Subtle quantum effects within atomic nuclei can dramatically affect how some nuclei break apart. By studying 100 isotopes with masses below that of lead, an international team of physicists uncovered a previously unknown region in the nuclear landscape where fragments of fission split in an unexpected way. This is driven not by the usual forces, but by shell effects rooted in quantum mechanics.

“When a nucleus splits apart into two fragments, the mass and charge distribution of these fission fragments exhibits the signature of the underlying nuclear structure effect in the fission process,” explains Pierre Morfouace of Université Paris-Saclay, who led the study. “In the exotic region of the nuclear chart that we studied, where nuclei do not have many neutrons, a symmetric split was previously expected. However, the asymmetric fission means that a new quantum effect is at stake.”

This unexpected discovery not only sheds light on the fine details of how nuclei break apart but also has far-reaching implications. These range from the development of safer nuclear energy to understanding how heavy elements are created during cataclysmic astrophysical events like stellar explosions.

Quantum puzzle

Fission is the process by which a heavy atomic nucleus splits into smaller fragments. It is governed by a complex interplay of forces. The strong nuclear force, which binds protons and neutrons together, competes with the electromagnetic repulsion between positively charged protons. The result is that certain nuclei are unstable and typically leads to a symmetric fission.

But there’s another, subtler phenomenon at play: quantum shell effects. These arise because protons and neutrons inside the nucleus tend to arrange themselves into discrete energy levels or “shells,” much like electrons do in atoms.

“Quantum shell effects [in atomic electrons] play a major role in chemistry, where they are responsible for the properties of noble gases,” says Cedric Simenel of the Australian National University, who was not involved in the study. “In nuclear physics, they provide extra stability to spherical nuclei with so-called ‘magic’ numbers of protons or neutrons. Such shell effects drive heavy nuclei to often fission asymmetrically.”

In the case of very heavy nuclei, such as uranium or plutonium, this asymmetry is well documented. But in lighter, neutron-deficient nuclei – those with fewer neutrons than their stable counterparts – researchers had long expected symmetric fission, where the nucleus breaks into two roughly equal parts. This new study challenges that view.

New fission landscape

To investigate fission in this less-explored part of the nuclear chart, scientists from the R3B-SOFIA collaboration carried out experiments at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. They focused on nuclei ranging from iridium to thorium, many of which had never been studied before. The nuclei were fired at high energies into a lead target to induce fission.

The fragments produced in each fission event were carefully analysed using a suite of high-resolution detectors. A double ionization chamber captured the number of protons in each product, while a superconducting magnet and time-of-flight detectors tracked their momentum, enabling a detailed reconstruction of how the split occurred.

Using this method, the researchers found that the lightest fission fragments were frequently formed with 36 protons, which is the atomic number of krypton. This pattern suggests the presence of a stabilizing shell effect at that specific proton number.

“Our data reveal the stabilizing effect of proton shells at Z=36,” explains Morfouace. “This marks the identification of a new ‘island’ of asymmetric fission, one driven by the light fragment, unlike the well-known behaviour in heavier actinides. It expands our understanding of how nuclear structure influences fission outcomes.”

Future prospects

“Experimentally, what makes this work unique is that they provide the distribution of protons in the fragments, while earlier measurements in sub-lead nuclei were essentially focused on the total number of nucleons,” comments Simenel.

Since quantum shell effects are tied to specific numbers of protons or neutrons, not just the overall mass, these new measurements offer direct evidence of how proton shell structure shapes the outcome of fission in lighter nuclei. This makes the results particularly valuable for testing and refining theoretical models of fission dynamics.

“This work will undoubtedly lead to further experimental studies, in particular with more exotic light nuclei,” Simenel adds. “However, to me, the ball is now in the camp of theorists who need to improve their modelling of nuclear fission to achieve the predictive power required to study the role of fission in regions of the nuclear chart not accessible experimentally, as in nuclei formed in the astrophysical processes.”

The research is described in Nature.

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New measurements by physicists from the University of Surrey in the UK have shed fresh light on where the universe’s heavy elements come from. The measurements, which were made by smashing high-energy protons into a uranium target to generate strontium ions, then accelerating these ions towards a second, helium-filled target, might also help improve nuclear reactors.

The origin of the elements that follow iron in the periodic table is one of the biggest mysteries in nuclear astrophysics. As Surrey’s Matthew Williams explains, the standard picture is that these elements were formed when other elements captured neutrons, then underwent beta decay. The two ways this can happen are known as the rapid (r) and slow (s) processes.

The s-process occurs in the cores of stars and is relatively well understood. The r-process is comparatively mysterious. It occurs during violent astrophysical events such as certain types of supernovae and neutron star mergers that create an abundance of free neutrons. In these neutron-rich environments, atomic nuclei essentially capture neutrons before the neutrons can turn into protons via beta-minus decay, which occurs when a neutron emits an electron and an antineutrino.

From the night sky to the laboratory

One way of studying the r-process is to observe older stars. “Studies on heavy element abundance patterns in extremely old stars provide important clues here because these stars formed at times too early for the s-process to have made a significant contribution,” Williams explains. “This means that the heavy element pattern in these old stars may have been preserved from material ejected by prior extreme supernovae or neutron star merger events, in which the r-process is thought to happen.”

Recent observations of this type have revealed that the r-process is not necessarily a single scenario with a single abundance pattern. It may also have a “weak” component that is responsible for making elements with atomic numbers ranging from 37 (rubidium) to 47 (silver), without getting all the way up to the heaviest elements such as gold (atomic number 79) or actinides like thorium (90) and uranium (92).

This weak r-process could occur in a variety of situations, Williams explains. One scenario involves radioactive isotopes (that is, those with a few more neutrons than their stable counterparts) forming in hot neutrino-driven winds streaming from supernovae. This “flow” of nucleosynthesis towards higher neutron numbers is caused by processes known as (alpha,n) reactions, which occur when a radioactive isotope fuses with a helium nucleus and spits out a neutron. “These reactions impact the final abundance pattern before the neutron flux dissipates and the radioactive nuclei decay back to stability,” Williams says. “So, to match predicted patterns to what is observed, we need to know how fast the (alpha,n) reactions are on radioactive isotopes a few neutrons away from stability.”

The ⁹⁴Sr(alpha,n)⁹⁷Zr reaction

To obtain this information, Williams and colleagues studied a reaction in which radioactive strontium-94 absorbs an alpha particle (a helium nucleus), then emits a neutron and transforms into zirconium-97. To produce the radioactive ⁹⁴Sr beam, they fired high-energy protons at a uranium target at TRIUMF, the Canadian national accelerator centre. Using lasers, they selectively ionized and extracted strontium from the resulting debris before filtering out ⁹⁴Sr ions with a magnetic spectrometer.

The team then accelerated a beam of these ⁹⁴Sr ions to energies representative of collisions that would happen when a massive star explodes as a supernova. Finally, they directed the beam onto a nanomaterial target made of a silicon thin film containing billions of small nanobubbles of helium. This target was made by researchers at the Materials Science Institute of Seville (CSIC) in Spain.

“This thin film crams far more helium into a small target foil than previous techniques allowed, thereby enabling the measurement of helium burning reactions with radioactive beams that characterize the weak r-process,” Williams explains.

To identify the ⁹⁴Sr(alpha,n)⁹⁷Zr reactions, the researchers used a mass spectrometer to select for ⁹⁷Zr while simultaneously using an array of gamma-ray detectors around the target to look for the gamma rays it emits. When they saw both a heavy ion with an atomic mass of 97 and a ⁹⁷Zr gamma ray, they knew they had identified the reaction of interest. In doing so, Williams says, they were able to measure the probability that this reaction occurs at the energies and temperatures present in supernovae.

Williams thinks that scientists should be able to measure many more weak r-process reactions using this technology. This should help them constrain where the weak r-process comes from. “Does it happen in supernovae winds? Or can it happen in a component of ejected material from neutron star mergers?” he asks.

As well as shedding light on the origins of heavy elements, the team’s findings might also help us better understand how materials respond to the high radiation environments in nuclear reactors. “By updating models of how readily nuclei react, especially radioactive nuclei, we can design components for these reactors that will operate and last longer before needing to be replaced,” Williams says.

The work is detailed in Physical Review Letters.

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