<P> Rare - earth elements , except scandium , are heavier than iron and thus are produced by supernova nucleosynthesis or the s - process in asymptotic giant branch stars . In nature , spontaneous fission of uranium - 238 produces trace amounts of radioactive promethium , but most promethium is synthetically produced in nuclear reactors . </P>
<P> Rare - earth elements , except scandium , are heavier than iron and thus are produced by supernova nucleosynthesis or the s - process in asymptotic giant branch stars . In nature , spontaneous fission of uranium - 238 produces trace amounts of radioactive promethium , but most promethium is synthetically produced in nuclear reactors . </P>
where do most rare earth elements come from
where do most rare earth elements come from
by supernova nucleosynthesis or the s - process in asymptotic giant branch stars
by supernova nucleosynthesis or the s - process in asymptotic giant branch stars