New insights into the mechanism of nuclear fission

Open questions since the 1930s

Nuclear fission, in which a heavy nucleus splits in two and releases energy, was already discovered at the end of the 1930s by the chemists Otto Hahn and Fritz Strassmann, and interpreted correctly by the physicists Lise Meitner and Otto Frisch. However, open questions about the process persist to this day. The new scientific study addresses the question of why, when a heavy atomic nucleus fissions, the resulting fragments are observed to emerge spinning, even when the original nucleus did not spin at all. There are many competing theories, but the majority of these state that the spin of the fission fragments is generated before the nucleus splits, leading to a clear correlation of the spins of the two partner fragments.

More than thousand hours of beamtime

To reveal the mechanism generating fragment spin, the team induced nuclear fission reactions at the ALTO facility and measured gamma rays, which are emitted in the process with "nu-ball" consisting of 184 detectors. More than 1200 hours of beamtime at the particle accelerator were available to irradiate samples of the uranium isotope 238U and the thorium isotope 232Th with a pulsed neutron beam. Scientists from TU Darmstadt were involved in the preparation of the experiment, participated in the measurements, analysed selected data and contributed to the scientific discussion. "My group at TU Darmstadt contributed the huge experience with fast scintillation detectors in combination with germanium detectors to investigate fission fragments," Professor Thorsten Kröll reported. "However, this experiment enabled for the first time to address also the dynamics of the fission process which proceeds on a 10-21 seconds time scale inaccessible for any direct observation."

The new comprehensive data shows that the spin in fission is actually generated after the nucleus splits. This is indicated by analysis of the measured gamma rays. The experiments showed that the average spin has a saw-tooth dependence on the fragment mass. However, the two fragments, which can split in different mass ratios, have average spins which do not appear to depend on the mass of their partner fragment.

The main author of the study, Dr Jonathan Wilson from the IJC Laboratory in Orsay, said: "What really surprised me was the lack of significant dependence of the average spin observed in one fragment on the minimum spin demanded in the partner fragment. Most theories hypothesizing that spin is generated before fission would have predicted a strong correlation. Our results show that the fragment spin emerges after the splitting. It can be illustrated with by the snapping of a stretched elastic band which results in a turning force, or torque."

Expertise from TU Darmstadt

These experiments became feasible based on the experience gained in previous campaignes aiming at prompt spectroscopy of fission fragments with multi-detector spectrometers where already scientists from TU Darmstadt contributed their expertise in detector technology and data analysis.

New horizons for physics research

These new insights into the role of angular momentum in nuclear fission are important for the fundamental understanding and theoretical description of the fission process. However, they also have consequences for other research areas, such as the study of the structure of neutron-rich isotopes and the synthesis and stability of super-heavy elements. Moreover, there are implications for practical applications such as the gamma-ray heating problem in nuclear reactors. Knowledge of the gamma rays emitted in nuclear fission (and parameters such as the number of gamma rays emitted) are important for calculating heating effects.

This type of experiments opens new horizons also for experiments with neutrons from laser-driven neutron sources as they are currently under development at TU Darmstadt within the "LOEWE Research Cluster "Nuclear Photonics."