Radioactive Decay Eric R. Christian Elements 2002 Workshop
Th234 a What is Radioactive Decay? • Some atoms are not stable, which means that even if they are completely left to themselves, they will not last forever. • Radioactive Decay is when an unstable atom of one element SPONTANEOUSLY changes to another element (Wow! Alchemy!). • Because of certain conservation laws (you should always conserve!), this necessarily includes the release of other particles. U238 2
Radioactive Decay relates to Isotopes • The type and speed of a radioactive decay depends upon the isotope rather than the element (in other words, the number of neutrons determines how stable the nucleus is). • An element may not have ANY stable isotopes (true of all the heaviest elements such as Uranium), or it may have one, or several, or as many as six stable isotopes. This is determined by whether it has the “right” number of neutrons to make it stable. • Every element has radioactive isotopes (the “wrong” number of neutrons). Element 1: Hydrogen ALL Hydrogen has 1 Proton per atom There are 3 Isotopes of Hydrogen: Hydrogen - No Neutrons - Stable Deuterium - 1 Neutron - Stable Tritium - 2 Neutrons - Unstable P P N P N N
Line of Stability • Protons are all positively charged and therefore electrically repulsive to one another. This is compensated by the attractive “Strong Nuclear Force.” You need enough neutrons so that the strong nuclear force balances the tendency of the protons to push apart. • If you put the stable isotopes on a plot of Number of Protons (Z) vs. the Number of Neutrons (N), you find that they cluster around a curve that is known as the “Line of Stability.” • Light Elements are stable when the number of neutrons roughly equals the number of protons (N = Z). • Heavy Elements need approximately 1.5 times as many neutrons as protons in order to be stable.
Little Stuff Little Stuff Types of Radioactive Decay Beta Decay • There are several different types of radioactive decay, but they group into two basic varieties: • Beta Decay • An electron or anti-electron (positron) is emitted or captured. The total number of nucleons (protons + neutrons) is the same in the old and the new element but either an proton has changed into a neutron or vice versa. • Fission • The nucleus splits into pieces. The total number of nucleons remains the same, but they are split into two or more smaller nuclei (elements) Little Stuff means electrons, positrons, and neutrinos Fission
e- P N N P N P P N Anti-neutrino N P Boron-10 5 Protons 5 Neutrons e+ P N N P N Neutrino N P Lithium-7 3 Protons 4 Neutrons Beta Decay Electron Emission • There are two basic types of Beta Decay: • Electron emission: an electron (negatively charged) and an anti-neutrino are released and a neutron is changed into a proton. The total number of nucleons and the charge is conserved. The element moves up one space on the periodic chart (since it now has one more proton). • Positron emission: a positron (anti-electron, positively charged) and a neutrino are released and proton is changed into a neutron. The total number of nucleons and the charge is conserved. The element moves down one on the periodic chart (since it now has one less proton). P N N P N P N N N P Beryllium-10 4 Protons 6 Neutrons Positron Emission P N N P P N P Beryllium-7 4 Protons 3 Neutrons
e- P N N P N Neutrino N P Lithium-7 3 Protons 4 Neutrons Electron Capture • There is another way for isotopes that decay by positron emission to decay. A proton in the nucleus can capture an electron (usually one of the orbiting electrons in the inner shell) and change into a neutron. The end result is the same as positron emission. • It is also possible for nuclei that decay via electron emission to have positron capture as well. But positrons are much rarer in the universe than electrons, and there are none orbiting close by, so positron emission is nearly impossible. Electron Capture P N N P P N P Beryllium-7 4 Protons 3 Neutrons
Thorium-234 90 Protons 144 Neutrons Little Stuff Alpha Particle (Helium nucleus) 2 Protons 2 Neutrons Curium-244 96 Protons 148 Neutrons Little Stuff Neon-20 10 Protons 10 Neutrons Fission Alpha Decay • Fission only happens with heavy elements. • The simplest type of fission is called alpha-decay. A group of two protons and two neutrons (called an “alpha particle”, which is basically a helium nucleus) splits off and the rest of the nucleus remains as a whole. • Fission can also result in the nucleus splitting into a bunch of fragments of varying sizes. • Fission is sometimes called Spontaneous Fission to distinguish it from Induced Fission, which is when you hit the nucleus with a projectile such as a neutron. Induced fission is responsible for most of the reactions in nuclear power plants and nuclear bombs. Uranium-238 92 Protons 146 Neutrons Little Stuff means electrons, positrons, neutrons, and neutrinos Spontaneous Fission Seaborgium-258 106 Protons 152 Neutrons
Radioactive Decay is a Random Process • You can NEVER tell when an individual atom is going to decay. You can figure out approximately how many atoms in a group are going to decay in a certain time, but you can’t tell which ones are going to blow. • The timescale for radioactive decay is described by the quantity called a “half-life”. • Half-lives can be VERY short (helium-5 decays in 7.6 x 10-22 seconds), or very long (thorium-232 decays in 1.4 billion years).
Time (T) = 0 N undecayed atoms N/2 undecayed atoms N/2 something else T =t½ N/4 undecayed atoms 3/4 x N something else T = 2 xt½ T = 3 xt½ N/8 undecayed atoms 7/8 x N something else . . . T = 10 xt½ N/1024 undecayed atoms 1023/1024 x N something else What is a Half-Life? • The half-life (t½) is the amount of time that it will take half of the atoms to decay. This does not mean that in twice that amount of time, all the atoms will decay. Since this is a random process, there is no history and you have to start over, so in the second half-life, half of the remaining atoms will decay, leaving a quarter of the original atoms. • Note: All the atoms will still be there, but the ones that have decayed will be a different element.
Radioactive Decay is Important for Which Elements? • During their nucleosynthesis in large stars and supernovae, many of the heavy elements (heavier than iron) are actually created as different isotopes that decay really quickly to something stable or, at least, less unstable. • On longer timescales, radioactive decay is important for lead, because it is one of the most stable of the heaviest elements and many heavier elements decay to it (sometimes via a long chain of radioactive decays: U238Th234 Pa234U234Th230Ra226Ru222Po218Pb214Bi214Po213 Pb210Bi210Po210Pb206 which is stable).