due to the excessive repulsion. In order to reduce the repulsion the atom emits an alpha particle. An alpha particle consists of two protons and two neutrons bound together as a stable entity. The loss of the alpha particle reduces the atomic number by two. A stream of alpha particles is referred to as alpha radiation or an alpha ray. In comparison to other subatomic particles alpha particles are quite large, as a result they lack penetrating power. They collide with molecules in the air and are stopped by two inches of air. They are unable to penetrate intact human skin or pass through a sheet of paper. They travel at approximately 10,000 miles per second. Alpha particles lose energy by gradually transferring small amounts of kinetic energy to the atomic electrons of the absorbing material until finally they are stopped and disappear. Alpha particles are considered heavy charged particles. Their main danger for humans occurs if they are inhaled or ingested, then they can be quite harmful.

When the neutron to proton ratio in the nucleus is too great a beta particle is emitted. In basic beta decay a neutron is transformed into a proton and an electron. The electron is then emitted as a beta particle. The path of this light charged particle is very irregular and it travels at nearly the speed of light. Streams of electrons form beta radiation, which has more speed and less mass than alpha radiation, allowing it to penetrate better. The mass number of the element remains unchanged by the loss of the beta particle. Beta radiation can be stopped by sheet metal but the particles are able to penetrate the human body to approximately an inch. They basically bounce along colliding with the atomic particles of the absorbing material losing energy with each collision until they exhaust their energy.

When the nucleus of an atom is at too high an energy it attempts to reach equilibrium by emitting a high-energy photon known as a gamma particle. Gamma radiation is a form of electromagnetic energy, a ray in the same sense as visible light or X rays. The photons of gamma rays are indistinguishable from X rays physically. Gamma radiation behaves differently than radiation consisting of charged particles. Rather than losing energy slowly as it ricochets off molecules in the absorbing material as alpha and beta radiation do, gamma photons lose all their energy at once either by being absorbed or by scattering. When scattering occurs it produces secondary photons from the atom struck which then continue on as gamma radiation until they in turn are either absorbed or scattered. Losing gamma photons does not alter the mass of the element; the atom simply passes to a lower state of excitation. Gamma rays have substantially more penetration power then either alpha or beta radiation. Gamma radiation can be stopped by lead or concrete in sufficient thickness or by glass made with lead.

Neutron emissions are caused by the nucleus of an atom being struck by a particle, which results in the disintegration of the bond binding the protons and neutrons of the nucleus together. It is the basis of all nuclear chain reactions. Neutron emissions affect the atom they strike in one of several ways. Elastic collision occurs when the neutron shares it kinetic energy with a nucleus without exciting the nucleus. Elements that have this sort of reaction when bombarded with neutrons are called moderators and are used to slow down the chain reaction in nuclear reactors. Graphite or deuterium are examples of moderators. Inelastic collision usually occurs with fast neutrons. In this case the bombarded atom becomes excited and to regain equilibrium it emits a gamma photon and a neutron. It then shares the remaining kinetic energy with the neutron that struck it. Radiative capture occurs when the neutron is absorbed by the atom it strikes and produces an excited compound nucleus, which attains stability by emission of a gamma. Charged particle ejection normally occurs with fast neutrons; in this case the collision of the neutron with the atom causes the atom to emit an alpha or beta particle to regain equilibrium. Fission reactions can occur with either fast or thermal neutrons. A thermal neutron is one which has lost some kinetic energy to a moderator, slowing it and making it more likely to produce fission.

The fission interaction is the basis of all nuclear chain reactions, without it there would be no nuclear reaction. In fission the neutron bombards an atom of fissionable material splitting it apart and producing additional free neutrons to perpetuate the reaction, as well as heat energy and radiation energy. A fissionable element will have a heavy nucleus from which an average of two or three neutrons are emitted when it is struck by a neutron. A diagram illustrating the fission reaction is available here. Neutron radiation penetrates very well. It is also capable of inducing radioactivity in certain non-radioactive elements through Radiative capture. Steel, which neutrons easily penetrate, is an example of a material capable of being made radioactive by neutron bombardment. The process of material being made radioactive by neutron bombardment is called activation, and the resulting radiation, residual radiation.

The units used to measure radiation are in the process of changing, the old units are being replaced by metric units. At this time both systems are being used simultaneously until the newer units become more familiar. The units vary in the aspect of radiation they measure, some measure the relative strength as a radioactive source and some the dose absorbed by a person in a given instance of exposure, others the relative effect of that dosage considering the type of radiation involved. They are used to determine the relative risk of a source of radioactivity and determine the probable outcomes of exposure. The relative strength of a source material is measured in Curies (Ci) or the new unit Becquerels (Bq). Both units are named for famous physicists. A curie is equivalent to 3.7 x 10 10 becquerels, the new SI or International System unit. A becquerel is one spontaneous nuclear transition per second, a curie is approximately equivalent to the number of nuclear transitions per second of one gram of Radium-226 or 37 billion disintegrations per second.

The curie was named for Pierre and Marie Curie (Photo) who discovered radium in 1898. The Becquerel is named for A.H. Becquerel (Photo) the discoverer of radioactivity. The Bq is such a small unit that it is often used with prefixes such as gigabecquerel (Gbq) or terrabequerel (Tbq) or 10 12 RAD is a measurement of the absorbed dose of radiation, in fact RAD is an acronym that stands for radiation absorbed dose. A RAD is equal to the absorption of 100 ergs per gram at the point of interest. The new measurement is the Gray, which was named for British radiobiologist L.H. Gray (Photo). The gray is the absorbed dose when the energy imparted per unit mass of matter by ionizing radiation is one joule per kilogram. One RAD, abbreviated Rd., is equal to 10-2 gray, abbreviated Gy. The erg and the joule are units of energy. The erg is equal to 10-7 joule. Both the RAD and the Gray apply to any material and any type of radiation. The RAD is 100 times less than the Gray. The average background radiation is estimated to be about 2 miligray (or 0.002 Gy) per year.

The REM is a measurement of the dose of radiation taking into account the differences in the various types of radiation, their relative biological effectiveness. A figure referred to as the Quality Factor is multiplied by the dose in RADS to give the dose in REM. The Quality Factor for X Rays, gamma rays or beta rays is 1, the QF for thermal neutrons is 2-5, the QF for fast neutrons is 5-10 and the QF for alpha rays is 20. The SI unit, the Sievert is used the same way and is equal to 1 Gy x QF. The same QF numbers are used. The higher the number, the more damage done to the organism. REM is an acronym that stands for Roentgen Equivalent Man, which refers to its original meaning, the unit dose of radiation required to produce the same biological effect as a one roentgen dose of X rays. Sievert is abbreviated Sv.

The Roentgen was the first unit of measurement for radioactive adopted in 1928. It is named for W.K. Roentgen (Photo) the discoverer of X rays. A Roentgen, abbreviated as R, is the quantity of X rays or gamma radiation required to produce ions carrying one electrostatic unit of electrical change in one cubic centimeter of dry air under average conditions. The equivalent SI unit is the coulomb per kilogram, the quantity of X rays or gamma radiation required to produce ion pairs carrying one coulomb of charge of either sign in one kilogram of pure dry air. The roentgen is equal to 2.58 x 10-4. The coulomb is named after Charles de Coulomb (Picture), a French physicist. A coulomb of energy is equal to the charge transferred by a current of one ampere in one second.