How Elements Heavier Than Iron
Were Formed
LIMITATIONS OF BIG BANG/STELLAR
NUCLEOSYNTHESIS
Fusion reactions above Fe is unfavorable
• Nuclear binding energy per nucleon holds the nucleus intact
• Smaller nuclear binding energy per nucleon
• Further fusion reactions with Fe require more energy
LIMITATIONS OF BIG BANG/STELLAR
NUCLEOSYNTHESIS
Nucleosynthesis of elements beyond Fe are nonspontaneous and
require different pathways
• Neutrinos released by a supernova help in forming neutrons
and protons which then get captured by nuclei residing in
nearby stars
• Neutron or proton capture processes help in achieving
higher-level nucleosynthesis
NEUTRON CAPTURE
Neutron capture starts with a neutron being added to a seed
nucleus
This starting reaction would then produce a heavier isotope of the
element
NEUTRON CAPTURE
Beta decay results in an increase in the number of protons of the
nucleus by one
A heavier nucleus of a new element is formed
NEUTRON CAPTURE: S-PROCESS
Slow neutron capture or s-process happens when there is a
small number of available neutrons
• The rate of neutron capture is slow compared to the rate of
beta decay (hence the term slow)
• If a beta decay occurs, it almost always occurs before another
neutron can be captured
NEUTRON CAPTURE: S-PROCESS
Slow neutron capture or s-process happens when there is a
small number of available neutrons
• These occur mainly on red giant or supergiant stars, with each
neutron capture taking a decade and the cascade of processes
taking thousands of years to complete
NEUTRON CAPTURE: R-PROCESS
Rapid neutron capture or r-process happens when there is a
large number of available neutrons
• The rate of neutron capture is fast that an unstable nucleus
may still be combined with another neutron prior to beta decay
(hence the term rapid)
NEUTRON CAPTURE: R-PROCESS
Rapid neutron capture or r-process happens when there is a
large number of available neutrons
• Associated with supernovae, in which the temperatures are
tremendously high that the neutrons are moving very fast
• Neutrons can immediately combine with isotopes that are
already heavy
PROTON CAPTURE
Proton capture or p-process starts with the addition of a p to a
nucleus after a supernova is formed
The tremendous amount of energy available allows the addition of
a p to the nucleus
PROTON CAPTURE
Proton capture or p-process starts with the addition of a p to a
nucleus after a supernova is formed
Produces a heavier nucleus that is different from the seed nucleus
1
Stellar nucleosynthesis fusion reactions cannot
produce nuclei higher than iron. Synthesis of heavier
nuclei happens via neutron or proton capture
processes.
2
In neutron capture, a neutron is added to a seed
nucleus. The addition of neutron produces a heavier
isotope of the element. Neutron capture can occur slowly
or rapidly.
3
Beta decay results in an increase in the number of
protons of the nucleus by one. Hence, a heavier nucleus
is formed.
4
Proton capture or p-process is the addition of a proton
in the nucleus.
GIVEN THE HALF-LIFE, ASSESS WHETHER THE
FOLLOWING NUCLIDES WILL UNDERGO SPROCESS, R-PROCESS, OR DECAY.
1. Fe-70 (77 ms)
2. Sr-88 (stable)
3. Ge-77 (11.21 h)
GIVEN THE FIGURE BELOW, WHICH ELEMENTS
ARE SYNTHESIZED VIA NUCLEAR FUSION
REACTIONS?