Nuclear reaction

A nuclear reaction is a process where nuclei undergo a fundamental change. This happens when two nuclei collide, or when a nucleus hits an external subatomic particle. For a process to be a true nuclear reaction, it must cause the transmutation of at least one nuclide into another. Transmutation is the transformation of one type of nucleus into a different one. If particles simply bounce off each other without changing their nature, scientists call this nuclear scattering instead of a reaction. While it is theoretically possible for three or more particles to collide at once, such events are exceptionally rare. Most reactions involve only two particles meeting at the same time and place.

To understand how these reactions occur, we must look at the forces involved. Most common nuclear particles carry a positive charge. Because like charges repel, these particles experience strong electrostatic repulsion when they approach one another. To overcome this push, the particles must be accelerated to very high energies. This can be achieved using particle accelerators or through extremely high temperatures, such as those found in thermonuclear reactions. Another way is through cosmic rays or natural nuclear decay. Neutrons are a special exception to this rule. Because neutrons have no electric charge, they do not experience electrostatic repulsion. This allows them to initiate reactions even at very low energies, such as those found at room temperature.

Nuclear reactions are often categorized by how the nuclei interact. In fusion reactions, two light nuclei join together to form a single, heavier nucleus. This process releases additional particles, such as protons or neutrons. This is the same process that powers the Sun and other stars. In fission reactions, a very heavy nucleus splits into two or three smaller pieces. This is often an induced reaction, meaning it happens after the nucleus absorbs a light particle like a neutron. There is also a process called spallation. This occurs when a particle with enough energy and momentum hits a nucleus and knocks out several small fragments.

Scientists have reached many milestones in understanding these processes. In 1919, Ernest Rutherford performed the first observation of an induced nuclear reaction. At the University of Manchester, he used alpha particles to transform nitrogen into oxygen. Later, in 1932, John Cockcroft and Ernest Walton achieved a fully artificial nuclear reaction at Cambridge University. They used accelerated protons to split lithium-7 into two alpha particles. This famous feat was often called "splitting the atom." Later, in 1938, German scientists Otto Hahn, Lise Meitner, and Fritz Strassmann discovered the fission of heavy elements.

These reactions can release massive amounts of energy, which is often measured as a Q-value. In an exothermic reaction, kinetic energy is released. In an endothermic reaction, kinetic energy must be supplied for the reaction to occur. This energy comes from the nuclear binding energy. During a reaction, the total rest mass of the particles can actually decrease. For example, in a reaction involving lithium-6 and deuterium, the starting mass is 8.029 daltons. The resulting helium-4 nuclei have a total mass of only 8.0052 daltons. The missing 0.0238 daltons of mass is converted into energy. Using Einstein's formula, $E=mc^2$, we can calculate that this specific reaction releases 22.2 MeV of energy.

This released energy can appear in several different forms. Much of it becomes the kinetic energy of the resulting particles. Some energy is emitted as high-energy photons known as gamma rays. Occasionally, energy remains within the nucleus for a short time. This is called a metastable state, often marked with an asterisk in scientific notation. Eventually, this energy is released through nuclear decay. A small amount of energy can also emerge as X-rays. This happens as electrons rearrange themselves into new shells following the change in the nucleus.

Nuclear reactions are also studied through specific types of particle transfers. In direct reactions, a projectile transfers energy or nucleons to a nucleus in a single, very quick event. This helps physicists probe the internal structure of the target nucleus. Nucleon transfer reactions involve moving one or more nucleons between the projectile and the target. These can be stripping reactions, where the target gains a particle, or pick-up reactions, where the projectile takes one. Scientists also use inelastic scattering to study the shapes and sizes of nuclei. By observing how particles like alpha particles scatter, researchers can learn about the surface and interior of the atom.