Physicists have found a new way to study the shape of atomic nuclei; by destroying them in high-energy collisions. The method could help scientists better understand the shapes of cores that affect the rate at which elements form in stars, for example, and determine which materials make the best nuclear fuel.

“The shape of the nuclei affects almost all aspects of atomic nuclei and nuclear processes,” says nuclear physicist Jie Meng of Peking University in Beijing. New imaging method published Nature He says the development on November 6 is a “significant and exciting advance.”

Atomic nucleus takes two shapes

At the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York, a team collided two beams of uranium-238 (and later two beams of gold) at extreme energies. They hit them “so hard that we melted the beans into soup,” says co-author Jiangyong Jia, a physicist at Stony Brook University in New York.

The hot plasma produced by the collisions expanded very rapidly under pressure, which was related to the initial shape of the nucleus. By using a detector called the Solenoidal Tracker at RHIC, or STAR, to find the momentum of each of the thousands of particles created by both types of collisions and match the results to the models, the team was able to “turn back the clock and extract its shape.” nuclei,” says Jia.

hidden figures

The atomic nucleus consists of protons and neutrons that live in energy shells like electrons. In general, particles take a shape that minimizes the energy of the system. Like a liquid droplet, the nucleus can take on a variety of shapes, such as a pear, an American football, or a peanut shell. The shape of the nucleus is “very difficult to predict from theory,” Jia says. Moreover change over time due to quantum fluctuations.

Past experiments investigating the shape involved deflecting low-energy ions from nuclei. This method, called Coulomb excitation, excites the nuclei, and the radiation they emit as they return to their ground state reveals the properties of their shape. But because the time scale is relatively long, this type of imaging can only provide a long-exposure shot that shows only the average of any fluctuations in shape.

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In contrast, the high-energy collision method gives a snapshot of the nucleus at the time of the collision. Jia says this is a more direct method that makes it more suitable for studying exotic shapes.

The technique confirmed that the gold had a nearly spherical shape that was consistent from one image to the next. In contrast, the uranium changed in the snapshots as the nuclei collided in different directions. This allowed the researchers to calculate the relative lengths of the nucleus in three spatial dimensions; This suggests that the uranium is not only stretched but also slightly compressed in one dimension, like a deflated American football.

“I find it fascinating that it works and that other nuclear processes do not affect the particle emission and do not hide the deformation,” says Magdalena Zielińska, a nuclear physicist at the French Commission for Alternative Energies and Atomic Energy near Paris.

Hard or soft?

Zielińska says this type of imaging can help with the challenging task of distinguishing between ‘hard’ nuclei with well-defined shapes and ‘soft’ undulating nuclei.

Jia said his team also wants to apply this method to study the differences between lighter ions such as oxygen and neon. Oxygen nuclei are nearly spherical, while neon nuclei, which carry an extra two protons and two neutrons, are thought to protrude. Comparing their shapes will allow researchers to understand how protons and neutrons form clusters in the nucleus, Jia says.

Information about shape can also reveal whether nuclei will interact or undergo nuclear fission, increasing the chances of detecting a process called neutrinoless couple β decayIt could help solve some long-standing mysteries in physics. About 99.9% of visible matter is found at the center of atoms, Jia says. “Understanding the nuclear building block is fundamentally central to understanding who we are.”