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Radioactive Decay Eric R. Christian Elements 2002 Workshop 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 U238 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. Th234 a 2 Radioactive Decay relates to Isotopes The type and speed of a radioactive Element 1: Hydrogen decay depends upon the isotope ALL Hydrogen has rather than the element (in other 1 Proton per atom words, the number of neutrons There are 3 Isotopes of Hydrogen: determines how stable the nucleus is). Hydrogen - No Neutrons - Stable An element may not have ANY stable isotopes (true of all the heaviest P elements such as Uranium), or it may have one, or several, or as Deuterium - 1 Neutron - Stable many as six stable isotopes. This is determined by whether it has the “right” number of neutrons to make PN it stable. Every element has radioactive isotopes Tritium - 2 Neutrons - Unstable (the “wrong” number of neutrons). PN 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. Types of Radioactive Decay Beta Decay There are several different types of radioactive decay, but they group into Little two basic varieties: Stuff 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 Little Stuff means electrons, positrons, new element but either an proton has and neutrinos changed into a neutron or vice versa. Fission Fission The nucleus splits into pieces. The total number of nucleons remains the Little same, but they are split into two or Stuff more smaller nuclei (elements) Beta Decay There are two basic types of Beta Decay: Electron Emission Electron emission: an electron (negatively P P charged) and an anti-neutrino are N N e- N P N N P N released and a neutron is changed into P N P P N N N P a proton. The total number of nucleons Anti-neutrino N P and the charge is conserved. The Beryllium-10 Boron-10 element moves up one space on the 4 Protons 5 Protons periodic chart (since it now has one 6 Neutrons 5 Neutrons more proton). Positron emission: a positron (anti- electron, positively charged) and a Positron Emission neutrino are released and proton is P P e+ changed into a neutron. The total N N N N P P P N number of nucleons and the charge is Neutrino N P N P conserved. The element moves down one on the periodic chart (since it now Beryllium-7 Lithium-7 has one less proton). 4 Protons 3 Protons 3 Neutrons 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 Capture electron (usually one of the orbiting electrons in the inner shell) and change P e- P N N into a neutron. The end result is the P N P P N N same as positron emission. N P Neutrino N P It is also possible for nuclei that decay via Lithium-7 Beryllium-7 electron emission to have positron 4 Protons 3 Protons capture as well. But positrons are much 3 Neutrons 4 Neutrons rarer in the universe than electrons, and there are none orbiting close by, so positron emission is nearly impossible. Fission Fission only happens with heavy Alpha Decay elements. The simplest type of fission is called Thorium-234 alpha-decay. A group of two protons Little 90 Protons and two neutrons (called an “alpha Stuff 144 Neutrons particle”, which is basically a helium Uranium-238 92 Protons Alpha Particle nucleus) splits off and the rest of the 146 Neutrons (Helium nucleus) nucleus remains as a whole. 2 Protons Fission can also result in the nucleus 2 Neutrons splitting into a bunch of fragments of Little Stuff means electrons, positrons, varying sizes. neutrons, and neutrinos Fission is sometimes called Spontaneous Fission Spontaneous Fission to distinguish it from Induced Fission, which is when you hit the nucleus with a projectile Little Curium-244 Stuff 96 Protons such as a neutron. Induced fission is 148 Neutrons responsible for most of the reactions Seaborgium-258 106 Protons in nuclear power plants and nuclear Neon-20 152 Neutrons bombs. 10 Protons 10 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). What is a Half-Life? Time (T) = 0 N undecayed atoms The half-life (t½) is the amount of time that it will take half of the t½ N/2 undecayed atoms T= N/2 something else atoms to decay. This does not mean that in twice that amount of time, all the atoms will decay. T=2x t½ N/4 undecayed atoms 3/4 x N something else Since this is a random process, there is no history and you have T=3x t½ N/8 undecayed atoms to start over, so in the second 7/8 x N something else 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. T = 10 x t½ N/1024 undecayed atoms 1023/1024 x N something else 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).
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