Impact of ab initio E1 strength on neutron capture rates along the nickel isotopic chain

Deep in the heart of stars, a cosmic alchemy is constantly at work, forging the heavy elements that make up our world. While lighter elements like carbon and oxygen are born in the steady burning of stellar furnaces, the heaviest elements, such as gold and platinum, are created in violent, extreme events where neutrons are captured at breakneck speeds. This process, known as the rapid neutron-capture process, or r-process, is the universe's primary factory for these heavy materials. To understand exactly how much of each element is produced, scientists must know the precise rates at which atomic nuclei capture neutrons. However, many of the unstable, neutron-rich nuclei involved in this cosmic dance exist for only fractions of a second, making them impossible to study directly in a laboratory. Consequently, researchers must rely on theoretical calculations to predict how these fleeting atoms behave, filling in the gaps where direct measurement is impossible.

A critical piece of this puzzle is how these nuclei interact with light, specifically how they absorb energy through a type of vibration called electric dipole strength. In simple terms, this describes how the protons and neutrons inside an atom shift relative to one another when hit by energy. For decades, scientists have used broad, generalized models to guess these behaviors for unstable atoms, but these guesses often lack the precision needed to accurately simulate the creation of elements in the universe. A new study by Francesca Bonaiti and Joanna E. Sobczyk tackles this challenge head-on by calculating these interactions from the ground up, using a method that starts with the fundamental forces between particles rather than relying on educated guesses.

The researchers focused on a specific family of atoms: the nickel isotopes. Nickel is a key player in the r-process, acting as a stepping stone that influences the abundance of elements with masses around one hundred. The team used a powerful computational approach called "ab initio," which means "from the beginning." Instead of fitting their calculations to existing data, they started with the fundamental laws of nuclear physics, specifically the interactions between protons and neutrons derived from a theory called chiral effective field theory. They applied this to a chain of nickel atoms, ranging from the stable nickel-58 to the extremely neutron-rich nickel-80. To handle the complex mathematics of these unstable, open-shell nuclei, they developed a new extension of a method known as the Lorentz integral transform coupled-cluster technique. This allowed them to simulate how these atoms respond to energy without needing to construct the impossible scattering states that usually complicate such calculations.

The results revealed a significant feature in the behavior of these neutron-rich atoms: a distinct, low-lying strength just above the point where a neutron can escape the nucleus. This phenomenon, often associated with a "pygmy dipole resonance," acts like a soft, low-energy vibration that becomes more pronounced as the nucleus gains more neutrons. In the case of nickel-80, this low-energy vibration is so strong and sits so close to the escape threshold that it begins to influence the atom's behavior at surprisingly low temperatures, around 0.5 billion degrees. For the other isotopes in the chain, this effect grows stronger as the number of neutrons increases, becoming a major factor at temperatures between 3 and 10 billion degrees.

When the researchers fed these new, first-principles calculations into the standard computer codes used to simulate stellar explosions, the impact was clear. The inclusion of this specific electric dipole strength significantly boosted the predicted rates at which these nickel nuclei capture neutrons. For the heavier isotopes, the rate of capture increased by factors ranging from 16 percent to three times the previous estimates, depending on the temperature and the specific isotope. This enhancement is not a minor adjustment; it represents a fundamental shift in our understanding of how these nuclei interact with their environment. The study also highlighted that the largest source of uncertainty in their results comes from the mathematical process of reversing their calculations to get the final answer, rather than from the underlying physics models themselves.

By providing the first rigorous, first-principles description of this electric dipole strength along an isotopic chain, the work offers a new, more reliable foundation for astrophysical simulations. It suggests that the production of heavy elements in the universe may be more efficient than previously thought, particularly for the heaviest nickel isotopes. While the researchers acknowledge that future work will need to refine the mathematical inversion process to reduce uncertainties, this study successfully bridges the gap between fundamental nuclear theory and the grand scale of cosmic element formation. It demonstrates that even for the most unstable atoms, we can now predict their behavior with a level of detail that was previously out of reach, bringing us closer to solving the mystery of where the heavy elements in our universe truly come from.