Nuclear Isomer

A nuclear isomer is a metastable state of an atomic nucleus in which one or more nucleons—protons or neutrons—occupy excited nuclear energy levels higher than those of the ground state. Unlike most excited nuclear states, which decay extremely rapidly, nuclear isomers are distinguished by their unusually long half-lives. In nuclear physics, a metastable state is generally defined as one with a half-life of at least 10⁻⁹ seconds, which is hundreds to thousands of times longer than the typical prompt decay times of ordinary excited nuclei, usually around 10⁻¹² seconds.
In many cases, nuclear isomer half-lives extend far beyond this threshold, lasting minutes, hours, years, or even longer. Some nuclear isomers are so long-lived that their half-lives exceed those of their corresponding ground states, making them of particular theoretical and practical significance.

Historical Discovery

The first recognised nuclear isomer–decay daughter system was discovered in 1921 by Otto Hahn during studies of uranium decay products. The substance initially referred to as uranium X₂, later identified as protactinium-234, demonstrated anomalously long-lived excited nuclear behaviour. This discovery provided early evidence that nuclei could exist in discrete, long-lived excited states distinct from the ground state.

Nuclear Structure and Excited States

The nucleus of a nuclear isomer exists at a higher energy configuration than the ground-state nucleus of the same nuclide. In these excited states, nucleons occupy higher-energy nuclear orbitals, analogous in principle to electrons occupying excited atomic orbitals. When excited atomic electrons return to lower energy states, they release energy via fluorescence, typically in the visible or ultraviolet range. In contrast, nuclear de-excitation involves far greater energies due to the strength of the nuclear binding force.
As a result, most nuclear excited states decay by emitting gamma rays, which carry energies ranging from tens of kiloelectronvolts (keV) to several megaelectronvolts (MeV). Nuclear isomers differ primarily in the timescale of this decay, not in the fundamental nature of the radiation emitted.

Origin of Metastability

The extended half-lives of nuclear isomers arise mainly from quantum mechanical selection rules, particularly those involving nuclear spin and parity. Gamma-ray emission requires conservation of angular momentum. When the spin difference between the isomeric state and the ground state is large, the transition becomes highly hindered or “forbidden”, significantly reducing the probability of decay.
Low excitation energy can further suppress decay by limiting the available decay pathways. Together, large spin differences and unfavourable energy conditions create metastable configurations that persist far longer than ordinary excited states.

Modes of Decay

Most nuclear isomers decay through isomeric transition, a form of gamma decay in which the nucleus emits one or more gamma rays to reach a lower energy state. Externally, this process resembles normal gamma decay, with the key difference being the long-lived nature of the parent state.
Another important decay mechanism is internal conversion, in which the energy of nuclear de-excitation is transferred directly to an inner-shell atomic electron rather than being emitted as a gamma photon. The electron is then ejected from the atom with high kinetic energy. This process occurs because inner atomic electrons penetrate the nuclear region and interact strongly with changes in the nuclear charge distribution.
In nuclei far from stability, additional decay modes may occur, including beta decay, electron capture, or spontaneous fission, depending on nuclear structure and energy balance.

Production of Nuclear Isomers

Nuclear isomers are commonly produced in nuclear reactions, including nuclear fission, fusion, and neutron capture. Following such reactions, nuclei are typically formed in highly excited states and undergo rapid de-excitation cascades. In some cases, the cascade ends in a metastable state rather than the ground state.
Fission products frequently include isomeric states, as the fragments are produced with high angular momentum. If the isomer half-life is sufficiently long, the relative populations of isomeric and ground states can be measured, yielding the isomeric yield ratio, an important parameter in reactor physics and nuclear data evaluation.

Designation and Classification

Metastable nuclear isomers are conventionally denoted by the letter m following the mass number of the isotope. For example, technetium-99m is written as ⁹⁹ᵐTc. When more than one metastable state exists for the same isotope, numerical indices are added, such as m₁, m₂, and m₃, with higher indices corresponding to higher excitation energies, as in hafnium-178m₂.
A distinct class of nuclear isomers is known as fission isomers or shape isomers. These occur primarily in heavy actinide nuclei, whose ground states are not spherical but instead elongated. Certain highly deformed configurations are separated from the ground state by substantial energy barriers, inhibiting de-excitation. Such isomers may decay slowly to the ground state or undergo spontaneous fission, typically with half-lives ranging from nanoseconds to microseconds.

Nearly Stable Nuclear Isomers

While most nuclear isomers eventually decay, a small number are extraordinarily long-lived. The most stable naturally occurring nuclear isomer is tantalum-180m, which exists in trace quantities in all natural tantalum samples. Its half-life is believed to exceed 10¹⁵ years, far longer than the age of the universe, and it has never been observed to decay spontaneously.
The metastability of tantalum-180m arises from a combination of low excitation energy and extreme spin mismatch with its ground state, which itself is radioactive with a relatively short half-life. The origin of this isomer is thought to lie in stellar nucleosynthesis processes, such as supernova explosions.
Another notable example is hafnium-178m₂, which has a half-life of approximately 31 years and stores an exceptionally large amount of excitation energy. This property has led to scientific interest in the possibility of induced gamma emission and applications such as gamma-ray lasers, though such technologies remain speculative.
Holmium-166m, with a half-life of about 1,200 years, and thorium-229, which possesses an extraordinarily low-lying metastable state only a few microelectronvolts above the ground state, further illustrate the diversity of isomeric behaviour.