Nuclear isomer

Everything is made of tiny bits. Inside these bits is a center. Sometimes the center has extra energy. This extra energy makes it special.

Most of the time, this energy leaves fast. But some centers hold it for a long time. They can hold it for years! This is very rare.

One type of center is shaped like a ball. Others look like a football. This shape helps them keep their energy.

Some of these bits are used in doctors' offices. They help see inside our bodies. This is a very helpful way to use science.

It is amazing how much energy is inside tiny bits.

Everything is made of tiny bits called atoms. At the center of each atom is a nucleus. Sometimes, the nucleus has extra power. We call this an excited state. Most nuclei lose this power very fast. But some stay excited for a long time. We call these special nuclei nuclear isomers.

An isomer can stay excited for minutes or even years. One type of isomer is called a fission isomer. These nuclei are not round like a ball. Instead, they look like a football. This shape makes it hard for them to lose their extra power.

How do they lose this power? They can let out gamma rays. These are tiny bursts of light. They can also let out an electron. This is called internal conversion. Sometimes, the change in spin makes this hard. Spin is a way the nucleus moves. If the spin must change a lot, the decay is "forbidden." This means it happens very slowly.

Scientists use some isomers in medicine. For example, lutetium-177m is used in medical tests. Other isomers might even help us make lasers.

Caption: This chart shows how some nuclei change states.

Everything in our world is made of atoms. At the center of every atom is a tiny nucleus. Most of the time, a nucleus sits in its lowest energy state, called the ground state. However, sometimes a nucleus has extra energy. This happens when its protons or neutrons move into higher energy levels. Most nuclei lose this extra energy almost instantly. But some nuclei stay in this high-energy state for a long time. We call these special nuclei nuclear isomers.

How does an isomer stay excited for so long? It usually happens because of something called spin. Spin is a type of movement or angular momentum within the nucleus. If a nucleus needs to change its spin by a large amount to reach the ground state, the decay is called "forbidden." This does not mean it is impossible, but it makes the process very slow. Another reason is low excitation energy. If the extra energy is very small, it is harder for the nucleus to release it. These factors together allow isomers to last for minutes, hours, or even years.

Scientists have been studying these nuclei for a long time. The first nuclear isomer was discovered by Otto Hahn in 1921. He found a system involving uranium. Since then, we have learned that isomers can be made in many ways. They can be created during nuclear fission or by hitting atoms with charged particles. Some isomers even exist naturally in the world. For example, a special kind of tantalum is found in nature. It is quite rare, appearing in only 1/8000 of all tantalum atoms.

There are many different types of isomers with unique facts. One type is called a fission isomer or shape isomer. These nuclei are not round like a ball. Instead, they look like a football or a rugby ball. This shape makes it very hard for them to return to a normal shape. Another famous example is lutetium-177m. This isomer is used in medical procedures. It has a half-life of about 6 hours. It releases energy as a gamma ray, which is a tiny burst of light.

Nuclear isomers help us understand how the smallest parts of our world work. Some scientists study thorium-229m because its energy is very low. This might help us build a nuclear clock that is incredibly accurate. Other isomers, like hafnium-178m2, have been studied as possible sources for gamma-ray lasers. These lasers could potentially produce extremely high power. By studying how these tiny nuclei hold onto their energy, we learn about the deep rules of physics. It is a small part of the atom, but it holds a lot of wonder.

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.