Nuclear force

The nuclear force is a powerful force that holds particles called nucleons together inside the tiny centers of atoms, known as atomic nuclei. Nucleons are made up of protons and neutrons. Even though protons all have a positive charge and normally push each other away, the nuclear force is so strong at very close distances that it can overcome this push and hold the nucleus together.

Force (as multiples of 10000 N) between two nucleons as a function of distance as computed from the Reid potential (1968). The spins of the neutron and proton are aligned, and they are in the S angular momentum state. The attractive (negative) force has a maximum at a distance of about 1 fm with a force of about 25000 N. Particles much closer than a distance of 0.8 fm experience a large repulsive (positive) force. Particles separated by a distance greater than 1 fm are still attracted (Yukawa potential), but the force falls as an exponential function of distance.

This force works best when nucleons are about 0.8 femtometres apart. If the nucleons get any closer, the force pushes them apart instead. This helps decide how big atomic nuclei can get. Because of this, atoms are much larger than their nuclei.

The nuclear force is very important for energy. When protons and neutrons come together to form a nucleus, some of their mass is turned into energy. This energy can be released in nuclear power plants or used in nuclear weapons.

Description

Comparison between the Nuclear Force and the Coulomb Force. a – residual strong force (nuclear force), rapidly decreases to insignificance at distances beyond about 2.5 fm, b – at distances less than ~ 0.7 fm between nucleons centres the nuclear force becomes repulsive, c – coulomb repulsion force between two protons (over 3 fm, force becomes the main), d – equilibrium position for proton – proton, r – radius of a nucleon (a cloud composed of three quarks). Note: 1 fm = 10−15 m

The nuclear force is a special power that works between tiny particles called hadrons, especially between protons and neutrons in atoms. These particles are influenced almost equally by this force. Even though protons push each other away because they have a positive charge, the nuclear force can pull them together when they are very close.

At very short distances, the nuclear force pushes the particles apart to keep them a certain distance from each other. But when the particles are a little farther apart, the force pulls them together strongly. This pulling force is what helps keep the nucleus of an atom together, even though the protons want to push each other away. The strength of this force changes depending on how the particles spin and how close they are to each other.

History

The nuclear force has been important in nuclear physics since 1932, when the neutron was discovered. Scientists have tried to understand how protons and neutrons stick together in the nucleus.

After the neutron was found, scientists like Werner Heisenberg suggested ideas about how protons and neutrons work together. In the 1930s, Hideki Yukawa tried to explain the nuclear force using tiny particles called mesons. Later, scientists made more detailed models to describe the nuclear force.

As a residual of strong force

An animation of the interaction. The coloured double circles are gluons. Anticolours are shown as per this diagram (larger version).

The nuclear force is a weaker effect of a more powerful force called the strong force. The strong force holds tiny particles named quarks together to make protons and neutrons. This strong force is one of nature's basic forces and works through particles called gluons. Gluons keep quarks together using something called colour charge, which is like electric charge but much stronger. While quarks and gluons stay inside protons and neutrons, some of their effects reach just beyond, creating the nuclear force.

This nuclear force between protons and neutrons is similar to forces between neutral atoms in chemistry, known as London dispersion forces. These forces are weaker and work over shorter distances than the forces that hold atoms together. In the same way, the nuclear force is much weaker than the strong force inside protons and neutrons, and it only works over very short distances. However, it is strong enough to hold neutrons and protons together in the nucleus, even though protons would normally push each other apart because of their similar electric charge.

Nucleon–nucleon potentials

Two-nucleon systems like the deuteron, the nucleus of a deuterium atom, help scientists study the special force between nucleons. They describe these systems by giving a special "potential" to the nucleons and use math equations. The shape of this potential comes from experiments, and theories about meson-exchange help build it. Scientists match the details of the potential to real experiment results.

The most used potentials for nucleons are the Paris potential, the Argonne AV18 potential, the CD-Bonn potential, and the Nijmegen potentials. A newer way uses special math theories to describe forces between nucleons and groups of three nucleons. One of these theories, called Quantum hadrodynamics, works like other theories that describe different forces in nature. Another theory looks at how nucleons interact using particles called pions.

From nucleons to nuclei

The big goal in nuclear physics is to explain all the forces inside nuclei by starting with the basic forces between nucleons. This is called the "microscopic" way of studying nuclear physics. There are two big challenges: doing very hard math for many particles at once, and including forces that involve three nucleons together.

This area is always changing, with new math methods helping scientists understand the structure of nuclei better. Scientists have used these methods for small nuclei with up to 12 nucleons.

Nuclear potentials

Another way to understand nuclear forces is to make one potential for the whole nucleus instead of each nucleon. This is called the "macroscopic" way. For example, when neutrons bounce off a nucleus, scientists can think of the nucleus as having a certain potential, like how light behaves when it hits a glass ball.

Nuclear potentials can be "local" or "global." Local potentials work only for a small range of energies or nucleus sizes, while global potentials can be used for more different situations but are usually less exact.