Scientists Solve a 20-Year Mystery About Gamma Rays

Scientists at a lab in Michigan have answered a question that puzzled nuclear physicists for more than 20 years. They found that a mysterious burst of energy released by atomic nuclei is magnetic in nature. The team studied a form of zinc metal and used special tools to measure how it gave off tiny packets of light called gamma rays. Their results were collected at the Facility for Rare Isotope Beams at Michigan State University.

Gamma rays are a powerful type of light that comes from the center of atoms, called the nucleus. When a nucleus has too much energy, it releases that extra energy as gamma rays. Scientists have studied gamma rays for over a hundred years. Ernest Rutherford first described them during radioactive decay back in 1904, and later showed they also appear during nuclear reactions. Today, gamma rays are used in medicine, food safety, and space research.

Every nucleus can release gamma rays in a pattern called its gamma-ray strength function. Think of this like a fingerprint — it shows how strong the gamma rays are at different energy levels. Most of the time, the strongest bursts happen at high energies. But scientists noticed something odd happening at very low energies, below 3 million electron volts. There was a surprise boost in gamma-ray strength that did not match their predictions.

This unexpected boost is called the low-energy enhancement, or LEE for short. It was first spotted in iron atoms around two decades ago, and then seen in many other types of atoms. Scientists were not sure what was causing it. They knew the LEE involved a type of change called a dipole transition, meaning one unit of spinning motion changed. But they could not figure out if the LEE was electric or magnetic in nature.

To answer that question, researchers used a clever trick. They fed energy into zinc nuclei using a process called beta decay, starting from two different forms of a copper atom. One copper form had a spin of 6, and the other had a spin of 1. Because beta decay is very picky about which states it can reach, each copper form lit up a different set of zinc states. This gave the scientists two separate gamma-ray fingerprints from the same zinc nucleus to compare.

When they compared the two fingerprints, they found a clear difference at low energies. The zinc gamma-ray pattern fed by the spin-1 copper was much weaker at high gamma-ray energies than the pattern fed by the spin-6 copper. The team figured out why: at low energies inside zinc, almost all the available quantum states have positive parity, which is like a positive spin direction. Electric gamma rays, called E1 transitions, need a negative-parity state to jump into. Since there were almost none available, the electric part of the signal was squeezed out.

The magnetic gamma rays, called M1 transitions, do not need a parity change. So they were not affected at all. When the scientists subtracted the suppressed electric signal, what was left — the low-energy enhancement — was purely magnetic. This matched the predictions made by many computer models over the years, but it had never been proven in an experiment before. Now it has.

This discovery matters beyond just knowing what kind of energy the LEE is. Gamma rays and nuclear reaction rates are closely tied to how stars create elements, a process called nucleosynthesis. When a star explodes or collapses, nuclei rapidly capture neutrons and build up heavier elements. The LEE affects how likely those captures are to happen. Knowing the LEE is magnetic will help scientists build better models of how stars forge the elements we find on Earth and throughout the universe.