Helium nuclei, which are abundant throughout the universe both as radioactive-decay products and as key participants in stellar fusion reactions. Alpha particles can also be generated in the laboratory, either by ionizing helium or from nuclear reactions. They expend their energy rapidly as they pass through matter, primarily by taking part in ionization processes, and consequently have short penetration ranges. Numerous technological applications of alpha particles can be found in fields as diverse as medicine, space exploration, and geology. Alpha particles are also major factors in the health concerns associated with nuclear waste and other radiation hazards.

An experimental technique that involves measuring the manner in which the likelihood of occurrence (or intensity or cross section) of a particular decay or collision process depends on the directions of two or more radiations associated with the process. Traditionally, these radiations are emissions from the decay or collision process. However, a variant on this technique in which the angular correlations are between an incident and emitted beam of radiation has been widely used; this variant is known as angular distribution.

The positively electrically charged central core of the atom. Atoms are analogous to the solar system, with an extremely dense central core that provides the electrical positive charge to hold the negatively charged electrons in orbits through the Coulomb force, somewhat as the Sun's gravitational force holds the planets in orbit. The nucleus is composed of protons, which carry a positive electric charge, and neutrons, which have a net zero electric charge. Collectively, protons and neutrons are called nucleons. Both these particles are composed of positive and negatively charged particles called quarks, and both have a magnetic field associated with them. The masses of the protons and neutrons are 1836.2 and 1838.7 times the mass of the electron, so essentially all the mass of an atom is in its very tiny nucleus. See also: Atomic structure and spectra; Electron; Neutron; Nucleon; Proton; Quarks

The name first applied in 1897 by Ernest Rutherford to one of the forms of radiation emitted by radioactive nuclei. Beta particles can occur with either negative or positive charge (denoted β⁻ or β⁺ and are now known to be either electrons or positrons, respectively. Electrons and positrons are now referred to as beta particles only if they are known to have originated from nuclear beta decay. Their observed kinetic energies range from zero up to about 5 MeV in the case of naturally occurring radioactive isotopes, but can reach values well over 10 MeV for some artificially produced isotopes. See also: Alpha particles; Electron; Gamma rays; Positron; Radioactivity

A group of nuclear reactions that involve the interaction of protons (nuclei of hydrogen atoms, designated by ¹H) with carbon, nitrogen, and oxygen nuclei. The cycle involving only isotopes of carbon and nitrogen is well known as the carbon-nitrogen (CN) cycle. These cycles are thought to be the main source of energy in main-sequence stars with mass 40% or more in excess of that of the Sun. Completion of any one of the cycles results in consumption of four protons (4 ¹H) and the production of a helium (⁴He) nucleus plus two positrons (e⁺) and two neutrinos (ν). The two positrons are annihilated with two electrons (e⁻), and the total energy release is 26.73 MeV. Approximately 1.7 MeV is released as neutrino energy and is not available as thermal energy in the star. The energy E = 26.73 MeV arises from the mass difference between four hydrogen atoms and the helium atom, and is calculated from the Einstein mass-energy equation E = Δmc², where Δm is the mass difference and c² is the square of the velocity of light. Completion of a chain can be thought of as conversion of four hydrogen atoms into a helium atom. Because the nuclear fuel that is consumed in these processes is hydrogen, they are referred to as hydrogen-burning processes by means of the carbon-nitrogen-oxygen (CNO) cycles. See also: Solar neutrinos

A nuclear fission process in which neutrons produced in earlier fission events cause successive fission events. Fission (splitting) of a large-mass atomic nucleus may be induced when the nucleus absorbs a neutron. Fission results in the release of approximately 200 MeV (200 × 10⁶ electronvolts) of energy, divided among the kinetic energy of two (and rarely, three) medium-size fragments, 1 to 5 neutrons (most often 2 or 3), and some gamma rays (∼8 MeV). The fission neutrons can interact with nearby fissionable nuclei (the fission fuel), producing more fissions and more neutrons (see Fig. 1). If there is enough fuel assembled to interact with the fission neutrons, the number of fissions per second in this chain reaction can be sustained or increase. See also: Atomic nucleus; Electronvolt; Energy; Gamma ray; Mass; Neutron; Nuclear fission; Nuclear physics

Charged particles can be accelerated to high velocities by electromagnetic fields. They are then able to travel through matter (termed an absorber), interacting with it, losing energy, and causing various effects important in many applications. In traveling through matter, charged particles interact with nuclei, producing nuclear reactions and elastic and inelastic collisions, and also collide with the electrons (electronic collisions) and with entire atoms of the absorber (atomic collisions).

Highly energetic collisions of elementary particles, namely, leptons and nucleons, which probe the nucleons' internal structure; or collisions between two heavy ions in which the two nuclei interact strongly while their nuclear surfaces overlap.

A neutron emitted spontaneously from a nucleus as a consequence of excitation remaining from a preceding radioactive decay event. Analogously, delayed emission of protons and alpha particles is observed, but the known delayed-neutron emitters are more numerous, and some of them have practical implications. In particular, they are important in the control of nuclear chain reactors. See also: Chain reaction (physics); Nuclear reactor

Energetic electrons ejected from atoms in matter by the passage of ionizing particles. In every primary ionizing collision between a charged particle and an atom, one or more electrons are ejected. Delta electrons are, by definition, that small fraction of these emitted electrons having energies which are large compared to the ionization potential. The name is a traditional one—comparable to alpha particles, for energetic helium nuclei, and beta particles, for energetic electrons emitted in radioactive decays. See also: Alpha particles; Beta particles; Charged particle interactions; Ionization