Atoms that are heavier than iron are generally produced by which process?

• A. spontaneous fission of uranium-238
• B. nuclear fusion in the cores of main-sequence stars
• C. neutron capture followed by beta decay (the s- and r-processes)
• D. alpha decay of heavy nuclei

Answer: (C)

Verified step by step guidance

1. Understand that elements heavier than iron (Fe) are not typically formed by fusion in normal stellar cores because fusion beyond iron is not energetically favorable; it requires energy input rather than releasing energy.
2. Recognize that spontaneous fission of uranium-238 and alpha decay of heavy nuclei are decay processes that break down heavy elements rather than build heavier ones.
3. Learn that neutron capture processes, specifically the slow (s-process) and rapid (r-process) neutron capture followed by beta decay, are responsible for creating elements heavier than iron in stars and during explosive astrophysical events.
4. The s-process occurs in relatively stable environments like asymptotic giant branch stars, where neutrons are captured slowly compared to beta decay, allowing the nucleus to move up the periodic table stepwise.
5. The r-process happens in more extreme environments such as supernovae or neutron star mergers, where a high flux of neutrons is captured rapidly before beta decay can occur, producing very heavy and neutron-rich nuclei.