Radioactivity and Radiation
Discussion of radioactivity and radiation, uranium and radioactivity, radiological health risks of uranium isotopes and decay products.
Radioactivity is the term used to describe the natural process by which some atoms spontaneously disintegrate, emitting both particles and energy as they transform into different, more stable atoms. This process, also called radioactive decay, occurs because unstable isotopes tend to transform into a more stable state. Radioactivity is measured in terms of disintegrations, or decays, per unit time. Common units of radioactivity are the Becquerel, equal to 1 decay per second, and the Curie, equal to 37 billion decays per second.
Radiation and Radionuclides
Radiation refers to the particles or energy released during radioactive decay. The radiation emitted may be in the form of particles, such as neutrons, alpha particles, and beta particles, or waves of pure energy, such as gamma and X-rays.
Each radioactive element, or radionuclide, has a characteristic half-life. Half-life is a measure of the time it takes for one half of the atoms of a particular radionuclide to disintegrate (or decay) into another nuclear form. Half-lives vary from millionths of a second to billions of years. Because radioactivity is a measure of the rate at which a radionuclide decays (for example, decays per second), the longer the half-life of a radionuclide, the less radioactive it is for a given mass.
Sources of Radiation
Everyone is exposed to radiation on a daily basis, primarily from naturally occurring cosmic rays, radioactive elements in the soil, and radioactive elements incorporated in the body. Man-made sources of radiation, such as medical X-rays or fallout from historical nuclear weapons testing, also contribute, but to a lesser extent. About 80% of background radiation originates from naturally occurring sources, with the remaining 20% resulting from man-made sources.
Uranium and Radioactivity
All isotopes of uranium are radioactive, with most having extremely long half-lives. Half-life is a measure of the time it takes for one half of the atoms of a particular radionuclide to disintegrate (or decay) into another nuclear form. Each radionuclide has a characteristic half-life. Half-lives vary from millionths of a second to billions of years. Because radioactivity is a measure of the rate at which a radionuclide decays (for example, decays per second), the longer the half-life of a radionuclide, the less radioactive it is for a given mass. The half live of uranium-238 is about 4.5 billion years, uranium-235 about 700 million years, and uranium-234 about 25 thousand years.
Decay Products of Uranium
Uranium atoms decay into other atoms, or radionuclides, that are also radioactive and commonly called "decay products." Uranium and its decay products primarily emit alpha radiation, however, lower levels of both beta and gamma radiation are also emitted. The total activity level of uranium depends on the isotopic composition and processing history. A sample of natural uranium (as mined) is composed of 99.3% uranium-238, 0.7% uranium-235, and a negligible amount of uranium-234 (by weight), as well as a number of radioactive decay products.
Radiological Health Risks of Uranium Isotopes and Decay Products
In general, uranium-235 and uranium-234 pose a greater radiological health risk than uranium-238 because they have much shorter half-lives, decay more quickly, and are thus "more radioactive." Because all uranium isotopes are primarily alpha emitters, they are only hazardous if ingested or inhaled. However, because several of the radioactive uranium decay products are gamma emitters, workers in the vicinity of large quantities of uranium in storage or in a processing facility can also be exposed to low levels of external radiation.