For decades, researchers have used statistical models, in particular the Hauser-Feshbach formalism and random matrix theory, to describe the behavior of excited atomic nuclei in regions with a high density of quantum states. These models assume that nuclear processes are chaotic and features due to nuclear structure become irrelevant.

The new experimental approach, developed at TU Darmstadt and carried out at the High Intensity Gamma-ray Source (HIγS) facility and TUNL at Duke University, requires a new perspective. Using polarized quasi-monoenergetic photon beams and high-resolution gamma-ray spectroscopy, the team investigated the decay behavior of the deformed nucleus neodymium-150 (¹⁵⁰Nd).

“We found strong evidence that the K quantum number, a measure of the alignment of the angular momentum of a nucleus along its axis of deformation, continues to have a measurable influence on the gamma-decay properties even when the nucleus already exhibits a very high level density”, explains Dr. Isaak. This observation suggests that the traditional statistical description may not fully account for the complexities of nuclear decay at these energies. The results indicate that certain aspects of nuclear structure remain relevant beyond what was thought to be a chaotic regime, dominated by pure statistics. This new insight has implications for how nuclear reactions are modeled, particularly those involved in the synthesis of elements in the Universe.

The new method is broadly applicable to stable isotopes, paving the way for further studies on both deformed and spherical nuclei. Future research will explore whether the observed deviations from statistical models are due to a partial conservation of the K quantum number or whether other nuclear physics phenomena influence the decay behavior.