Editing Nuclear isomer (section)

==Nuclei of nuclear isomers==
The nucleus of a nuclear isomer occupies a higher energy state than the non-excited nucleus existing in the [[ground state]]. In an excited state, one or more of the protons or neutrons in a nucleus occupy a [[nuclear orbital]] of higher energy than an available nuclear orbital. These states are analogous to excited states of electrons in atoms.

When excited atomic states decay, energy is released by [[fluorescence]]. In electronic transitions, this process usually involves emission of light near the [[visible light|visible]] range. The amount of energy released is related to [[bond-dissociation energy]] or [[ionization energy]] and is usually in the range of a few to few tens of eV per bond. However, a much stronger type of [[binding energy]], the [[nuclear binding energy]], is involved in nuclear processes. Due to this, most nuclear excited states decay by [[gamma ray]] emission. For example, a well-known nuclear isomer used in various medical procedures is [[technetium-99m|{{nuclide|Tc|99|m}}]], which decays with a half-life of about 6 hours by emitting a gamma ray of 140&nbsp;keV of energy; this is close to the energy of medical diagnostic X-rays.

Nuclear isomers have long half-lives because their gamma decay is "forbidden" from the large change in [[nuclear spin]] needed to emit a gamma ray. For example, {{nuclide|Ta|180|m}} has a spin of 9 and must gamma-decay to {{nuclide|Ta|180}} with a spin of 1. Similarly, {{nuclide|Tc|99|m}} has a spin of 1/2 and must gamma-decay to {{nuclide|Tc|99}} with a spin of 9/2.

While most metastable isomers decay through gamma-ray emission, they can also decay through [[internal conversion]]. During internal conversion, energy of nuclear de-excitation is not emitted as a gamma ray, but is instead used to accelerate one of the inner electrons of the atom. These excited electrons then leave at a high speed. This occurs because inner atomic electrons penetrate the nucleus where they are subject to the intense electric fields created when the protons of the nucleus rearrange in a different way.

In nuclei that are far from stability in energy, even more decay modes are known.

After fission, several of the [[Nuclear fission product|fission fragments]] that may be produced have a metastable isomeric state. These fragments are usually produced in a highly excited state, in terms of energy and [[angular momentum]], and go through a prompt de-excitation. At the end of this process, the nuclei can populate both the ground and the isomeric states. If the half-life of the isomers is long enough, it is possible to  measure their production rate and compare it to that of the ground state, calculating the so-called ''isomeric yield ratio''.<ref>{{Cite journal|last1=Rakopoulos|first1=V.|last2=Lantz|first2=M.|last3=Solders|first3=A.|last4=Al-Adili|first4=A.|last5=Mattera|first5=A.|last6=Canete|first6=L.|last7=Eronen|first7=T.|last8=Gorelov|first8=D.|last9=Jokinen|first9=A.|last10=Kankainen|first10=A.|last11=Kolhinen|first11=V. S.|date=2018-08-13|title=First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments|url=https://link.aps.org/doi/10.1103/PhysRevC.98.024612|journal=Physical Review C|language=en|volume=98|issue=2|pages=024612|doi=10.1103/PhysRevC.98.024612|s2cid=125464341 |issn=2469-9985}}</ref>