What Are Nuclear Isomers? The Real Physics of Science Fiction

Nuclear isomers sound like the kind of thing that belongs in a footnote. In practice, they already power tens of millions of medical imaging scans a year, and recent work suggests they could do far more. The question is no longer whether they matter, but whether they could turn science fiction into scientific reality.

Imagine "nuclear batteries" smaller than the eye can see, powering microscopic implants that enhance or fix problems in the human body. Imagine firing "gamma ray lasers" like in the movies, or using this technology to initiate nuclear fusion reactions that provide clean energy for the entire planet. Both of these technologies rely on the same underlying physics, and that physics can be understood with a few simple analogies.

What Are Nuclear Isomers?

If you heat a saucepan of water, it begins to boil because the water molecules (H2O) absorb heat energy from the stove and turn it into kinetic energy, smashing into each other to create the bubbling effect. When the stove is turned off the water returns to a state of calm. This is common sense. Now, imagine turning off the stove but the water keeps boiling. Impossible right? Yes, but not for a nuclear isomer! The key thing to understand about nuclear isomers is they stay hot for a long time. They store energy.

A nucleus can be heated by smashing things into it or hitting it with intense radiation. To create silver-108m, stable silver is bombarded with neutrons inside a nuclear reactor. A neutron is absorbed and the nucleus briefly becomes "hot." It cools quickly, but ~1% of the time, it doesn't cool all the way down. Instead, it gets stuck in a higher-energy state with more internal angular momentum, and there's no easy way to shed that extra angular momentum all at once. That state is the isomer, and it holds onto that energy for 438 years. When it finally does escape, the energy is released as gamma rays—invisible, high-energy forms of light that can be detected with specialised equipment.

The laws of quantum mechanics determine why some nucleons hold onto their energy while others do not. These laws are difficult to explain to non-physicists, but if you imagine throwing a Frisbee into a tree, sometimes it will become snagged on a branch, and sometimes it will fall to the ground. Depending on how it's snagged, it might fall in a second or take centuries to fall on its own.

Nuclear Batteries

If nuclear isomers are "stuck" in an energetic state, they are essentially nanoscopic energy storage devices, or nuclear batteries. For example, silver-108m could be thought of as a battery with a 438-year lifetime. Even more impressive: if a normal AA battery were composed entirely of silver-108m, it would store 800,000 times more energy, which is the same as two tons of TNT. Of course, that much energy isn't necessary. If the isomer battery stored the same energy as an AA battery, it would be the size of a grain of sand.

Unfortunately, this energy is useless if it can't be released in a controlled way and, currently, there is no easy way to do this. Theoretically, it should be possible to release an isomer’s energy by "nudging" nucleons out of their "hot" positions. In our analogy, this would involve throwing something into the tree to dislodge the Frisbee. To achieve this, a laser could be shone onto the isomer, giving the excited nucleons even more energy to force them into positions where they are less "stuck" and can decay faster, releasing all of their energy at once in a cascade of gamma rays without any radioactive waste.

The silver isomer may be problematic for this purpose, given that it has a radioactive intermediate state and would require a very powerful laser to "nudge" it down. However, other isomers, such as thorium-229m, have transition energies low enough that a conventional laser might one day be used to trigger their decay.

At present, the high energies needed to trigger this gamma ray cascade are often impractical. However, more efficient techniques may be developed, leading to the manufacture of nuclear batteries that are smaller than the eye can see—revolutionizing phones, computers, and the future of nanotechnology.

Gamma Ray Lasers

Once it is possible to stimulate the release of a nuclear isomer's energy, it won't be long before the technique is used to develop weapons, such as by enhancing the intensity and focus of a controlled burst of gamma rays. One problem with that is, when a nucleus emits a gamma ray, it kicks back—much like a gun recoils when it fires a bullet. This kick slightly shifts the gamma ray's energy, meaning it can no longer trigger other excited nuclei to emit their own gamma rays. Without that chain reaction, there's no laser.

One possible solution is to embed the isomer atoms in a crystal lattice. The recoil is then shared by the entire crystal—trillions of atoms absorbing the kick collectively—so the gamma ray is emitted at exactly the right energy to stimulate a cascade. This is known as the Mössbauer effect. Unlike Star Trek or Star Wars, you won't be able to see or hear the laser burst, although a separate sound could be emitted to signal it.

A controlled source of gamma rays could also be used to heat up air, powering the jet engines on planes. This is known as a quantum nucleonic reactor. Furthermore, a gamma ray laser could compress fuel capsules for nuclear fusion reactors, providing vast amounts of clean energy to power the planet.

Nuclear Isomers Are Already Used in Medicine

Isomers already have several applications. For example, an isomer of technetium (technetium-99m) is used in medical imaging. Once injected into a patient, it binds chemically to specific molecules depending on what it is combined with. The isomer emits gamma rays which are detected by a specialised camera (a SPECT scan), producing images that highlight health problems. For instance, when a tin compound is used to prepare the patient's red blood cells, the technetium isomer is drawn into them and binds to haemoglobin, allowing the scan to track blood flow and identify problems such as internal bleeding. The gamma rays are low in energy and the isomer decays within hours, so the radiation dose to the patient is small.

In sum, nuclear isomers promise to provide the human race with a clean use for nuclear energy. Whether this is through the invention of nuclear batteries, or the initiation of nuclear fusion reactions with gamma ray lasers, we have only scratched the surface of what is possible.