A nuclear isomer is a
metastable
In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy.
A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...
state of an
atomic nucleus
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden experiments, Geiger–Marsden gold foil experiment. After th ...
, in which one or more
nucleon
In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number).
Until the 1960s, nucleons w ...
s (protons or neutrons) occupy
higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have
half-lives
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10
−12 seconds). The term "metastable" is usually restricted to isomers with half-lives of 10
−9 seconds or longer. Some references recommend 5 × 10
−9 seconds to distinguish the metastable half life from the normal "prompt"
gamma-emission half-life.
Occasionally the half-lives are far longer than this and can last minutes, hours, or years. For example, the
nuclear isomer survives so long (at least 10
15 years) that it has never been observed to decay spontaneously. The half-life of a nuclear isomer can even exceed that of the ground state of the same nuclide, as shown by as well as
,
,
and multiple
holmium isomers.
Sometimes, the