B. The Role of Radioactive Decay|
One process that alters the composition we might expect from cosmic events discussed above is radioactive decay. The processes already discussed produce all the natural elements, through plutonium, on the periodic chart. However, the amounts of these elements in nature cannot be entirely explained by those processes. Lead, in particular, is much more abundant than expected, if it is only produced in supernova explosions. What else could make lead, and how?
Figure 8: Unstable elements such as uranium 238 decay over time, resulting in a stable element: lead. (Click on image for larger version.)|
The secret is in the very heavy elements made in supernova explosions. Many of the elements heavier than lead have nuclei so large that they are fairly unstable. Due to the instability, over time they eject a neutron or proton, or a neutron in the nucleus decays into a proton and electron. This is called radioactive decay, since the original nucleus is "decaying" into a more stable one. Frequently, the decay results in a new element with a lower atomic number. (See "Binding Energy Per Nucleon" Figure 4.)
Lead is not radioactive, and so does not spontaneously decay into lighter elements. Radioactive elements heavier than lead undergo a series of decays, each time changing from a heavier element to a lighter or more stable one. Once the element decays into lead, though, the process stops. So, over billions of years, the amount of lead in the Universe has increased, due to the decay of numerous radioactive elements. Lead is still produced in supernova explosions, but it also slowly accumulates through the radioactive decay of other elements. This is why the total amount of lead we observe today is greater than can be explained by supernova production alone.
The explosive power of supernovae also create radioactive isotopes of a number of elements. These isotopes, such as 56Ni, 22Na, 44Ti, 27Al, decay into 56Fe, 22Ne, 44Ca, and 27Mg, respectively. This decay is accompanied by emission of gamma rays. Each of these elements decays on a different time scale, ranging from 100 days for 56Ni --> 56Fe, to 1 million years for 27Al --> 27Mg. By watching the light intensity from a fading supernova, astronomers can detect these time scales and determine the abundances of these elements. By observing the gamma ray lines with gamma ray observatories such as the European INTEGRAL mission, astronomers can study the sites of this element formation, the rate at which the formation occurs, and compare that with models of element formation from supernovae.
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