Physicists find atomic nucleus with a ‘bubble’ in the middle

Scientists have found the first experimental evidence that an atomic nucleus can harbor bubbles.

The unstable isotope silicon-34 has a bubblelike center with a paucity of protons, scientists report October 24 in Nature Physics. This unusual “bubble nucleus” could help scientists understand how heavy elements are born in the universe, and help scientists find new, ultraheavy stable isotopes.

In their quirky quantum way, protons and neutrons in a nucleus refuse to exist in only one place at a time. Instead, they are spread out across the nucleus in nuclear orbitals, which describe the probability that each proton or neutron will be found in a particular spot. Normally, due to the strong nuclear force that holds the two types of particles together, nuclei have a fairly constant density in their centers, regardless of the number of protons and neutrons they contain. In silicon-34, however, some scientists predicted that one of the proton orbitals that fills the center of the nucleus would be almost empty, creating a bubble nucleus. But not all theories agreed. “This was the reason for doing the experiment,” says coauthor Olivier Sorlin, a nuclear physicist at the National Large Heavy Ion Accelerator, GANIL, in Caen, France. “Some people didn’t believe that it would exist.”

In pursuit of the bubble nucleus, the scientists smashed silicon-34 nuclei into a beryllium target, which knocked single protons out of the nuclei to create aluminum-33. The resulting aluminum-33 nuclei were in excited, or high-energy, states and quickly dropped down to a lower energy by emitting photons, or light particles. By observing the energy of those photons, Sorlin and colleagues could reconstruct the orbital of the proton that had been kicked out of the nucleus.

The scientists found that they ejected few protons from the central orbital that theorists had predicted would be empty. While the orbital can theoretically hold up to two protons, it held only 0.17 protons on average. In silicon-34, the central proton density is about half that of a comparable nucleus, the scientists calculated, after taking into account other central orbitals that contain normal numbers of protons. (The density of neutrons in silicon-34’s center, however, is normal.)

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“What they are doing is extremely difficult,” says theoretical nuclear physicist Paul-Henri Heenen of the Université libre de Bruxelles in Belgium. Silicon-34 isn’t stable, he notes. It has a half-life of less than three seconds, making it a challenge to work with.

As protons are added to nuclei, they fill orbitals in a sequential manner, according to the energy levels of the orbitals. Silicon-34 is special — it has a certain “magic” number of protons and neutrons in its nucleus. There are a variety of such magic numbers, which enhance the stability of atomic nuclei. A magic number of protons means that the energy needed to boost a proton into the next orbital is particularly high. This explains the bubble’s origin. For a proton to jump into the unfilled central orbital, it needs significantly more energy. So silicon-34’s center remains sparsely populated.  

“It’s an interesting paper and indeed provides evidence” for a bubble nucleus, says nuclear physicist Jiangming Yao of the University of North Carolina, Chapel Hill. But, he says, the evidence is “not direct,” because it relies on nuclear models to calculate density. To directly measure the density of protons will require using electrons to probe the inner workings of the nucleus.

Still, the research could help scientists understand the spin-orbit interaction, the interplay between a proton’s angular momentum in its orbital and its intrinsic angular momentum, or spin. The effect is important for keeping heavy nuclei stable. Figuring out the impact of that interaction in this unusual nucleus could help scientists better predict the potential location of the “island of stability,” a theorized region of the periodic table with heavy elements that may be stable for long periods of time (SN: 6/5/10, p. 26).

What’s more, a better grasp of the spin-orbit interaction could also help scientists learn how elements are forged in rare cosmic cataclysms such as the merging of two neutron stars. There, nuclei undergo a complex chain of reactions, swallowing up neutrons and undergoing radioactive decay. Modeling this process requires a precise understanding of the stability of various nuclei — a property affected by the spin-orbit interaction.