Physical Science: Formation of Heavy Elements

Study Notes

Stellar Nucleosynthesis

Stellar nucleosynthesis produced nuclei heavier than helium-4 by nuclear fusion, starting with the fusion of two helium-4 nuclei to form beryllium-8, releasing energy in the form of gamma radiation.
This process continues up to nickel-56, which is unstable and undergoes positron emission, resulting in a nucleus with lower atomic number.

Limitations of Fusion Reactions

Fusion reactions cannot produce nuclei heavier than iron-56 because the nuclear binding energy per nucleon decreases after iron-56, making fusion reactions unfavorable.
Different pathways are needed for the synthesis of heavier nuclei.

Neutron Capture

Neutron capture is a process where a neutron is added to a seed nucleus, producing a heavier isotope of the element.
The generated isotope, when unstable, undergoes beta decay, resulting in an increase in the number of protons of the nucleus by 1, forming a heavier nucleus.
Neutron capture can either be slow (s-process) or rapid (r-process).

Slow Neutron Capture (s-process)

Slow neutron capture occurs when there is a small number of neutrons, and the rate of neutron capture is slow compared to the rate of β-1 decay.
β-1 decay occurs before another neutron can be captured.

Rapid Neutron Capture (r-process)

Rapid neutron capture occurs when there is a large number of neutrons, and the rate of neutron capture is fast.
The r-process is associated with a supernova, where the temperature is extremely high, and neutrons are moving very fast.
Unstable nuclei may combine with another neutron just before undergoing β-1 decay.

Proton Capture (p-process)

Proton capture is the addition of a proton to the nucleus, producing a heavier nucleus that is different from the seed nucleus.
It occurs after a supernova, where there is a tremendous amount of energy available.
The addition of a proton to the nucleus is not favorable due to Coulombic repulsion.

Key Points

Fusion reactions cannot account for the synthesis of nuclei heavier than iron-56.
Neutron and proton capture processes are responsible for the synthesis of nuclei heavier than iron-56.
Seed nucleus is the starting material for the formation of heavier isotopes or new nuclei.
Neutron capture process can be either slow (s-process) or rapid (r-process).
The r-process and proton capture are processes that happen in a supernova because a tremendous amount of energy is needed for them to occur.

Description

Explore the process of how elements heavier than iron are formed through stellar nucleosynthesis. Learn about nuclear fusion, gamma radiation, and positron emission in the creation of elements up to nickel-56.