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In this explosion various radioactive and high-mass nuclei are formed by
the constituent nuclei colliding together with a high enough relative
kinetic energy that the endothermic fusion reactions which form these
nuclei can proceed as they absorb the needed energy from the explosion's
excessive vigor.
David Bowman
dbowman@georgetowncollege.edu
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I have deleted most of David's excellent discussion in order to focus
on this one point, given above. In supernovae, elements with A
greater than 60 may be built up via r-process (i.e., rapid process)
neutron capture, rather than the endothermic process David discussed.