Date: Sat Mar 26 08:34:39 2011
Posted By: Matthew Buynoski, Process Integration Engineer (retired)
Area of science: Physics
Radioactive atoms have the energy put into them in several ways:
Back at the very beginnings of the universe, some very small amounts of very light radioactive
atoms may have been formed (e.g. tritium) but it's impossible to tell because if any were formed has
long ago decayed away. In this possible case, the energy came from the initial energy of the newly
Action of extremely energetic particles (cosmic rays) colliding with atoms can cause radioactivity.
This is how carbon-14 (used for radioactive dating) is formed. In this case, the energy is supplied by
the energetic particle and the original nucleus that was modified.
Inside of stars. Giant, massive stars can add neutrons to atoms by two methods.
The s (for slow) process occurs mainly in red giants and can make elements up to about
bismuth in atomic mass. Many of the nuclei formed by this method are initially radioactive and decay
(typically by beta particle) to make, eventually, stable nuclei. However, some have very long half-lives
and can persist for billions of years.
The r (for rapid) process occurs in supernovae, and adds neutrons to atoms so fast that the
atomic masses go up to at least that of uranium and probably beyond it. Some of these atoms are
radioactive with very long half-lives (e.g. uranium 238 with a 4.4 billion year half-life). In this case the
energy in those atoms comes from the original nuclei and the neutrons that the supernova explosion
injects into them.
Decay products of other radioactive nuclei. Here the energy is inherent in the original nucleus.
Radium is an example.
Understand that any nucleus has a lot of energy in it. Sometimes modification of a nucleus, by a slow
neutron, e.g., will cause the original nucleus to become unstable, thus allowing the energy already in
that nucleus to be released. Fission of uranium illustrates this; absorbtion of one neutron destabilizes
the uranium nucleus, causing it to split and release daughter elements plus additional neutrons. More
energy is released than was supplied by the incoming neutron, so the rest must have been inherent in
the uranium nucleus. So we must add a 5th source of energy:
- Formation of elements up to about iron in atomic mass, in massive stars, as a result of nuclear
fusion in the core of the star. So far as I know, this method does not create any radioactive elements
directly (or at least none of my books mention it), but in the mish-mash of reactions that constitute
what is known as "silicon burning", where photodisintegration and fusion reactions are competing, it is
at least possible that some long-lived isotopes like potassium-40 get formed. Maybe some short-
lived ones, too, but those wouldn't last very long in that environment. All these elements have a lot of
mass-energy in them. Later on, the s-process and r-process can take the elements so formed and
create heavier elements, which we know today as radioactive (uranium, thorium, etc).
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