Natural uranium consists of three major isotopes: uranium 238 (99.28% natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%). All three are radioactive. Uranium-238 is the most stable isotope, with a half-lifeof 4.51 × 109 years (close to the age of the Earth). Uranium-235 has a half-life of 7.13 × 108 years, and uranium-234 has a half-life of 2.48 × 105 years.
Uranium-238 is an α emitter, decaying through the 18-member uranium natural decay series into lead-206. The decay series of uranium-235 (also called actino-uranium) has 15 members that ends in lead-207. The constant rates of decay in these series makes comparison of the ratios of parent to daughter elements useful in radiometric dating. Uranium-234 decays to lead-206 through a series of short-lived intermediaries. Uranium-233 is made from thorium-232 by neutron bombardment; its decay series ends with thallium-205.
The isotope uranium-235 is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, can be broken apart by thermal neutrons. The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which is also fissile.
Alpha particles don't get very far in the environment. Once emitted, they travel relatively slowly (at approximately one-twentieth the speed of light) due to their electric charge and large mass. They lose energy rapidly in air, usually expending it within a few centimeters. Because alpha particles are not radioactive, once they have lost their energy, they pick up free electrons and become helium.Alpha particles also cannot penetrate most matter they encounter. Even a piece of paper, or the dead outer layers of human skin is sufficient to stop alpha particles.
The health effects of alpha particles depend heavily upon how exposure takes place. External exposure (external to the body) is of far less concern than internal exposure, because alpha particles lack the energy to penetrate the outer dead layer of skin. However, if alpha emitters have been inhaled, ingested (swallowed), or absorbed into the blood stream, sensitive living tissue can be exposed to alpha radiation. The resulting biological damage increases the risk of cancer; in particular, alpha radiation is known to cause lung cancer in humans when alpha emitters are inhaled.
