Nuclear Physics

The Atomic Nucleus, Radioactivity, Fission and Fussion

M. Rocha   

Physics 2B

The Atomic Nucleus

Rutherford Scattering

The Rutherford scattering experiment (1911) demonstrated that atoms must have a core (Nucleus) 

The Atomic Nucleus

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Today we know the atomic nucleus is made out of protons and neutrons, which subsequently are made out of quarks

Nuclear Forces

Electric Force

Strong Force

vs.

Electric force pushes protons apart

But the strong force pulls the quarks inside the nucleons together

The electric force has a long range

The strong force has a short range

The strong force is stronger than the electric force only when the nucleons are very close together (short distances)

Heavy Nuclei

As nuclei get heavier, they need more neutrons than protons in order for the attractive strong force to dominate over the repulsive electric force

Unstable Nuclei

Radioactivity

Isotopes Review

= Protons + Neutrons

= Number of Protons 

Hydrogen-1

Hydrogen-2

Hydrogen-3

Checkpoint 1

Carbon-14 is a radioactive Carbon isotope with 6 protons. Carbon-14 decays into Nitrogen-14 by one of its neutrons decaying into a proton. How many neutrons does Nitrogen-14 has?

Neutron Decay

Neutrons are unstable by themselves but stable in nuclei.
A high neutron/proton ratio however makes nuclei unstable, causing the neutrons to decay into protons

Neutrons decay into a proton, an electron and an antineutrino

Radioactive Decay

Unstable atoms undergo radioactive decay in order to find stability

Beta Decay:  A neutron decays into a proton emitting an electron (Beta Particle)

Alpha Decay: A Helium atom (alpha particle) is ejected from the parent nucleus

Checkpoint 2

If an atom decays emitting alpha radiation, would the decay product (daughter atom) be an isotope of the original atom, or a new element?

Gamma Decay

Gamma decay is the emission of a gamma photon from the transition of a nucleus in an excited energy state to a lower (relaxed) energy state

There is no change in the atomic or mass number of the nucleus after the gamma decay

X-Rays

X-Rays are photons with a lot less energy than gamma rays. X-Rays originate from high energy electronic energy transitions, not from nuclear radioactive decays

X-Rays penetrate through skin but are  stopped by bones

Radiation Penetration Power

• Alpha particles have the most charge and the least speed, so they can be easily stopped. A few centimeters of air or a sheet of paper is enough to stop alpha rays.
• Beta particles travel faster and have less charge, so they are harder to stop. Aluminum foil or any other thin metal sheet will stop them.
• You need a thick layer of lead to stop gamma rays.

Radioactive Ray Dispersion

You can separate the alpha, beta and gamma rays from a sample with a magnetic field

Summary of Nuclear Radioactive Decays

Radiation Exposure

Units of Radiation Exposure

RADS (Radiation Absorbed Dose): Is a unit of radiation dosage that measures the amount of energy in Joules absorbed per 1 kg of tissue 

REMS (Roentgen Equivalent Man) and Sieverts: Units of radiation potential damage. They correlate the absorbed dose of any radiation to the biological effect of that radiation. Since not all radiation has the same biological effect, the dosage is multiplied by a "quality factor" Q.

Q = 1  for X, Gamma and Beta radiation

Q = 10  for Alpha  radiation

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(SI unit)

Radon Gas

We all get about a 100-150 millirem radiation dose per year from radon gas 

Radiation Exposure

Radiation exposure/dose per year depends on life choices—however the average, healthy US citizen receives about 360 millirem of dose per year.

1 millisievert (mSv) = 100 millirem

1 millisievert (mSv) = 100 millirem

Extreme radiation exposure can damage DNA molecules in cells and produce mutations

Radiation Detection

Scintillators

Scintillators produce photons from the fluorescence of a gas or a phosphor as the radiation excites electrons in the atoms

Scintillators are more sensitive to gamma rays

Geiger Counters

Geiger Counters produce currents from the ionization of a gas as the radiation knocks some electrons off from the gas atoms

Radioactive Half-Life

The half-life of a radioactive material is the time needed for half of its mass to decay

Since some radioactive nuclei are more stable than others,  different radioactive materials have different half-lifes

For example Uranium-238 has a half life of 4.5 billion years, whereas Helium-6 has a half-life of ~1 second

Checkpoint 1

If a 10 kg sample of Carbon-14 reduces in weight by 1 g per yr, what would be the half life of Carbon-14?

Radioactive Half-Life

Radium-226 has a half-life of 1620 years

An initial 1 Kg Radium-226 sample will decay to 1/2 Kg in 1620 years. In another 1620 years there will be only 1/4 Kg left

Radiometric Dating

Carbon Dating

• Organic material emits radiation due to Carbon-14 decay 

• When living organism die they stop taking Carbon-14 and the radiation rates go down with time

• Radiation rates tell us how long ago organic material died

Carbon Dating

The radioactive carbon isotopes in the skeleton diminish by one half every 5730 years. The red arrows symbolize relative amounts of carbon-14

Uranium Dating

The current ratio of lead to uranium in a mineral can be used to determine its age

Nuclear Fission and Fusion

Nuclear Binding Energy

The mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it

 The difference is a measure of the nuclear binding energy which holds the nucleus together

Nuclear binding energy = Δmc^2

Nuclear Binding Energy

Another way to see the same concept is by looking at the mass per nucleon

 The higher the binding energy per nucleon the lower the mass per nucleon

Nuclear Fusion

In nuclear fusion, energy is released when light nuclei fuse together

Nuclear Binding Energy

The binding energy per nucleon increases from Hydrogen to Iron, but then decreases as the repulsive electric force between protons starts to dominate in heavy elements

Checkpoint 2

If there are 10^21 atoms in 1 gram of iron, with about 500 Mev of binding energy per atom. For how many hours can you keep a 60 Watt light bulb on with the binding energy of 1 gram piece of iron?

Nuclear Fission

Nuclear fission is the splitting of a heavy nucleus into  lighter nuclei plus energy

Nuclear Fission

Nuclear deformation leads to fission when repelling electrical forces dominate over attracting nuclear forces

Nuclear Fission

The absorption of a neutron by a uranium nucleus supplies enough energy to cause such an elongation

One neutron starts the fission of the uranium atom and three more neutrons are produced when the uranium fissions

Chain Reaction

If there are other fissionable uranium atoms around a chain reaction starts,  and a fission explosion occurs

The fission of one U-235 atom releases about seven million times the energy released by the explosion of one TNT molecule

Chain reactions do not occur in uranium deposits because only the rare isotope U-235 is fissionable by absorption of a neutron

Only 0.7% or 1 part in 140 of uranium is U-235. The prevalent isotope, U-238, absorbs neutrons but does not undergo fission

U-235 vs U-238

Critical Mass

a. If the piece of uranium is too small, a neutron is likely to escape through the surface before it “finds” another nucleus and the chain reaction dies out.

b. For there to be a sustained chain reaction a pice of uranium with a mass above the critical mass is required

Nuclear Fission Bomb

In a fission bomb two subcritical pieces of uranium are put together to form a super critical piece for detonation

Nuclear Fission Reactors

A nuclear fission reaction can be controlled to produce generate usable energy

The reaction is controlled by using Uranium with low percentages of U-235, and control rods that can be moved in and out and control the number of neutrons

Transmutation of Elements

Radioactive elements transform to other elements as they climb down the transmutation latter in the search for stabitlity

Waste Products of Fission

A major drawback to fission power is the generation of radioactive waste products

Towards Cleaner Nuclear Energy

Nuclear Fusion

In nuclear fusion, energy is released when light nuclei fuse together

Thermonuclear Fusion

For fusion to occur, nuclei must collide at very high speeds to overcome electrical repulsion.

Fusion brought about by high temperatures is called thermonuclear fusion

In the central part of the sun, about 657 million tons of hydrogen are converted into 653 million tons of helium each second

The missing 4 million tons of mass is discharged as radiant energy

Natural Fusion Reactors

Fusion Reactors

The dream of clean energy production! No radioactive waste nor air pollution, and no risk of getting out of control

A real engineering challenge though! We haven't been able to produce sustainable fusion that produces more energy than what is input

The End

Good job for hanging in there, we made it!

1. Physics is the study of energy, forces, matter            and the dynamics caused by their interactions

2. Energy and matter, waves and particles, they are all the same, just different representations of energy fields

3. Everything is vibrating, and vibrations produce waves of energy

Wish you all good vibes and positive energy!

If nothing else I hope you all take home these 3 things: