Radioactivity and radionuclides

There are 118 known chemical elements and around 3300 known atomic nuclei. About 3000 of these nuclei are radioactive and such atoms are called radionuclides. Radionuclides can be natural or anthropogenic.

• Naturally occurring radionuclidescan be found in the Earth's crust and the atmosphere. Most naturally occurring radionuclides were created when the Earth was formed, while some are produced in the upper atmosphere as a result of nuclear reactions with cosmic rays.
• Anthropogenic (artificial) radionuclidesare produced in nuclear reactions. We can induce a nuclear reaction by shooting fast-charged particles or neutrons at atomic nuclei. Thus, a new, radioactive nucleus is created. The nuclear reaction called nuclear fission occurs when slow-moving neutrons collide with the heavy nucleus of uranium.
• Some nuclei that were formed through radioactive decay can be radioactive as well and they continue to decay until a stable nucleus is formed. This series of radionuclide decays is called a decay chain. Most known are the uranium (U-238) and thorium (Th-232) decay series.

Radiation types and half-life

Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. When radioactive nuclei decay, they radiate different mass particles with high kinetic energy or they emit energy in the form of electromagnetic waves. Three of the most common types of decay are alpha decay, beta decay, and gamma decay, all of which involve emitting one or more particles and photons.

Alpha particles are twice positively charged heavy particles consisting of two protons and two neutrons. They are in fact the nuclei of helium.

Beta particles are either negatively or positively charged light particles with the same mass as electrons. Gamma rays are electromagnetic waves with short wavelengths and high energies.

Radioactive isotopes are stable enough to exist for varying lengst of time before they eventually decay to produce other nuclei.  The time in which half of the original number of nuclei decay is called the half-life. The length of a half-life can vary significantly, it can be less than a one thousandth of a second or a hundred billion years.

The activity of a source is measured as the number of decays of radioactive nuclei per unit of time. The unit of activity is the becquerel (Bq). One becquerel equals one decay per second. An activity of 50 Bq means that 50 radioactive nuclei decay in one second. From about 3000 known radionuclides, around 2400 have half-lives of less than one hour. Radionuclides with half-lives of less than a few days are called short-lived radionuclides. The most well-known short-lived radionuclides are the natural radionuclide radon Rn-222 and artificial radionuclide iodine I-131 (half-life of 8.04 days). Radionuclides with half-lives of more than a few decades are called long-lived radionuclides. The most well-known artificial long-lived radionuclides in the environment are fission products Cs-137 and Sr-90 (half-lives of 30.17 years and 28.8 years, respectively). The half-lives of the natural long‑lived radionuclides uranium U-238, thorium Th-232 and potassium K-40 are several billions of years.

Radioactivity in the environment

Radioactivity is present everywhere in the environment. In the world that surrounds us, the vast majority of substances are radioactive, including hard earthen materials such as rocks and soil, but also freshwater and saltwater, as well as gases and particles in the atmosphere. Consequently, plants, animals and human beings are radioactive as well. Natural radionuclides in the human body are the long-lived isotope of potassium, K-40, as well as radium Ra-226, carbon C-14 and tritium H-3.