Decay chain
From Free net encyclopedia
Nearly all the decay products of radioactive decay are themselves radioactive. Because of this, most radioactive substances do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached. Such series are known as decay chains.
A parent isotope is one that undergoes decay to form a daughter isotope. The daughter isotope may be stable or it may decay to form a daughter isotope of its own. The daughter of a daughter isotope is sometimes called a granddaughter isotope.
The intermediate stages are often far more dangerous than the original radioisotope. For example, pure natural uranium metal is not dangerously radioactive, but many lumps of pitchblende, a uranium ore, are dangerously radioactive because of the radium and other daughter nuclei they contain. Radium itself is extremely dangerous for its radioactivity alone, but its chief danger is the gaseous radon it generates as the next stage in the decay chain.
The four most common modes of radioactive decay are: alpha decay, beta minus decay, beta plus decay and isomeric transition. Electron capture also occurs, as does (rarely) neutron emission. Of these decay processes, alpha decay changes the atomic mass number of the nucleus, and always decreases it by four, while the very rare process neutron emission decreases it by one. Because of this, almost any decay will result in a nucleus whose atomic mass number has the same residue mod 4, dividing all nuclides into four classes. The members of any possible decay chain must be drawn entirely from one of these classes.
Three main decay chains (or families) are observed in nature, commonly called the thorium series, the radium series (not uranium series), and the actinium series, representing three of these four classes, and ending in three different, stable isotopes of lead. The mass number of every isotope in these chains can be represented as A=4n, A=4n+2 and A=4n+3, respectively. The starting isotopes of these three have existed since the formation of the earth. The fourth chain, the neptunium series with A=4n+1, is already extinct due to quite short half life time of its starting isotope 237Np. The ending isotope of this chain is 205Tl. Some older sources give the final isotope as 209Bi. This is a consequence of the relatively recent discovery of its extremely weak radioactivity.
There are also many shorter chains, for example carbon-14. On the earth, most of the starting isotopes of these chains are generated by cosmic radiation. Primary cosmic ray particles are stripped of their electrons; this makes the cosmic ray Beryllium-7, for example, unable to decay in flight, because its only decay mode is electron capture. The nucleus has no orbital electrons and passes by matter too quickly to capture electrons from it.[1]
In the tables below, the minor branches of decay (with the branching ratio of less than 0.0001%) are omitted. The energy release includes the total kinetic energy of all the emitted particles (electrons, alpha particles, gamma quanta, neutrinos, Auger electrons and X-rays) and the recoil nucleus.
External links
- Decay chains
- Uranium-238 decay chain
- Some numerical calculations based on the Uranium-232 decay chain
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