Singular observation of shape and spin hindrance in metastable nuclear decay of $^{197}$Au

Inside the heart of every atom lies a nucleus, a tightly packed cluster of protons and neutrons that usually behaves with predictable simplicity. When these nuclei are excited, they typically release their extra energy almost instantly, flashing it away in a fraction of a second. However, nature occasionally creates a rare exception: a nuclear isomer. This is a state where the nucleus gets stuck, holding onto its energy for a surprisingly long time, sometimes seconds or even years. This delay happens because the nucleus has changed its shape or its spin in a way that makes it very difficult to return to a calmer state. Scientists study these stubborn states because they act as sensitive probes, revealing how the fundamental building blocks of matter interact and how the rules of the quantum world shape the physical universe.

For decades, researchers have known of two main reasons why a nucleus might get stuck. Sometimes, the nucleus changes its physical shape, like a ball stretching into a football, and the energy required to snap it back is too high. Other times, the nucleus spins so fast or in such a specific direction that it cannot easily slow down without breaking the rules of physics. Until now, these two reasons for delay were thought to be separate phenomena, occurring in different types of atoms. A team of physicists has now discovered a unique case where both reasons happen at the same time in a single atom, creating a complex puzzle that challenges our current understanding of how atomic nuclei are built.

The researchers focused their attention on gold, specifically the stable isotope gold-197. While gold is a familiar metal, its atomic nucleus can be coaxed into an excited state using high-energy particle beams. In experiments conducted at the ATLAS accelerator in Illinois, scientists fired beams of heavy atoms at a thick gold foil to shake the gold nuclei into these high-energy states. They then waited, using highly sensitive detectors to listen for the faint signals of gamma rays—the light emitted when the nucleus finally relaxes. By carefully timing these signals, they were able to identify a new, long-lived state within the gold nucleus that had never been seen before.

This newly found state sits at an energy level of 2102 kiloelectronvolts and has a spin value of 29/2, a measure of its angular momentum. What makes this discovery remarkable is how the nucleus decides to let go of its energy. Instead of taking a single, straightforward path, it faces a strange dilemma. It can release energy by changing its shape, a process that is usually efficient, but in this case, that path is heavily blocked. At the same time, it can release energy by spinning down, a process that typically proceeds with near-single-particle strength, yet here it competes with the blocked shape change. The nucleus ends up splitting its decay into two competing branches: one that is strongly hindered by a change in shape, and another that follows a spin-related path with comparable intensity, despite the unusual coexistence of these mechanisms.

The team measured the lifetime of this state to be longer than ten seconds, which is an eternity in the world of nuclear physics. They observed that the nucleus emits a gamma ray with an energy of 266.6 kiloelectronvolts as it drops to a lower energy level. This specific emission is a type of transition that is normally very rare and slow, yet it competes directly with a much weaker, shape-changing transition that is almost completely suppressed. This unusual coexistence of a blocked shape change and a competing spin change has never been observed in a long-lived nuclear state before. It suggests that the gold nucleus is caught between two different structural realities, unable to fully commit to one or the other.

To understand why this is happening, the scientists ran detailed computer simulations using two different models of nuclear structure. One model treated the nucleus as a deformed, spinning object, while the other looked at the individual particles inside. Both models agreed that the nucleus has a moderate, flattened shape, but they also revealed a conflict. The calculations showed that the transition involving the change in shape should be much stronger than what was observed, while the spin-related transition matched the data better. This discrepancy indicates that the simple models are missing something important about how the shape and the individual particles mix together in this specific state. The results suggest that even near stable, well-understood regions of the periodic table, the behavior of atomic nuclei can be far more intricate than previously thought.

This finding is significant because it forces a reevaluation of how scientists describe the internal life of an atom. Current theories often treat the collective shape of a nucleus and the behavior of its individual particles as separate stories. The gold-197 isomer shows that these stories are deeply intertwined, with the shape hindering the decay while the spin-related transition proceeds with near-single-particle strength. This creates a unique benchmark for testing modern theories, as any successful model must now be able to explain how these two different types of resistance work together in a single atom. The discovery does not just add a new entry to a list of nuclear states; it opens a window into the complex interplay between the collective motion of the nucleus and the quantum mechanics of its constituents, proving that even in the familiar realm of gold, there are still profound mysteries waiting to be solved.