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An Introduction to Nuclear Physics

Atomic nucleus


Nuclear fission

Nuclear fusion

Nuclear Energy

Strong force

Weak force

Nuclear decay

Accelerated decay

Nuclear physics: The study in physics of the atomic nucleus.

Nuclear physics mainly deals with the neutrons and protons within the nucleus and how they interact with each and external influence. It is the area of physics dealing with nuclear energy including reactor and bombs. The nucleus of an atom is the relatively small dense positively charged mass at the atom’s center. It consists of two types of particles: Positively charged Protons and neutrally charged (no charge) Neutrons and they are bound together in the nucleus by way of residual strong nuclear force between these particles.

The simplest atomic nucleus is found in hydrogen and consists of a single photon.

Others are more complicate including helium which consists of two photons and two neurons.

The atomic nucleus is the densest part of the atom and the element of each  is atoms determined by the number of protons in the nucleus. The number of protons in the nucleus of an atom is called the atomic number and it is unique to each element. The number of neutrons can vary in what are called Isotopes.

Isotopes: The variations within atoms of the same element distinguished by the number of neutrons in the nucleus.

In writing isotopes are distinguished by a particle number that equals the total number of protons and neutrons in the nucleus. The written designation is given by the particle number and the element symbol.


Uranium-235 235U
Uranium-238 238U
Carbon-12 12C
Carbon-14 14C

Each isotope has its own set of unique properties. The particle number is number of protons and neutrons in the nucleus and the atomic number is number of protons in the nucleus. It unique to the element. The atomic weight is the mass of the isotope in atomic mass units and 1 atomic mass unit = 1.66053892110−27 kg. The half-life of an isotope the time is takes half of a radiometric isotope to decay with stable isotopes having an infinite half-life.

Nuclear Fission

Illustration of Uranium-235 fission.

Nuclear Fission is the process of splitting an atomic nucleus to two smaller atomic nuclei. The two isotopes most commonly used in fission are Uranium-235 and Plutonium-239. A chain reaction is the process of the neutrons one nuclear fission event causing others forming a chain of fission events that can runaway if the number of neutrons is not controlled. The number of neutrons is controlled in a nuclear reactor but not controlled in a bomb. Now controlled Uranium fission is a major source of Energy in several nations including the U.S. in fact several billion Uranium-235 atoms probably gave their all to bring you this web page.

Nuclear Fusion

This is an illustration of tritium fusion to from helium-4

Nuclear fusion is the joining of small atomic nuclei into larger atomic nuclei. Now given enough energy theoretically any two nuclei can fuse but most of possible fusion reactions can not be used to produce energy.  For a fusion reaction to be a usable source of energy, it has to meet several criteria.

Exothermic: ( Produce heat) While this is obvious it keeps the reactants to nuclei with low numbers of protons. They also tend to produce product nuclei of vary tight binding such as helium 4.
Use low Z nuclei: (low numbers of protons) Electrostatic repulsion of protons needs to be overcome for nuclei to get close enough to fuse. With high z nuclei electrostatic repulsion it uses to much energy to be exothermic.
Two reactants: At densities less than that of the core of a star, three body collisions are too improbable for energy production.
Produce at least two products: This makes energy and momentum conservation both possible in the same reaction.
Conservation of protons and neutrons: Their weak interaction cross sections are too small to be practical for an energy source.

Nuclear Fusion is the energy source for stars and it is also the one energy source that has the potential to be a virtually limitless clean source of energy. It is the ultimate form of nuclear energy since its fuel is plentiful and it by product is non radioactive helium.

Nuclear Energy

Nuclear energy is the use of nuclear fission or fusion to derives energy from these reactions. There are two ways of tapping nuclear energy: nuclear bombs and nuclear reactors. In needs to be noted that nuclear reactors are controlled reactions that even in a worst case scenario lack sufficiently concentrated nuclear fuel to become a nuclear bomb.

Video of a Nuclear bomb explosion.

There are two types of nuclear bombs fission bombs that uses ether uranium or plutonium and fusion bombs that use tritium fusion. They are actually triggered by small fission bombs. They are mainly used as weapons having been uses in war only twice at the end of WWII. However it has also been proposed to use them to propel spacecraft in what is call a nuclear pulse engine.

Orion nuclear pulse engine concept spacecraft.

Generation of electricity is the main peaceful use of nuclear energy. In fact the electricity used to make the page came from such a power plant. The main use of a nuclear reactor is generating electricity.
One type of nuclear reactor called a breeder reactor makes plutonium producing more nuclear fuel than it consumes. While electricity is the main use of nuclear energy, nuclear reactors can also be used for transportation.

Nuclear subs are commonly used for military purposes. Their big advantage is that they can go a long time without refueling or resurfacing.

Nuclear rockets are another possible use for a nuclear reactor. One design called NERVA that was a solid core nuclear rocket was actually built in 1964 and tested but none has never flow in space. However liquid core and gas core engines have also been explored in theory though never built.

Nuclear energy has many possible applications some of the crazier of which include nuclear cars and planes. While not without risks, it is the safest and most efficient energy source in use today.

Strong Nuclear Force

Strong Nuclear Force: The fundamental force of nature that is the binding force of the atomic nucleus. The other three fundamental forces are electromagnetism, the weak force and gravity.

On the atomic scale Strong Nuclear Force is about 100 times stronger than electromagnetism and orders of magnitude stronger than the weak force and gravity. Strong Nuclear Force mainly acts on the quarks that make up Protons and Neutrons.

Two up quarks and one down quark make up a Proton and two down quarks and one up quark make up a Neutron. In both cases the gluons go back and forth between the quarks binding them together. This is the Beginning of the binding force of the Atomic nucleus.

By Manishearth (Own work) [CC-BY-SA-3.0 or GFDL], via Wikimedia Commons

The gluons (shown here as small colored double circles) that bind quarks of the proton and neutron together also hold a quark-antiquark pair called a pion together. A gluon can decay or interact with each other so as to form a pion. The pion help transmit a residual part of the strong force between the protons and neutrons. The quark of the pion swaps places with a quark in the receiving proton or neutron. The new quark-antiquark pair then annihilate each other to form a gluon. This residual part of the strong nuclear force is called the residual strong force or just the nuclear force. This nuclear force binds protons and neutrons into atomic nuclei.

Weak Nuclear Force

Weak Nuclear Force: The fundamental force of nature that is responsible for several forms of particle decay including the beta decay of atomic nuclei. The other three fundamental forces are strong nuclear force, electromagnetism, and gravity.

Negative Beta decay is the process where a neutron is turned into a proton while emitting an electron an electron anti-neutrino. The process involves a down quark decaying into an up quark, an electron  and an electron anti-neutrino. The immediate particle of the weak nuclear force is called a W boson. While Negative Bata decay is not the only process involving the W boson it is the most common since Negative Beta decay is a main from of atomic nuclear decay.

This is a Feynman diagram of negative beta decay of a neutron into a proton, electron and electron anti-neutrino, by way of a W− boson.
p  Proton
n Neutron
u Up Quark
d Down Quark
e Electron
W W− boson
νe electron anti-neutrino

The process starts with a down quark in the neutron decaying into a up quark and W− boson, turning the neutron into a proton. The W− boson quickly decays into an electron and an electron anti-neutrino.

Nuclear Decay

Nuclear Decay: The process where an unstable atomic nucleus loses energy by emitting ionizing particles with a net loss of rest mass to the nucleus.

While there are many types of nuclear decay there are two main ones found in nature, Alpha decay and Negative Beta decay.

In alpha decay the nucleus ejects an alpha particle.
An alpha particle is a helium atomic nucleus consisting of 2 protons and 2 neutrons.
The process reduces the atomic number of atom by 2 changing the element.

In negative Beta decay a neutron emits a Beta particle and an electron anti-neutrino to become a proton.
Beta particles are actually negatively charged electrons.
This process increases the atomic number of the nucleus by one.

For collections of radioactive nuclei there is a characteristic decay rate usually designated by the isotope's half-life. The isotope's half-life is the time needed for half of the isotope to decay. This half-life is independent of the amount of the isotope present because the more atoms of the isotope there are the more nuclei will decay in a given unit of time making the half-life is independent of the amount of the present isotope. Nuclear decay is often used in determining the age of a sample however there are assumptions involved in the process. It is especially assumed that the half-life of an  isotope is constant.

Accelerated Nuclear Decay

Accelerated Nuclear Decay is the process by which nuclear decay proceeds at a faster than normal rate. Now small amounts of accelerated nuclear decay have been observed in Beta-decay under some circumstances, while accelerated alpha decay has never been directly observed evidence for it exists in the retention of radiogenic helium by zircon crystals. The main arguments against the hypothesis that the retention of radiogenic helium by zircon crystals shows substantial accelerated alpha decay is heat and the lack of an observed cause of accelerated alpha decay. However there are theoretical answers to both. The real reason for resistance to the hypothesis that the retention of radiogenic helium by zircon crystals shows substantial accelerated alpha decay is the fact that it would drastically reduce radiometric ages. However none of these arguments change the fact that measured helium diffusion rates in zircon crystals are a perfect match to a model showing accelerated alpha decay about six thousand years ago.


Nuclear physics is basically the science of the nucleus of the atom. It describes varieties of atoms within the same element called isotopes It also describes binding forces of the nucleus as well as how and why they decay. Nuclear physics has made available a powerful and relatively clean source of energy. However because that energy can also be used to create a devastating bomb it is also feared. Yet future developments in fusion have the potential of producing an even safer and cleaner source of energy.


Nuclear Physics Past, Present and Future

Nuclear Fusion

Nuclear Fusion

What is Nuclear Energy?

Have the decay rates been constant?


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