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 – Atomic Mass (# of protons & neutrons)• Z – Atomic Number (# of protons)• X – Chemical Symbol

The Nucleus• For example, let’s look at the following atom:

• 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:

Number of Radioactive Atoms Currently Present

Number of Radioactive Atoms Originally 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

• 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

• Positron Emission

• Electron Capture

• 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.

antineutrino

neutrino

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:

• 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!)

• 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!

Energy released Change in mass

Speed of light (c = 3.0 x 108)

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: