Neutron

A neutron is a subatomic particle, symbol n or n⁰. It has no electric charge, and its mass is slightly more than that of a proton. Neutrons are found with a similar number of protons in the nuclei of atoms. Atoms of a chemical element that have different numbers of neutrons are called isotopes.

The neutron was discovered by James Chadwick in 1932. This led to important discoveries like nuclear fission, the first self-sustaining nuclear reactor (Chicago Pile-1, 1942), and the first nuclear weapon (Trinity, 1945).

Neutrons are important in stars and powerful space events. They help create chemical elements in stars through processes like fission, fusion, and neutron capture. Neutron stars are formed from huge collapsing stars. They are made of neutrons and have a mass bigger than the Sun.

Neutrons are needed for making nuclear power. Special neutron sources such as neutron generators, research reactors, and spallation sources create free neutrons for scientific work. Free neutrons can be a health risk, depending on how much of them a person is exposed to. There is also a small natural amount of free neutrons on Earth, caused by cosmic rays and the natural radioactivity in the Earth's crust.

Discovery

Main article: Discovery of the neutron

The discovery of the neutron was an important moment in science. It changed how we understand atoms. The word "neutron" comes from an old word meaning "neutral."

In 1911, a scientist named Ernest Rutherford described atoms as having a small, positive center with negative particles around it. Later, he thought the center might have positive particles called protons.

In 1932, James Chadwick proved that atoms have a new kind of particle with no charge — the neutron. This discovery helped explain many things about atoms and led to important developments.

Occurrence

Atomic nucleus

Main articles: Atomic nucleus and Nuclear physics

See also: Valley of stability, Beta-decay stable isobars, and Neutron emission

An atomic nucleus is made of protons and neutrons. They are held together by a strong force. Protons and neutrons have almost the same weight. The number of protons tells us what element it is, while the number of neutrons helps decide which version of the element it is, called an isotope.

Most atoms have both protons and neutrons. For example, the simplest atom, hydrogen, usually has just a proton, but heavier versions can have one or two neutrons. Bigger atoms, like lead, have many protons and even more neutrons.

Free neutron

Main article: Free neutron decay

Neutrons are usually found inside atoms, where they stay stable. If a neutron leaves an atom, it will change over time. This change happens in about 15 minutes. Because of this, free neutrons are not found in everyday life.

Dineutrons and tetraneutrons

Main articles: Dineutron and Tetraneutron

Scientists think very small groups of neutrons might exist for very short times. These groups, made of two or four neutrons, break apart after a tiny fraction of a second.

Neutron stars and neutron matter

In places with extremely high pressure and temperature, like inside stars called neutron stars, normal matter can change into something called neutron matter. Here, atoms are crushed down into just neutrons. The huge pressure inside these stars might even change the shape of the neutrons to fit them even closer together.

Composition

Main article: Standard Model

A neutron is made of tiny parts called quarks. It has two down quarks and one up quark. These quarks stay together because of a strong force in nature.

Neutrons can change into protons through a process called beta decay. This happens when a down quark changes into an up quark. During this change, the neutron becomes a proton, and other tiny particles are also made. This process helps atoms change over time.

The change of a proton into a neutron can also happen, but only inside an atom's nucleus. This also makes new particles.

Beta decay

Main article: Beta decay

Neutrons and protons inside an atom can change into each other through beta decay. This process is guided by a special force and makes electrons or positrons, along with tiny particles called neutrinos.

The reactions look like this:

n⁰
→ p⁺
+ e⁻
+ ν
_e

and

p⁺
→ n⁰
+ e⁺
+ ν
_e

In these reactions, new particles are made when the original particle changes.

Properties

The neutron is a tiny particle with no electric charge. It is a little heavier than a proton, another particle found in atoms.

Neutrons have a property called "spin." This makes them act like tiny magnets. Scientists found this by seeing how neutrons act around magnetic fields. Because of their spin, neutrons follow special rules. This helps keep neutron stars from getting even smaller.

Detection

Neutrons are hard to detect because they don’t create ionization tracks like charged particles do. Scientists detect neutrons by letting them interact with atomic nuclei. Two main methods are used: neutron capture and elastic scattering.

In neutron capture, neutrons are absorbed by certain materials, releasing energy that can be turned into electrical signals. Materials like helium-3, lithium-6, boron-10, and some heavy elements work well for this. In elastic scattering, neutrons bounce off nuclei, causing them to move and create detectable signals. This method can help measure the neutron’s energy and direction.

Sources and production

Free neutrons do not stay the same and live for only about 10 minutes. Because of this, they can only come from places that make them all the time.

There is a small amount of natural neutron background everywhere on Earth. This comes from muons made when cosmic rays hit the atmosphere. These muons can go deep into water and soil and knock neutrons loose from atoms there. Another source is from the natural breakdown of uranium and thorium in rocks. These natural neutrons are not strong enough to harm people, but they are important for some very sensitive science experiments.

For research, scientists use special materials and reactions to create neutrons. Some use radioactive materials that break apart and release neutrons. Others use small machines called particle accelerators that crash particles together to make neutrons. The most common lab sources use radioactive decay to make neutrons.

Nuclear fission reactors make lots of neutrons as they work. These neutrons can also be used to create other useful materials. Experimental nuclear fusion reactors also make neutrons, but turning this energy into something useful is still a big challenge.

Free neutron beams come from special places that have strong neutron sources, like research reactors or spallation sources. Because neutrons have no electric charge, they are hard to control with electric or magnetic fields. But scientists can guide them using other methods, like slowing them down or bouncing them off certain materials.

Applications

Nuclear energy

Neutrons are important for nuclear energy. When a neutron is absorbed by certain heavy atoms, like uranium-235, the atom can become unstable and break apart. This process, called nuclear fission, releases a lot of energy and more neutrons. These new neutrons can cause more atoms to split, creating a chain reaction. This is how nuclear reactors work.

Other uses

Neutrons are used in many other ways. They help scientists study the structure of materials by bouncing off atoms. Neutrons can also help find out what elements are in a sample by causing them to emit gamma rays. This helps in studying many things, from small objects to rocks underground. Neutrons can even measure how much water is in soil by looking at how they bounce off hydrogen atoms in water.

Medical therapies

Neutrons can help treat some kinds of cancer. Neutrons can go deep into the body and affect cells, so doctors can aim them at cancerous areas. One way is called fast neutron therapy. This uses high-energy neutrons to damage cancer cells faster than normal cells.

Another method is called boron neutron capture therapy. In this treatment, a special drug with boron is given to the patient. The boron collects in the cancer cells. Then, low-energy neutrons are aimed at the tumor. When these neutrons meet the boron, they create particles that can kill the cancer cells without harming nearby healthy cells too much. This needs very strong neutron sources, usually from special research reactors.

Health risks

Being around free neutrons can be dangerous because neutrons can disturb molecules and atoms in the body and create other types of radiation. To stay safe, it’s important to avoid neutron exposure, stay far away from the source, and limit the time spent near it. Unlike other types of radiation, such as alpha, beta, or gamma rays—which are best blocked by dense materials like lead—neutrons need special shielding. Materials that contain hydrogen, like ordinary water or concrete blocks, are often used because they can slow down and absorb neutrons effectively.

In nuclear reactors, regular water absorbs neutrons so strongly that the fuel needs to be enriched to keep the reaction going. Heavy water, which contains deuterium instead of normal hydrogen, absorbs fewer neutrons and is used in certain types of reactors to help control the reaction better.

Neutron temperature

Neutrons have different energies based on their temperature. Thermal neutrons have the right energy to easily react with atoms in materials. This helps them be absorbed by atomic nuclei, creating heavier isotopes. Most fission reactors use special materials to slow down neutrons so they can cause more fission reactions.

Cold neutrons are made using very cold materials, and high-energy neutrons come from nuclear reactions and cosmic rays.