Types of Radiation
There are generally four types of radiation associated with radioactive decay:
Alpha particles -- An alpha particle is a positively charged particle emitted in the radioactive decay of some unstable atoms. It is made up of two protons and two neutrons (it is essentially the nucleus of a helium atom) and is thus heavier and slower-movingthan other decay emissions. Alpha particles do not penetrate far into a material and can be stopped quite easily; however, they are capable of breaking chemical bonds (which can cause chemical or biological damage) when they strike a molecule because of their size, mass and charge. (Penetration distance of alpha particles depends upon the energy with which they are emitted and the materialthrough which they are passing). Thus, while alpha particles can be stopped by thin barriers such as a piece of paper or skin, alpha emitters are mostly damaging if they are ingested or inhaled into the lungs.
Beta particles -- A beta particle is emitted during the radioactive decay of some unstable atoms. Beta particles can have either a negative charge or a positive charge and they have the samevery small mass (1/2000 the mass of a neutron) regardless of charge. A negatively charged beta particle is infrequently referred to as a negatron, but is usually called an electron, and a positively charged beta particle is called a positron. Most beta particles that are produced in the decay of naturally occurring radioisotopes are electrons. Positrons are usually the result of the decay ofman-made radioisotopes such as radioisotopes used in Positron Emission Tomography (PET scan), which has become a mainstay of nuclear medicine diagnoses. Beta particles can penetrate farther than alpha particles (again, penetration distance depends upon the energy with which they are emitted and the material through which they are passing); however, they can be stopped fairly easily by a sheet ofaluminum. Positrons are less penetrating than electrons because they interact with an electron and both particles are annihilated, which produces two 511 keV photons (a photon is a quantum, or unit, of electromagnetic radiation and 511 keV is the rest mass energy of electrons and positrons; thus, annihilation is the process where the total mass of the positron and electron is converted to energy).Electromagnetic radiation -- Two types of electromagnetic radiation (see The Radiation Spectrum) are associated with radioactive decay. Electromagnetic radiation originating in the nucleus of an atom is referred to as a gamma ray (this happens when the nucleus transitions from a higher energy level to a lower energy level). Electromagnetic radiation emitted by an atomic electron changingenergy levels is referred to as an x-ray. Gamma rays usually have higher energies than x-rays and both can penetrate matter farther than can particles. They can be stopped by high density materials such as several feet of concrete or lead.
Neutrons -- Neutrons are particles having a mass 1/4 that of an alpha particle and 2000 times that of a beta particle. The neutron has no electrical charge.It has the potential to penetrate matter deeper than can charged particles but this depends greatly on the physical and atomic nature of the matter being penetrated.
Decay and Half Lives
The Atom : Structure of the Atom
Allmatter (solid, liquid or gaseous) consists of elements, of which there are more than 100. If, in theory, we cut a block of iron into smaller and smaller pieces, we would finally end up with the smallest piece possible that still has all the characteristics of the iron element. That smallest piece is called an iron atom. An atom is very, very small. In fact, the size of an atom compared to the...