The Nickel Alchemy: Ab Initio Calculations Reveal the Origin of Heavy Elements

The Enigma of the Cosmic Forge and the r-process

The origin of heavy elements in the universe, such as gold, platinum, and uranium, remains one of the most fascinating mysteries in modern astrophysics. We know these elements are forged in cataclysmic cosmic events, such as neutron star mergers or supernovae, through a mechanism known as the rapid neutron capture process, or r-process. However, accurately simulating these events requires deep knowledge of neutron capture rates by highly unstable, neutron-rich atomic nuclei.

The great challenge lies in the fact that many of these isotopes are so short-lived that they cannot be produced or measured in terrestrial laboratories. This is where the fundamental work of Francesca Bonaiti and Joanna E. Sobczyk comes in. In a newly published study, the researchers applied cutting-edge theoretical methods to predict the behavior of the nickel isotopic chain, offering a new lens into how matter transforms in the heart of stars.

Ab Initio Methodology: Building from Scratch

The approach used by Francesca Bonaiti and Joanna E. Sobczyk is what physicists call an “ab initio” calculation (Latin for “from the beginning”). Unlike phenomenological models that rely on adjustments based on previous experimental data, the ab initio method builds the understanding of the atomic nucleus from the fundamental interactions between protons and neutrons.

The authors utilized chiral effective field theory to describe two- and three-nucleon interactions. To solve the resulting complex equations, they employed the Lorentz Integral Transform Coupled-Cluster (LIT-CC) method. This technique was extended to axially symmetric reference states, allowing, for the first time, the calculation of the electric dipole (E1) strength in open-shell nuclei along the nickel chain, from nickel-58 to nickel-80.

The importance of the E1 strength lies in its ability to dictate how a nucleus absorbs energy and, consequently, how it interacts with environmental neutrons. By validating their models against known data for nickel-58 and nickel-68, the researchers ensured the robustness necessary to explore territories where experiments cannot yet reach.

The Pygmy Resonance and Its Impact on Neutron Capture

One of the most significant findings of the study is the presence of the so-called “pygmy dipole resonance” in heavier nickel isotopes. While the giant dipole resonance is a well-known phenomenon, the pygmy resonance occurs at lower energies, near the neutron emission threshold. Francesca Bonaiti and Joanna E. Sobczyk demonstrated that as the neutron number increases in the nickel chain, this low-energy strength grows significantly.

By integrating these data into the TALYS reaction code, the researchers observed a dramatic increase in neutron capture rates. For isotopes between nickel-68 and nickel-78, the capture rate was enhanced by factors of 3 to 10 at temperatures ranging between 3 and 10 GK (gigakelvin). In the case of nickel-80, the effect was even earlier, manifesting as low as 0.5 GK.

Mathematically, the relationship between photonuclear strength and the capture rate can be expressed by integrating the cross-section over the Maxwell-Boltzmann distribution, where the reaction rate $\langle \sigma v \rangle$ is proportional to $\int_0^\infty \sigma(E) E e^{-E/kT} dE$. The increase in E1 strength at low energies directly alters the value of this integral, accelerating the production of heavier elements.

Implications for Deep Tech Astrophysics

The practical impact of this research for astrophysics is profound. Nucleosynthesis models that previously underestimated the speed of heavy element formation can now be refined with high-precision theoretical data. This allows for more accurate predictions of element abundances observed in ancient stars and kilonova remnants.

Furthermore, the work of Francesca Bonaiti and Joanna E. Sobczyk sets a new standard for theoretical nuclear physics. By demonstrating that ab initio calculations can provide reliable inputs for isotopic chains far from stability, they pave the way for other essential r-process elements to be mapped. We are one step closer to understanding not only where we came from but the fundamental limits of matter’s stability in the cosmos.

Scientific Source: Impact of ab initio E1 strength on neutron capture rates along the nickel isotopic chain — Authors: Francesca Bonaiti, Joanna E. Sobczyk. Available on arXiv.