How does successive neutron capture produce heavier transuranium elements?

Successive neutron capture produces heavier transuranium elements through a repeating cycle of neutron absorption and radioactive decay. This process generally follows these steps:12

• Neutron Capture: A target nucleus absorbs a neutron, increasing its mass number by one while its atomic number remains the same.13
• Beta Decay: The resulting isotope is often unstable and undergoes negative beta decay (${\beta }^{-}$-decay). During this process, a neutron within the nucleus is transformed into a proton, which increases the atomic number by one.124

This sequence of neutron capture (increasing mass) followed by beta decay (increasing atomic number) allows for the synthesis of successively heavier elements. Two distinct rates of this process are recognized:1

• Slow Neutron Capture (s-process): Neutrons are added at a rate that allows unstable nuclei sufficient time to undergo beta decay before another neutron is captured.12
• Rapid Neutron Capture (r-process): Neutrons are added so quickly that unstable nuclei capture multiple neutrons before they have the opportunity to beta-decay. This process typically occurs in explosive environments, such as nuclear explosions or stellar events.12

This major path of synthesis is limited; for example, it terminates at fermium-257 because the subsequent isotope, fermium-258, undergoes spontaneous fission too rapidly to allow for further element production.1