Surreal illustration of an atomic nucleus emitting gamma rays.

Unlocking Nuclear Secrets: How a Tiny Atom Could Rewrite Physics Textbooks

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Lena Voss in Science & Nature August 2025 • 4 min read.

"Scientists are testing the limits of the Brink-Axel hypothesis, revealing new insights into the behavior of atomic nuclei and its impact on our understanding of nuclear physics."


For decades, scientists have leaned on certain assumptions about how atomic nuclei behave, especially when they're in an excited state. One of the most vital of these assumptions is the idea that the way a nucleus releases energy—specifically through gamma decay—doesn't depend on how that nucleus was initially energized. This concept, known as the generalized Brink-Axel (gBA) hypothesis, simplifies a lot of complex calculations, making it easier to predict nuclear reactions. However, like any good scientific theory, it needs constant testing.

Imagine the nucleus of an atom as a black box. You pump energy into it, and it spits that energy out in the form of gamma rays. The gBA hypothesis says that the characteristics of those gamma rays depend only on their energy and not on the specific way you excited the nucleus in the first place. This is a handy rule, but what if it's not always true? What if the internal structure of the nucleus and its energy state do influence the gamma decay process? That's what a team of physicists recently set out to investigate.

In a study published in Physical Review C, researchers meticulously examined the gamma decay patterns of nickel isotopes, specifically 64Ni and 65Ni. By smashing particles into these isotopes and then carefully measuring the emitted gamma rays, they sought to determine whether the gBA hypothesis holds up under close scrutiny. Their findings could have significant implications for fields ranging from nuclear astrophysics to reactor physics, potentially reshaping how we model nuclear behavior in extreme conditions.

The Experiment: Probing Nickel with Particles

Surreal illustration of an atomic nucleus emitting gamma rays.

The team's approach involved analyzing existing data from experiments conducted at the University of Oslo. These experiments used particle-gamma coincidence techniques. Think of it this way: they shot a beam of particles at a target of nickel isotopes and measured not only the outgoing particles but also the gamma rays emitted in coincidence. This allowed them to correlate the energy of the incoming particles with the energy and characteristics of the emitted gamma rays, providing a detailed picture of the nuclear decay process.

To test the gBA hypothesis, the researchers focused on how the gamma strength function (ySF) changes with the initial and final excitation energies of the nickel nuclei. The ySF essentially describes the probability of gamma decay at different energy levels. If the gBA hypothesis is correct, the ySF should depend only on the gamma-ray energy and not on the specific energy levels of the initial or final states of the nucleus.

Here's what they looked at:
  • The relationship between initial excitation energy and gamma-ray strength.
  • How well the data aligned with the generalized Brink-Axel hypothesis.
  • Fluctuations in transition strengths as a function of gamma-ray and excitation energy.
The team applied a method developed at the University of Oslo, which involves unfolding the measured gamma-ray spectra and extracting the primary gamma rays—those emitted directly after the initial nuclear reaction. This is crucial because it allows them to isolate the gamma rays that are most directly related to the nuclear decay process and minimize the influence of secondary decays. By carefully selecting specific regions in the data, they could focus on gamma decay from compound nuclear states, where the gBA hypothesis is expected to be most valid.

What It All Means: A Step Forward, But More to Explore

The research supports the idea that the gamma strength functions of 64Ni and 65Ni are largely independent of the initial excitation energy, aligning with the generalized Brink-Axel hypothesis. However, they also observed significant fluctuations, particularly when looking at decays to the ground state and first excited states. These fluctuations suggest that the nuclear structure and the specific energy levels involved can indeed influence the gamma decay process, especially at lower excitation energies. This nuanced understanding is crucial for refining nuclear models and improving predictions in various applications.

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Everything You Need To Know

1

What is the generalized Brink-Axel hypothesis, and how does this research challenge or refine it?

The generalized Brink-Axel hypothesis suggests that the way an atomic nucleus releases energy through gamma decay depends only on the energy of the emitted gamma rays and not on how the nucleus was initially energized. This simplifies calculations of nuclear reactions. However, the research indicates that the gamma decay process can be influenced by the internal structure of the nucleus and its energy state, especially at lower excitation energies, suggesting that gBA may not always hold true.

2

How did the researchers investigate the generalized Brink-Axel hypothesis using nickel isotopes?

Researchers examined the gamma decay patterns of nickel isotopes, specifically 64Ni and 65Ni. They did this by smashing particles into these isotopes and carefully measuring the emitted gamma rays. By correlating the energy of the incoming particles with the energy and characteristics of the emitted gamma rays, they sought to determine whether the generalized Brink-Axel hypothesis holds up under close scrutiny.

3

What is the gamma strength function (ySF), and how does it relate to testing the generalized Brink-Axel hypothesis?

The gamma strength function (ySF) describes the probability of gamma decay at different energy levels. According to the generalized Brink-Axel hypothesis, the ySF should depend only on the gamma-ray energy and not on the specific energy levels of the initial or final states of the nucleus. The research focused on how the ySF changes with the initial and final excitation energies of the nickel nuclei.

4

To what extent did this research support or contradict the generalized Brink-Axel hypothesis, and what implications do the findings have?

The research primarily supports the generalized Brink-Axel hypothesis, indicating that the gamma strength functions of 64Ni and 65Ni are largely independent of the initial excitation energy. However, the observation of significant fluctuations, particularly at lower excitation energies and decays to the ground and first excited states, suggests that nuclear structure and specific energy levels can influence the gamma decay process. This calls for refining nuclear models to improve predictions, especially in extreme conditions.

5

Can you explain the particle-gamma coincidence techniques used and the method for extracting primary gamma rays in the experiment?

The particle-gamma coincidence techniques involved shooting a beam of particles at a target of nickel isotopes and measuring both the outgoing particles and the emitted gamma rays. This allowed the researchers to correlate the energy of the incoming particles with the energy and characteristics of the emitted gamma rays. The unfolding method developed at the University of Oslo was used to extract primary gamma rays emitted directly after the initial nuclear reaction and isolate gamma rays most directly related to the nuclear decay process, minimizing the influence of secondary decays.

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