Nuclear isomer

A nuclear isomer is a special state of an atomic nucleus. In this state, one or more nucleons—the protons or neutrons inside the nucleus—occupy excited state levels. These are higher energy levels than the nucleus usually holds. Most nuclei release this extra energy almost immediately through a process called prompt decay. However, some nuclei enter a metastable state. This means they stay in that high-energy state for a much longer time. The term metastable describes nuclei with half-lives of 10⁻⁹ seconds or longer. Many scientists use a threshold of 10⁻⁷ seconds to distinguish these from prompt decays. Some isomers are incredibly stable, lasting for minutes, hours, or even years.

How does a nucleus stay excited for so long? The main reason is a concept called forbiddenness. This usually involves a large change in nuclear spin, which is a type of angular momentum. To reach the ground state, a nucleus must release its energy, often as a gamma ray. A gamma ray carries a spin of 1 unit. If the nucleus must change its spin by much more than one unit to reach the ground state, the transition is considered forbidden. For example, $^{180m}$Ta has a spin of 9, while its lower states have spins of 1 or 2. This massive difference makes the decay very slow. Low excitation energy also helps. If the energy above the ground state is very small, the transition rate slows down even further.

There are several ways these isomers can release their energy. The most common method is isomeric transition (IT). This is when the nucleus emits a gamma ray to drop to a lower energy level. Another process is internal conversion. In this case, the transition energy is used to eject an electron from the atom instead of emitting light. These two processes often compete with each other. For instance, in $^{177m}$Lu, the nucleus may decay via internal conversion to a specific state before emitting a gamma ray. Some isomers are so unique that they cannot decay via gamma rays at all. In $^{242m}$Am, the forbiddenness is so high that the nucleus only undergoes alpha decay.

Isomers can be classified into different types based on their structure. A notable category is the fission isomer, also called a shape isomer. Most actinide nuclei are not perfectly spherical in their ground state. Instead, they are prolate spheroidal, meaning they are shaped like a rugby ball or an American football. Shape isomers occur when the distribution of protons and neutrons is pushed even further from a spherical shape. This unusual geometry makes it very difficult for the nucleus to return to its normal state. These isomers may de-excite slowly or undergo spontaneous fission. Fission isomers are often labeled with a superscript "f," such as $^{240f}$Pu.

Humans have been studying these phenomena since the early 20th century. The first nuclear isomer and decay-daughter system was discovered by Otto Hahn in 1921. He identified a system involving uranium that provided early clues about these states. Today, we know that metastable isomers can be produced through many different nuclear reactions. These include radioactive decay, neutron capture, and nuclear fission. They can also be made by bombarding atoms with accelerated charged particles. After fission, several fragments may be produced in a metastable state. Scientists can measure how many isomers are created compared to ground states by using the isomeric yield ratio.

Specific examples of isomers show just how diverse these nuclei can be. $^{177m}$Lu is used in various medical procedures. It has a half-life of about 6 hours and emits a gamma ray with an energy of 140.5 keV. This energy is similar to the X-rays used in medical diagnostics. Another remarkable example is $^{229m}$Th. It has an extremely low excitation energy. This low energy allows for the development of a nuclear clock with unprecedented accuracy. Researchers spent twenty years, from 2003 to 2024, working to achieve the precise estimates needed for spectroscopy of this nucleus. Other isomers, like $^{178m2}$Hf, have been studied as potential sources for gamma-ray lasers. These lasers could theoretically produce extremely high powers, on the order of exawatts.

Understanding nuclear isomers connects many different fields of science. It links the study of subatomic particles to practical applications in medicine and timekeeping. The study of $^{229m}$Th connects nuclear physics to ultra-precise spectroscopy. Meanwhile, the study of shape isomers helps us understand the quantum-mechanical forces that hold the nucleus together. Even the way we name these particles follows strict rules. We use an "m" after the mass number, such as $^{58m}$Co. If there are multiple isomers, we use indices like m1, m2, or m3 to show increasing levels of excitation energy. This organized system helps scientists track the many different ways energy can be stored inside an atom.