3. Nucleosynthesis
• Nucleosynthesis is the process of element (nuclei) formation
which creates new atomic nucleus from preexisting nucleons,
primarily protons and neutrons.
• Three types: Big Bang nucleosynthesis
Stellar (star) nucleosynthesis
Supernova nucleosynthesis
• Today, only stellar and supernova nucleosynthesis are
occurring in our universe.
• Element formation in our universe relies on nuclear fusion
reactions.
(fusion = come together)
4. The Big Bang
• The Big Bang Theory is the most widely
accepted scientific theory about the origin of
the universe. It is supported by multiple lines
of evidence.
• The “Big Bang” was a phenomenally energetic
explosion that initiated the expansion of the
universe.
• At the moment prior to the Big Bang explosion,
all matter and energy were compressed at a
single point (a singularity – a point of infinite
density).
• We do not know what was before…..?
• The universe has been expanding ever since,
with galaxies moving farther and farther apart.
• Using the rates of expansion measured in the
universe and astronomical distances, the age
of the universe can be calculated back to the
time of the Big Bang. The age of the universe
is calculated at about 13.7 billion years old. By
contrast, our Sun and its surrounding planets
(i.e. our Solar System) is 4.65 billion years old.
5.
6. Big Bang Nucleosynthesis
• All Hydrogen and most Helium in the universe was produced
during the Big Bang Event, starting ~100 seconds after the
explosion. A small amount of Lithium was also produced.
• Big Bang nucleosynthesis ceased within a few minutes after
the Big Bang because the universe had expanded and cooled
sufficiently by then such that the temperatures and pressures
were too low to support additional nuclear fusion reactions.
7. Nuclear Fusion
• In nuclear fusion, smaller nuclei collide together
to make larger nuclei, and energy is released in
the form of electromagnetic radiation.
• Requires extremely high temperatures and
pressures beyond those found on or within
Earth. However, these temperatures and
pressures are found inside stars and did occur
during the initial formation of our universe
(during the Big Bang event).
• Fusion involves only the nuclei of atoms. At the
temperatures at which fusion can occur, matter
exists as a plasma. This is the state of matter
where the electrons have been stripped off of
the atoms. Plasma is basically a super high
energy, electrically charged gas.
• When nuclei collide, some of the mass of the
nuclei is converted to energy by Einstein’s
famous equation, E=mc2. Nuclear fusion
releases a lot of energy per gram of material;
much more energy than is released by burning a
comparable amount of wood, coal, oil, or
gasoline!
8. Nuclear Fusion Requirements
(in stars)
Fusion Fusion By-product
Minimum Core
Temperature
Minimum Core
Density
Minimum Stellar
Mass*
Hydrogen He 13 million K 100 gm/cc 0.08 solar masses
Helium C, O 100 million K 100,000 gm/cc 0.5 solar masses
Carbon O, Ne, Mg, Na 500 million K 200,000 gm/cc 4 solar masses
Neon O, Mg 1.2 billion K 4 million gm/cc about 8 solar masses
Oxygen Mg, Si, S, P 1.5 billion K 10 million gm/cc about 8 solar masses
Silicon
Si, S, Ar, Ca, Ti, Cr, Fe,
Ni
around 3 billion K 30 million gm/cc about 8 solar masses
gm/cc = grams per cubic centimeter (units of density)
https://sites.uni.edu/morgans/astro/course/Notes/section2/fusion.html
9. Nuclear fusion
Is the process by which light nuclei fuse together to form a heavier
nucleus. When this happens, a tremendous amount of energy is released.
Through nuclear fusion, the light elements – hydrogen (H), helium (He),
and small amounts of lithium (Li) and beryllium (Be)-were formed. The isotope
is a form of an element that has the same atomic no. of the original element
but with a different atomic mass or mass no.
10.
11. 1. What was formed as the universe expanded and cooled down?
2. What do you call the process of creating new atomic nuclei from
preexisting nucleons?
3. What is the process by which light nuclei join together to form a
heavier nucleus?
Hydrogen (H) and Helium (He)
Nucleosynthesis
Nuclear Fusion reactions power the Sun and other stars
12. 1. Fusion of two deuterium isotopes to form helium-3
2. Fusion of deuterium and a neutron to form tritium
3. Fusion of deuterium and a proton to form helium-3
Proton-proton chain
They create a helium nucleus, which has two protons and two neutrons.
Nuclear Fusion reactions power the Sun and other stars. In a fusion
reaction, two light nuclei merge to form a single heavier nucleus. The
process releases energy because the total mass of the resulting single
nucleus is less than the mass of the two original nuclei.
13. Stellar Nucleosynthesis
• A star is a very hot ball of gas (plasma). Stars create elements by combining lighter nuclei into
heavier nuclei via nuclear fusion reactions in their cores and releasing energy in the process.
They are natural nuclear reactors!
• Enormous temperatures (15,000,000 K), pressures, and densities of matter are needed to
initiate the fusion (thermonuclear) reactions which squeeze nuclei together and release energy.
• The basic nuclear reaction in the Sun converts hydrogen to helium and releases energy in the
form of electromagnetic radiation (see the basic fusion reaction below). This is why our Sun
shines!
• Our Sun is only large enough to fuse hydrogen into helium within its core.
14. Stellar Nucleosynthesis
• Stars much larger than our Sun can
fuse heavier elements from lighter
elements.
• These giant stars have an “onion
layer” structure.
• As you proceed deeper into the
star, temperatures and pressures
increase, and heavier and heavier
nuclei are fused together.
• The heaviest element that can be
made in a star is iron. Elements
heavier than iron have fusion
reactions with temperature and
pressure requirements greater than
those that can occur within the
core of a giant star.
• Note: In the adjacent diagrams, the
term “burning” really means
nuclear fusion!
15. Supernova Nucleosynthesis
• Elements heavier than Iron (Z = 26) are
made primarily when giant stars explode
in supernovae.
• Even the largest stars do not have core
temperatures and pressures high enough
to fuse iron into heavier elements.
Therefore, when a star runs out of
nuclear fuel (lighter nuclei) and can no
longer undergo fusion reactions, gravity
causes the star to collapse. The
gravitational collapse triggers a
phenomenally large explosion called a
supernova. The explosion of the star
momentarily generates high enough
temperatures and pressures to cause
nuclear fusion reactions that make
elements with atomic numbers 27-92
(Cobalt to Uranium).
• Since only the largest stars can explode in
supernovae events, elements with
atomic numbers 27-92 are rarer than
elements with atomic numbers 1-26
(see abundance diagram to right)
An exploded star
(supernova)
Relative Abundance of the Elements in our Universe
16. Formation of Heavy Elements: Neutron Capture
Reaction
• Elements heavier than iron cannot be formed through fusion as
tremendous amounts of energy are needed for the reaction to occur.
• Heavy elements are formed in a supernova, a massive explosion of a
star.
• In a supernova, neutron capture reactions take place, leading to the
formation of heavy elements.
• In a neutron capture reaction, heavy elements are created by the
addition of more neutrons to existing nuclei instead of fusion of light
nuclei
17. • Adding neutrons to a nucleus
does not change an element.
Rather, a more massive isotope
of the same element is produced.
• Eventually, many neutrons will be
added to the nucleus that it
becomes unstable, and then it
decays radioactively to form a
stable nucleus of some other
element.
• Elements with an atomic mass
higher than iron required
tremendous amount of energy to
be formed. Thus, they were
produced from a neutron capture
reaction in a supernova.
19. Summary:
In this module, you have learned how elements are formed. There are
three (3) reactions that led to the formation of elements:
nucleosynthesis, fusion, and neutron capture reaction.
These reactions required a certain amount of energy to proceed, which
was obtained from the heat of the continuously expanding universe.
Thus, energy in the form of heat does not only produce work but also
the elements that make up matter that we have today
20. The reactions involved in the formation of these elements are
dependent on the atomic mass of the elements. More energy, and thus
higher temperature, is need to form heavier element.
Nucleosynthesis formed light elements, whereas fusion in stars
formed elements with an atomic mass that is within the range of
beryllium and iron. Thus, any element with an atomic mass higher than
iron, which required tremendous amount of energy to be formed, was
produced from a neutron capture reaction in a supernova
21. Task
• You are a graphic artist. For the National Science and Technology
Week, the Philippine Nuclear Research Institute (PNRI) commissioned
you to make a brochure on the applications of nuclear fusion in
various industries. You have to include relevant picture in your
brochure. The brochure will be evaluated based on accuracy of
details, appropriateness of pictures, correctness of grammar, and
creativity
25. Nuclear Fission
• We have learned that elements form in the universe by nuclear fusion reactions which assemble larger
nuclei by forcing smaller nuclei together under tremendous temperatures and pressures.
• However, elements can also form when a large, unstable nucleus breaks apart in an attempt to achieve
a more stable, lower energy state.
• The splitting of a nucleus to form two or more smaller, more stable nuclei is called nuclear fission.
(fission = split)
• Fission may occur spontaneously (without energy being added) or it may be prompted by firing a
nuclear bullet (like a proton or neutron) at an unstable nucleus, as seen in the example below.
• Like fusion, fission also releases energy stored in the nucleus of an atom. However, not as much energy
is released from fission as from fusion. Still, the energy released per gram of material by fission is
considerably more than the energy released by burning a comparable amount of wood, oil, gasoline,
etc. Fission of uranium-235 atoms is used in nuclear power plants to produce energy.
• Fission also occurs naturally within the layers of the earth as radioactive elements in rocks
spontaneously decay to more stable elements, creating a natural source of heat within the earth. You
also contain a small proportion of radioactive isotopes within your body. These isotopes decay
naturally, releasing radiation. Therefore, you are slightly radioactive too! So is the banana you ate for
breakfast!
Nuclear
bullet
26. Radioactivity
• Radioactivity is the release of energy,
in the form of energetic particles and
waves, from the nuclei of unstable
(radioactive) isotopes. Radioactive
atoms undergo fission-type reactions
in order to try to become more stable
nuclei with lower energies.
Radioactive atoms are called
radioisotopes.
• The nuclei of unstable, radioactive
isotopes have the wrong ratio of
neutrons to protons (n/p). Generally,
it is too high. When n/p of an isotope
falls between 1 to 1.5, the nucleus is
stable (within the “Band of Stability”
on a n0 vs. p+ plot). Outside of that
range, nuclei tend to be unstable and
break apart over time. This “breaking
apart” of unstable nuclei over time
and the accompanying release of
nuclear particles and energy is called
radioactive decay.
27. Types of Radioactive Decay – Alpha Decay
In alpha decay, an unstable
nucleus releases two
neutrons and two protons.
This is called an alpha ()
particle. It is equivalent to a
4
2He nuclei. Energy is also
released in the process.
As a result, the mass
number of the remaining
nucleus decreases by 4 and
the atomic number
decreases by 2. A new
element is formed in the
process!
Credit: Khan Academy
28. Types of Radioactive Decay – Beta Decay
In beta decay of an unstable
nucleus, a neutron suddenly
changes to a proton, releasing an
electron, a ghostly, low mass
particle called a neutrino (not
pictured), and energy!
As a result, the atomic number of
the remaining nucleus increases
by 1 but the mass number does
not change. A new element is
formed!
Note: The released electron did
not come from outside the
nucleus. It came from inside the
nucleus. It is called a beta ()
particle. Credit: Khan Academy
29. Types of Radioactive Decay – Gamma Decay
In gamma decay, an unstable
nucleus releases a high energy form
of electromagnetic radiation (light)
called a gamma () particle or a
gamma ray. This particle of light is
also known as a photon.
The energy is released as the
protons and neutrons in the
unstable nucleus reposition
themselves in an attempt to find a
lower energy arrangement.
Since no protons or neutrons are
released, the mass number and
atomic number of the nucleus
remain unchanged, and no new
element is formed. Gamma decay
usually accompanies alpha and beta
decay. Credit: Khan Academy
30. Nuclear Reactions can be Represented by
Nuclear Equations
• Fusion
Making a
larger nucleus
from two or
more smaller
nuclei
• Fission
Making two or
more smaller
nuclei from a
larger nucleus
31. Important Symbols Used in Nuclear Equations
• To write a nuclear
reaction, you
must remember
how to read and
use isotope
symbol notation
• You must know
the symbols used
for various
subatomic
particles like
protons,
neutrons, etc.
Particle How written in a nuclear reaction
Proton 1 1
p or H
1 1
Neutron 1
n
0
Electron
(Beta particle)
0 0
e or β
-1 -1
Alpha Particle
(Helium nuclei)
4 4
or He
2 2
Gamma Particle or
Ray
(a massless packet of pure
electromagnetic radiation, a form of
energy)
32.
33. 238
92U
32
15P
10
5B
Check the math on
these examples of
nuclear equations to
see if the sums of the
mass numbers and
the atomic numbers
are the same on each
side of the
equations.
Can you figure out
which equations are
fission and which are
fusion?
Balancing Nuclear Reactions
34. Transmutation
Transmutation is a general term for the changing of chemical element
or isotope to another by changing the number of protons and/or
neutrons. Fusion and fission reactions both qualify as transmutations.
The bombardment of a nucleus by a nuclear bullet in order to change it
into another element also counts as transmutation.
35. Synthetic Elements
• Elements with atomic numbers Z ≥ 93
are synthetic (man-made)
• These elements have been made in
particle accelerators, either by
smashing smaller nuclei together or
else by shooting nuclear bullets at
large nuclei.
• These elements are all radioactive.
They decay over time to more stable
elements, releasing radiation (particles
and energy) from their nuclei. Some
have very short half-lives and have
only existed for fractions of a second.
• Some synthetic elements have uses for
mankind. Americium (Am) is used in
smoke detectors. Others have no
current use but were made during
basic research to better understand
atomic nuclei and the forces that hold
them together. The heaviest synthetic
element has an atomic number of 118.
It has no uses at present.
