<P> Along with all elements having atomic weights higher than that of iron , uranium is only naturally formed in supernovae . Primordial thorium and uranium are only produced in the r - process ( rapid neutron capture ) , because the s - process ( slow neutron capture ) is too slow and can not pass the gap of instability after bismuth . Besides the two extant primordial uranium isotopes , U and U , the r - process also produced significant quantities of U , which has a shorter half - life and has long since decayed completely to Th , which was itself enriched by the decay of Pu , accounting for the observed higher - than - expected abundance of thorium and lower - than - expected abundance of uranium . While the natural abundance of uranium has been supplemented by the decay of extinct Pu ( half - life 0.375 million years ) and Cm ( half - life 16 million years ) , producing U and U respectively , this occurred to an almost negligible extent due to the shorter half - lives of these parents and their lower production than U and Pu , the parents of thorium : the Cm : U ratio at the formation of the Solar System was ( 7.0 ± 1.6 ) × 10 . </P>
<P> Along with all elements having atomic weights higher than that of iron , uranium is only naturally formed in supernovae . Primordial thorium and uranium are only produced in the r - process ( rapid neutron capture ) , because the s - process ( slow neutron capture ) is too slow and can not pass the gap of instability after bismuth . Besides the two extant primordial uranium isotopes , U and U , the r - process also produced significant quantities of U , which has a shorter half - life and has long since decayed completely to Th , which was itself enriched by the decay of Pu , accounting for the observed higher - than - expected abundance of thorium and lower - than - expected abundance of uranium . While the natural abundance of uranium has been supplemented by the decay of extinct Pu ( half - life 0.375 million years ) and Cm ( half - life 16 million years ) , producing U and U respectively , this occurred to an almost negligible extent due to the shorter half - lives of these parents and their lower production than U and Pu , the parents of thorium : the Cm : U ratio at the formation of the Solar System was ( 7.0 ± 1.6 ) × 10 . </P>
where did the uranium on earth come from
where did the uranium on earth come from
supernovae
supernovae