An Etymological Dictionary of Astronomy and Astrophysics
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Apparent cross-section possessed by an atomic nucleus when it undergoes a particular type of collision process.

→ nuclear; → cross section.

The density of an atomic nucleus (about 10¹⁴ g/cm³).

→ nuclear; → density.

Energy released during a nuclear reaction as the result of the conversion of mass into energy. → mass-energy equivalence.

→ nuclear; → energy.

A → nuclear reaction in which a heavy atomic nucleus splits into two or more approximately equal parts, usually as the result of the capture of a slow, or → thermal neutron by the nucleus. It is normally accompanied by the emission of further neutrons or → gamma rays and very large amounts of energy. The neutrons can continue the process as a → chain reaction, so that it becomes the source of energy in a → nuclear reactor or an atomic bomb. It may also be a trigger for → nuclear fusion in a hydrogen bomb. Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors.

→ nuclear; → fission.

The attractive force which acts between nucleons when they are extremely close together (closer than 10^-13 cm).

→ nuclear; → force.

A substance, such as uranium-238 or plutonium-239, which undergoes nuclear fission in a nuclear reactor.

→ nuclear; → fuel.

A → nuclear reaction between atomic nuclei as a result of which a heavier → atomic nucleus is formed, a small fraction of mass is lost, and a large quantity of energy is released. Nuclear fusion is the source of the energy of stars.

→ nuclear; → fusion.

An analysis technique applied to some atomic nuclei that have the property to behave as small magnets and respond to the application of a magnetic field by absorbing or emitting electromagnetic radiation. When nuclei which have a magnetic moment (such as ¹H, ¹³C, ²⁹Si, or ³¹P) are submitted to a constant magnetic field and at the same time to a radio-frequency alternating magnetic field, the nuclear magnetic moment is excited to higher energy states if the alternating field has the specific resonance frequency. This technique is especially used in spectroscopic studies of molecular structure and in particular provides valuable information in medicine that can be used to deduce the structure of organic compounds.

→ nuclear; → magnetic; → resonance.

The magnetism associated with the magnetic field generated by atomic nuclei.

→ nuclear; → magnetism.

The quantity of matter in a nucleus, which is less than the total mass of its nucleons by its binding energy divided by the square of the speed of light.

→ nuclear; → mass.

The branch of physics which is concerned with the study of atomic nuclei, subatomic particles, and their exploitation.

→ nuclear; → physics.

Electric or motive power whose primary source is nuclear energy.

→ nuclear; → power.

A process in which an → atomic nucleus changes, including → radioactive decay of naturally occurring and man-made → isotopes, → nuclear fission, and → nuclear fusion.

→ nuclear; → process.

A process in which the energy, composition, or structure of an atomic nucleus changes.

→ nuclear; → reaction.

A device in which a nuclear fission chain reaction is maintained and controlled for the production of nuclear energy or radioactive isotopes.

→ nuclear; → reactor.

The total → angular momentum of a → nucleus, represented by symbol I. The nucleus, which is composed of neutrons and protons, acts as if it is a single entity which has intrinsic angular momentum. The nuclear spin depends on the → mass number; if the mass number is odd then the nucleus has half-integer spin like the electron while if the nucleus has even mass number then its spin will be integer spin.

→ nuclear; → spin.

In → nucleosynthesis models, the condition in which all nuclear species are in equilibrium under exchange of → protons and → neutrons. Matter in nuclear statistical equilibrium is characterized by a large abundance of iron peak nuclei. In such equilibria abundance of each nuclide can be calculated from repeated application of → Saha equation.

→ nuclear; → statistical; → equilibrium.

The time required for a star to exhaust its hydrogen (H) supply in → nuclear fusion. The nuclear time scale is given by the relation t = E/L, where E is the total nuclear energy that can be generated by a star and L is the stellar → luminosity. Assuming that the end point of fusion is → iron (Fe), the → atomic mass difference between H and Fe is Δm = 0.008 m_H. Therefore, the maximum amount of energy a star with a hydrogen mass M can release is Δ M = 0.008 Mc². The nuclear time scale is then: t = 0.008 c²M/L. However, stars use up only a fraction of their hydrogen supply, because only the inner part of the star is hot enough for fusion. For example, the Sun will spend only about 10% of its hydrogen supply before evolving into a → red giant. In other words, the solar life time on the → main sequence is about 10¹⁰ years.

→ nuclear; → time; → scale.

A change in the → energy level or state of an atomic → nucleus involving a → quantum of energy.

→ nuclear; → transition.

The changing of atoms of one element into those of another by suitable nuclear reactions.

→ nuclear; → transmutation.