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Why are some materials more radioactive than others?

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March 30, 2008 11:54PM

Radioactivity stems from the instability of the nucleus of a given atom. Remember that in an atomic nucleus, protons and neutrons are held together with nuclear glue or binding energy (1H being the exception). Protons don't like each other to begin with. But under the most extraordinary conditions (like in a star), protons and neutrons can be forced together and fused (fusion) to create more complex nuclei. And in a supernova, elements heavier than iron (the heaviest "regular" element that a star makes during "normal" fusion) are created. In all this "creativity" and among all the products that result, some atomic nuclei that are formed aren't really happy with their arrangement. They are unstable, and at some time in the future they will spontaneously break apart. In some arrangements of nucleons (the particles that make up an atomic nucleus, the protons and neutrons), the ratio of the two types of particles, the ratio of protons to neutrons, is one that "strains" the combinational power that holds them together and other arrangements are possible. It is the number and type of nucleons that make up a nucleus that determines how stable it is. There are many stable nuclei. There are many combinations that are not possible - they will never form, they cannot form - and then there are the unstable nuclei. The different numbers of protons and neutrons that make up a nucleus make for a different "dynamic" in each atomic nucleus in which they are confined. Some are structures that will stay together, and in some of the structures formed, the nucleons can "shift" and break the structure of the nucleus, thereby allowing the nucleons to move to a lower energy level state. In radioactive decay, a shift in the nuclear structure and the release of a particle (or particles) and/or energy, allows the remaining nucleons to "rewrite" the terms and conditions of their "confinement" in the nucleus. This spontaneous transition is what radioactive decay is. The possibilities are why some nuclei are stable and some are not, and why some are more stable than others. It is impossible to say when any given unstable atom will decay, but over a large number of them, an "avarage" rate of decay can be quantified. That will allow us to know the half life of that radionuclide.