Nobel Prize winner Otto Hahn is credited with the discovery of nuclear fission. Fission is one of the most important discoveries of the 20th century, but Hahn saw something else as his best scientific work.
In 1921, while studying radioactivity at the Kaiser Wilhelm Institute of Chemistry in Berlin, Germany, he noticed something he couldn’t explain. One of the elements he was working with was not behaving the way it have. Hahn had unknowingly discovered the first nuclear isomer, an atomic nucleus whose protons and neutrons are arranged differently from the element’s common shape, giving it unusual properties. It took another 15 years of discoveries in nuclear physics to be able to explain Hahn’s observations.
The most common place to find isomers is inside stars, where they play a role in the nuclear reactions that create new elements. In recent years, researchers have begun to explore how isomers can be used to benefit mankind. They are already used in medicine and could one day offer powerful options for energy storage in the form of nuclear batteries.
This video shows radioactive uranium-238 in a fog-filled chamber. The streaks are created when particles are emitted from the radioactive sample and pass through the water vapor.
In search of radioactive isotopes
In the early 1900s, scientists were looking for new radioactive elements. An element is considered radioactive if it spontaneously releases particles in a process called radioactive decay. When this happens, the item changes over time into a different item.
At that time, scientists relied on three criteria to discover and describe a new radioactive element. One was to look at chemical properties – how the new element reacts with other substances. They also measured the type and energy of particles released during radioactive decay. Finally, they measured how quickly an element decomposed. Decay rates are described using the term half-life, which is the time it takes for half of the original radioactive element to decay into something else.
By the 1920s, physicists had discovered certain radioactive substances with identical chemical properties but different half-lives. These are called isotopes. Isotopes are different versions of the same element that have the same number of protons in their nucleus but different numbers of neutrons.
Uranium is a radioactive element with many isotopes, two of which occur naturally on Earth. These natural isotopes of uranium decay into the element thorium, which in turn decays into protactinium, and each has its own isotopes. Hahn and his colleague Lise Meitner were the first to discover and identify many different isotopes from the decay of the element uranium.
All of the isotopes they studied behaved as expected except one. This isotope seemed to have the same properties as one of the others, but its half-life was longer. It didn’t make sense, because Hahn and Meitner had put all the known isotopes of uranium into a neat classification, and there were no gaps to accommodate a new isotope. They called this substance “uranium Z”.
The radioactive signal from uranium Z was approximately 500 times weaker than the radioactivity of the other isotopes in the sample, so Hahn decided to confirm his observations using more material. He purchased and chemically separated uranium from 220 pounds (100 kilograms) of highly toxic and rare uranium salt. The surprising result of this second, more precise experiment suggested that the mysterious uranium Z, now known as protactinium-234, was a previously known isotope but with a very different half-life. It was the first case of an isotope with two different half-lives. Hahn published his discovery of the first nuclear isomereven if he couldn’t fully explain it.
Neutrons complete the story
At the time of Hahn’s experiments in the 1920s, scientists still thought of atoms as a cluster of protons surrounded by an equal number of electrons. It was not until 1932 that James Chadwick suggested that a third particle – neutrons – was also part of the core.
With this new information, physicists were immediately able to explain isotopes – they are nuclei with the same number of protons and a different number of neutrons. Armed with this knowledge, the scientific community finally had the tools to understand uranium Z.
In 1936 Carl Friedrich von Weizsäcker proposed that two different substances could have the same number of protons and neutrons in their nuclei but in different arrangements and with different half-lives. The arrangement of protons and neutrons that gives the lowest energy is the most stable material and is called the ground state. Arrangements resulting in less stable and higher energies of an isotope are called isomeric states.
At first, nuclear isomers were only useful in the scientific community as a way to understand the behavior of nuclei. But once you understand the properties of an isomer, it’s possible to start wondering how it can be used.
Isomers in medicine and astronomy
The isomers have important applications in medicine and are used in tens of millions of diagnostic procedures each year. Since isomers undergo radioactive decay, special cameras can track them as they move through the body.
For example, technetium-99m is an isomer of technetium-99. When the isomer decays, it emits photons. Using photon detectors, doctors can track the evolution of technetium-99m moves throughout the body and create pictures of the heart, brain, lungs and other critical organs to help diagnose diseases, including cancer. Radioactive elements and isotopes are normally dangerous because they emit charged particles that damage body tissue. Isomers like technetium are safe for medical use because they only emit one harmless photon at a time and nothing else when they decay.
Isomers are also important in astronomy and astrophysics. Stars are powered by the energy released during nuclear reactions. Since the isomers are present in the stars, the nuclear reactions are different than if the material were in its ground state. This makes the study of isomers essential to understanding how stars produce all the elements in the universe.
The isomers of the future
A century after Hahn first discovered isomers, scientists are still discovering new isomers through powerful research facilities around the world, including Facility for rare isotope beams at Michigan State University. This facility went live in May 2022 and we hope it will unlock over 1,000 new isotopes and isomers.
Scientists are also investigating whether nuclear isomers could be used to build the most accurate clock in the world or whether isomers could one day be the basis for the next generation of Battery. More than 100 years after the detection of a small anomaly in uranium salt, scientists are still searching for new isomers and are only just beginning to reveal the full potential of these fascinating elements of physics.
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