A nucleogenic isotope, or nuclide, is one that is produced by a natural terrestrial nuclear reaction, other than a reaction beginning with cosmic rays (the latter nuclides by convention are called by the different term cosmogenic). The nuclear reaction that produces nucleogenic nuclides is usually interaction with an alpha particle or the neutron capture of nuclear fission or thermal . Some nucleogenic isotopes are stable and others are radioactive.
Example
An example of a nucleogenic nuclide is neon-21 produced from neon-20 that absorbs a thermal neutron (though some neon-21 is also primordial).
[[1] Neon-21 production from absorption of neutrons from alpha-spallation on oxygen-18] Other nucleogenic reactions that produce heavy neon isotopes are (fast neutron capture, alpha emission) reactions, starting with magnesium-24 and magnesium-25, respectively.
[[2] USGS explanation of the nucleogenic sources of Ne-21 and Ne-22] The source of the neutrons in these reactions is often secondary neutrons produced by alpha radiation from natural
uranium and
thorium in rock.
Types
Because nucleogenic isotopes have been produced later than the birth of the
Solar System (and the nucleosynthetic events that preceded it), nucleogenic isotopes, by definition, are not primordial nuclides. However, nucleogenic isotopes should not be confused with much more common
radiogenic nuclides that are also younger than primordial nuclides, but which arise as simple
from radioactive decay. Nucleogenic isotopes, as noted, are the result of a more complicated nuclear reaction, although such reactions may begin with a radioactive decay event.
Alpha particles that produce nucleogenic reactions come from natural alpha particle emitters in uranium and thorium decay chains. Neutrons to produce nucleogenic nuclides may be produced by a number of processes, but due to the short half-life of free neutrons, all of these reactions occur on Earth. Among the most common are cosmic ray spallation production of neutrons from elements near the surface of the Earth. Alpha emission produced by some radioactive decay also produces neutrons by spallation knockout of neutron rich isotopes, such as the reaction of alpha particles with oxygen-18. Neutrons are also produced by neutron emission (a form of radioactive decay in some neutron-rich nuclides) and spontaneous fission of fissile isotopes on Earth (particularly uranium-235).
Nucleogenesis
Nucleogenesis (also known as
nucleosynthesis) as a general phenomenon is a process usually associated with production of nuclides in the
Big Bang or in stars, by nuclear reactions there. Some of these neutron reactions (such as the
r-process and
s-process) involve absorption by atomic nuclei of high-temperature (high energy) neutrons from the star. These processes produce most of the
in the universe heavier than
zirconium (element 40), because nuclear fusion processes become increasingly inefficient and unlikely for elements heavier than this. By convention, such heavier elements produced in normal elemental abundance, are
not referred to as "nucleogenic". Instead, this term is reserved for nuclides (isotopes) made on Earth from natural nuclear reactions.
Also, the term "nucleogenic" by convention excludes artificially produced , for example tritium, many of which are produced in large amounts by a similar artificial processes, but using the copious neutron flux produced by conventional .
