Physical Science: Formation of Heavy Elements

12 Questions

What happens during slow neutron capture (s-process)?

A small number of neutrons are captured slowly.

Why is the rapid neutron capture process (r-process) called 'rapid'?

The rate of neutron capture is fast.

What is the main factor that makes proton capture (p-process) unfavorable?

Coulombic repulsion

Why does the r-process occur in supernovae?

Tremendous energy is available after a supernova.

Which type of nucleosynthesis is associated with the rapid neutron capture process?

Supernova nucleosynthesis

What happens if a 0−1β decay occurs during slow neutron capture?

It almost always occurs before another neutron can be captured.

What is the product of fusing two helium-4 nuclei according to the text?

Beryllium-8

Why do fusion reactions become unfavorable after iron-56?

Decrease in nuclear binding energy per nucleon

In neutron capture, what happens when a neutron is added to a seed nucleus?

Generation of a heavier element

What is the result of nickel-56 undergoing positron emission?

Formation of iron-56

How are heavier nuclei synthesized after iron-56?

Through neutron or proton capture processes

What is the consequence of an unstable isotope undergoing beta decay?

Increase in protons of the nucleus by 1

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.