Radiation
Radiation refers to many types of energy that travel through space, including light and radio waves. Ionizing radiation is a more dangerous and particular form of radiation. The radiation that most people think of is ionizing radiation. Elements are said to be radioactive when their atoms are unstable. Ionizing radiation occurs when unstable atoms release energy or mass to achieve stability. Eventually, these atoms may become stable.
Radioactive Decay
The process of losing mass or energy in radioactive materials is called radioactive decay. Radioactive decay occurs on an atomic level. At the smallest level, radioactive decay occurs one nucleus at a time. The basic measurement of radioactive decay is one Becquerel, or one atomic decay per second. Radiation is very difficult to measure at these minuscule levels, so a system of larger measurements is usually used.
International System of Units
The International System of Units (SI) is an internationally accepted standard for measuring weights, distances, quantities and in the case of radioactive decay, events. One Bq describes one atomic decay per second. Scientists commonly measure radioactive decay in larger quantities, such as the microcurie (uCi), which is equal to 37,000 Bq, or the millicurie (mCi), which is equal to 37 million Bq. The former U.S. standard for measuring radioactive decay was the curie (ci), which is equal to 37 billion Bq.
Half-Life
Another common measure of radioactive decay is the half-life. A radioactive element's half-life is the amount of time it takes for half of its atoms to decay. Some elements have very long half-lives, while some elements have extremely brief half-lives. The 4.5-billion-year half-life of uranium-238 compared to the 160-microsecond half-life of polonium-214 represent two extremes in the spectrum of radioactive half-lives.