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. It can be illustrated with by the snapping of a stretched elastic band which results in a turning force, or torque." Our results show that the fragment spin emerges after the splitting. Most theories hypothesizing that spin is generated before fission would have predicted a strong correlation. 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. 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 experiments showed that the average spin has a saw-tooth dependence on the fragment mass. This is indicated by analysis of the measured gamma rays. The new comprehensive data shows that the spin in fission is actually generated after the nucleus splits. "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." "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. 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.
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.
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. 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. 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.
However, open questions about the process persist to this day. 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.
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