Little-known forces aid in maintaining atomic stability: what physicists have discovered.

Protons and neutrons together form a stable atomic nucleus due to strong interactions. For decades, the two-nucleon force has been viewed as the primary player that simultaneously attracts these particles and prevents them from colliding too closely. However, physicists have now concluded that the less-explored three-nucleon force also significantly influences the stability of atomic nuclei. This research has been published in the journal Physics Letters B, reports Earth.

Nucleons are protons and neutrons, which make up the nuclei of atoms. These elementary particles determine whether a chemical element is stable or prone to decay. According to physicists, three forces act within atomic nuclei:

• The strong nuclear interaction and electrostatic force, which together form the two-nucleon force;
• The weaker three-nucleon force.

Physicists base the current understanding of atomic nucleus structure on the shell model of the atomic nucleus. In this model, nucleons fill specific energy levels, similar to how electrons do, although the underlying physics of this process is more complex.

Nuclear forces govern how atoms evolve in the cores of stars, where new chemical elements are created through nuclear fusion. This process leads to the production of heavier atoms, which ultimately disperse into space. Understanding how nuclear forces shape this process could enhance models of stellar evolution and the formation of chemical elements.

For many decades, physicists have worked to create a model for the operation of the two-nucleon force. However, the three-nucleon force, which comes into play when three nucleons interact simultaneously, remains poorly understood.

The authors of the study discovered that this additional force surprisingly plays a significant role in maintaining the stability of the atomic nucleus. Physicists employed simulations to observe how these interactions alter energy levels within the atomic nucleus.

When nucleons align their spins (intrinsic angular momentum) with their direction of motion, they transition to a lower energy state. Conversely, when the spins of nucleons do not align with their direction of motion, the particles move to a higher energy state.

Different energy levels create shells within the nucleus. Physicists found that the three-nucleon force increases the energy gap between shells in larger atomic nuclei. Consequently, this force has an almost equivalent impact on the stability of the atomic nucleus as the influence of the two-nucleon force.

Heavier atoms contain more nucleons, making these interactions even more critical. If an atomic nucleus has a stable structure, the arrangement of additional neutrons becomes more complex.

When chemical elements are formed through fusion within stars, increased stability in the nucleus complicates the capture of additional neutrons. As a result, some chemical elements may form more slowly or under specific conditions, physicists say.

The results of this study could influence predictions of isotope stability (different versions of chemical elements) and enhance the understanding of radioactive decay. Physicists believe that a greater number of nucleons likely further strengthens the three-nucleon force, potentially making some atomic nuclei more stable than current models suggest.