Free neutron in the context of "Discovery of the neutron"

Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides—which, in turn, may trigger further neutron radiation. Free neutrons are unstable, decaying into a proton, an electron, plus an electron antineutrino. Free neutrons have a mean lifetime of 887 seconds (14 minutes, 47 seconds).

Neutron radiation is distinct from alpha, beta and gamma radiation.

Radioactive contamination, also called radiological pollution, is the deposition of, or presence of radioactive substances on surfaces or within solids, liquids, or gases (including the human body), where their presence is unintended or undesirable (from the International Atomic Energy Agency (IAEA) definition).

Such contamination presents a hazard because the radioactive decay of the contaminants produces ionizing radiation (namely alpha, beta, gamma rays and free neutrons). The degree of hazard is determined by the concentration of the contaminants, the energy of the radiation being emitted, the type of radiation, and the proximity of the contamination to organs of the body. It is important to be clear that the contamination gives rise to the radiation hazard, and the terms "radiation" and "contamination" are not interchangeable.

In nuclear astrophysics, the rapid neutron-capture process, also known as the r-process, is a set of nuclear reactions that is responsible for the creation of approximately half of the atomic nuclei heavier than iron, the "heavy elements", with the other half produced largely by the s-process. The r-process synthesizes the more neutron-rich of the stable isotopes of even elements, and those separated from the beta-stable isotopes by those that are not often have very low s-process yields and are considered r-only nuclei; the heaviest isotopes of most even elements from zinc to mercury fall into this category. Abundance peaks for the r-process occur near mass numbers A = 82 (elements Se, Br, and Kr), A = 130 (elements Te, I, and Xe) and A = 196 (elements Os, Ir, and Pt). Further, all the elements heavier than bismuth, including natural thorium and uranium (and other actinides) must ultimately originate in an r-process nucleus.

The r-process entails a succession of rapid neutron captures (hence the name) by one or more heavy seed nuclei, typically beginning with nuclei in the abundance peak centered on Fe. The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay (typically via β decay) before another neutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (the neutron drip line) to physically retain neutrons as governed by the short range nuclear force. The r-process therefore must occur in locations where there exists a high density of free neutrons. At some time following the neutron captures, the nucleus beta-decays back to the line of stability (just as with fission products) resulting in a stable isotope of the same mass number A, and normally the most neutron-rich of those.