Neutron

A neutron is a subatomic particle, symbol n or n⁰, that has no electric charge, and a mass slightly greater than that of a proton. Neutrons are found, together with a similar number of protons, in the nuclei of atoms. Atoms of a chemical element that differ only in neutron number are called isotopes.

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

Neutrons play a key role in stars and powerful events in space. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes. Neutron stars, formed from massive collapsing stars, consist of neutrons at the density of atomic nuclei but have a total mass more than the Sun.

Neutrons are essential for producing nuclear power. Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in scientific experiments. While free neutrons do not directly ionize atoms, they can indirectly cause ionizing radiation, so they can be a biological hazard, depending on the amount exposed to. There is also a small natural "neutron background" flux of free neutrons on Earth, caused by cosmic rays, and by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.

Discovery

Main article: Discovery of the neutron

The discovery of the neutron was a key moment in the history of science, leading to big changes in how we understand atoms. The word "neutron" comes from an old word meaning "neutral" and a ending used for tiny parts of atoms.

In 1911, a scientist named Ernest Rutherford described atoms as having a small, positive center surrounded by negative particles. Later, he thought the center might contain positive particles called protons and neutral particles made of a proton and an electron together. But this idea didn’t fully work with new ideas in physics. In 1932, James Chadwick proved that atoms have a new kind of particle with no charge and a mass similar to the proton — the neutron. This discovery helped explain many things about atoms and led to important developments, including the first nuclear reactor in 1942 and the first atomic bomb test in 1945.

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 up of protons and neutrons held together by a strong force. Protons and neutrons each have about the same mass. The number of protons decides the type of element, while the number of neutrons helps determine the specific version of that element, called an isotope.

Most atoms have both protons and neutrons in their nuclei. For example, the simplest atom, hydrogen, usually has just a proton, but heavier versions can have one or two neutrons. Larger 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 are very stable. If a neutron is freed from an atom, it will change over time. This change happens in about 15 minutes on average. Because of this, free neutrons are not common in everyday life.

Dineutrons and tetraneutrons

Main articles: Dineutron and Tetraneutron

Scientists have found clues that very small groups of neutrons might exist for very short moments. These groups, made of two or four neutrons, last only a tiny fraction of a second before breaking apart.

Neutron stars and neutron matter

In places with incredibly high pressure and temperature, like inside stars called neutron stars, normal matter can change into something called neutron matter. This is where atoms are crushed down into just neutrons. The extreme pressure inside these stars might even change the shape of the neutrons to pack them even tighter together.

Composition

Main article: Standard Model

A neutron is made up of smaller parts called quarks. It has two down quarks and one up quark. These quarks stick 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 created. This process is important in how some atoms change over time.

The decay of a proton into a neutron can also happen, but only inside an atom's nucleus. This also creates 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 creates 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 has a mass just a bit more than a proton, another type of particle found in atoms.

Neutrons have a property called "spin," which means they act like tiny magnets. Scientists discovered this by watching how neutrons react to magnetic fields. Because of their spin, neutrons follow special rules that prevent them from being in the same state as other neutrons. This helps keep stars made mostly of neutrons, called neutron stars, from collapsing into even smaller points.

Detection

Neutrons are tricky to detect because they don’t create ionization tracks like charged particles do. Instead, scientists often 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 without needing extra materials to slow them down.

Sources and production

Free neutrons are not stable and have a short life of 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 very 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 and nuclear weapons work, as they release energy millions of times greater than normal chemical reactions.

Other uses

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

Medical therapies

Neutrons can be used to help treat some kinds of cancer. Because neutrons can go deep into the body and affect cells, doctors can aim them at cancerous areas. One way is called fast neutron therapy, where high-energy neutrons are used to damage cancer cells more quickly 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 depending on their temperature. For example, thermal neutrons have just the right energy to easily react with atoms in many materials. This makes them more likely to be absorbed by atomic nuclei, creating heavier and often unstable isotopes. Most fission reactors use special materials to slow down the neutrons from fission so they can cause more fission reactions.

Cold neutrons can be made by using very cold materials, while high-energy neutrons come from nuclear reactions and cosmic rays.