Nuclear Physics
Nuclear Physics
• Nuclear Physics is the study of the atom.
• This is a larger part of modern physics study, however we will
only look at basic energy exchanges occurring in the atom.
Subatomic Particles
• We know that an atom can be broken down into smaller
particles. The important particles we should know:
Name
Symbol
Charge (C)
Mass (kg)
Proton
p
+e
1.67 x 10-27
Neutron
n
0
1.67 x 10-27
Electron
e-
-e
9.11 x 10-31
Positron
e+
+e
9.11 x 10-31
Photon
γ
0
0
The Nucleus
• From your studies in Chemistry you should recall that the
nucleus:
• is located at the center of the atom.
• is made up of protons and neutrons.
• tells us what element we are looking at.
• We use the following model to describe the atom we are
studying:
𝐴
𝑍𝑋
• A – Atomic Mass (# of protons & neutrons)
• Z – Atomic Number (# of protons)
• X – Chemical Symbol
The Nucleus
• For example, let’s look at the following atom:
12
6𝐶
• From this we know:
•
•
•
•
The atom is classified as carbon
It has 6 protons
It has 6 neutrons
If it is neutral it will have 6 electrons
• An Isotope has the same number of protons, but a different
number of neutrons than the most abundant form of that
atom.
• This change in neutrons leads to different nuclear properties
Radioactivity
• Radioactivity is the spontaneous emission of radiation
resulting from changes in the nuclei of the atom.
• Basically, it is an atom giving away extra energy that it does not
need.
• There are 3 different types of radioactivity:
• Alpha Decay
• Beta Decay
• Gamma Decay
Half-Life
• Radioactive Half-Life is the time required for half of the
radioactive substance’s mass to decay.
• One use of half-life is to date things such as artifacts and
fossils to determine how old they are.
• The equation that we use for half-life is:
𝑁 = 𝑁𝑜
𝑡
−0.693 𝑡
1 2
𝑒
Number of Radioactive Number of Radioactive
Atoms Currently
Atoms Originally
Present
Present
Time Elapsed
Radioactive half-life
of the specific
isotope
Alpha Decay
• In an alpha decay, a helium nucleus is ejected from the nucleus.
• The original nucleus is called the ‘parent’ nuclei
• The now smaller nucleus is called the ‘daughter’ nuclei
𝐴
𝑍𝑃𝑎𝑟𝑒𝑛𝑡
=
𝐴−4
𝑍−2𝐷𝑎𝑢𝑔ℎ𝑡𝑒𝑟
+ 42𝐻𝑒
• This allows the parent nuclei to become smaller which will lower the
amount of energy that atom has.
• This type of decay occurs with heavier elements ( Z > 52 Tellurium)
Beta Decay
• In beta decay, the nucleus will capture or emit electrons to change
the make up of the nucleus.
• There are three varieties:
• Beta Emission
𝑛
𝑝 + 𝑒− + 𝜐−
𝑝
𝑛 + 𝑒+ + 𝜐
antineutrino
• Positron Emission
• Electron Capture
neutrino
𝑝 + 𝑒−
𝑛+𝜐
• Again, the point of this is to lower the energy in the atom.
• Atoms would undergo Beta Decay to achieve the optimal proton to
neutron ratio. Unlike Alpha Decay, any atom can undergo Beta Decay.
Gamma Decay
• In gamma decay, a short wavelength form of light called a
gamma ray is ejected from the nucleus.
• Like Beta Decay, any atom can undergo Gamma Decay. This is
the most energetic and most dangerous form of radioactivity.
Rules for Radioactivity
• Two quantities are conserved when radioactive decay occurs:
• Conservation of Charge
• Conservation of Atomic Mass
• Therefore to determine if a radioactive decay is possible, be
sure that the total charge stays the same and the number of
nucleons (protons and neutrons) remains constant!
• For example:
238
92𝑈
234
90𝑇ℎ
+ 42𝐻𝑒
• In this alpha decay of Uranium, it ejects a helium nucleus and
leaves behind a daughter Thorium atom.
Nuclear Forces
• There are four fundamental forces in nature:
•
•
•
•
Gravity
Electromagnet
Weak Nuclear
Strong Nuclear
• The Weak Nuclear Force is the one responsible for the
radioactive decay process. It is now believed to actually be a
part of the electromagnet force (which would combine them
to make an electroweak force!).
• The Strong Nuclear Force is the most important one of all
since it is responsible for keeping the nucleus together.
Strong Nuclear Force
• Since most nuclei have more than one proton in them, the
Electromagnet force is trying to repel/push those positive
protons away from each other.
• The Strong Nuclear Force opposes the Electromagnet force
and keeps the nucleus from separating.
• It is 100 times strong than the Electromagnet Force
• It is 1,000,000,000,000,000,000,000,000,000,000,000,000,000
times stronger than Gravity.
• It has a very, very small range that it can apply its force within.
Nuclear Fission
• Nuclear fission is the splitting of an atomic nucleus into
smaller nuclei.
• Radioactivity is a natural form of fission, however we only
refer to fission as the man made variety using a chain reaction.
• Fission releases A LOT of energy when the atom is split.
• Which is why fission is used in nuclear reactors and atomic
bombs.
Nuclear Fission
• In this example a neutron collides with a Uranium atom and
starts a chain reaction:
Energy Released in Fission
• This brings us to Einstein’s famous equation (which you can
now brag about the fact you know what it means!)
Δ𝐸 = Δ𝑚𝑐
Energy released
2
Speed of light
(c = 3.0 x 108)
Change in mass
• By figuring out how much mass has been ‘lost’ we can figure
out the amount of energy that was released.
• What Einstein’s equation proves is that mass and energy are
the same thing in two different forms!
Nuclear Fusion
• Nuclear fusion is the combination of two smaller particles to
make a bigger particle.
• Nuclear fusion is what the sun does to create its energy
(which we notice in the forms of heat and light).
• Nuclear fusion has also been used as an atomic bomb, but has
yet to be used to produce power in nuclear reactors.
Nuclear Fusion
• In this example, two Hydrogen atoms fuse to form a Helium
atom: