When an over-inflated balloon bursts, its pieces fly away in opposite directions performing various air evolution. The process of nuclear fission, in which a nucleus is split into two parts accompanied by the emission of several neutrons, proceeds similarly. UW scientists have contributed to the research presented in the newly-published article in “Nature”.
“Angular momentum generation in nuclear fission” is the article published on 24th February in the scientific journal “Nature”. Researchers from the UW Faculty of Physics: Prof. Agnieszka Kargul, Prof. Krzysztof Miernik, Dr. Victor Gaudilla, and doctoral candidates Monika Piersa and Ewa Adamska are among the authors of the publication.
Angular momentum generation in nuclear fission
When an over-inflated balloon bursts, its pieces fly away in opposite directions performing various air evolution. The process of nuclear fission, in which a nucleus is split into two parts accompanied by the emission of several neutrons, proceeds similarly. The energy released in this process manifests itself not only in the form of the kinetic energy of the created fragments but also in the form of rotation and other nuclear excitations. One of the accompanying phenomena is the emission of gamma-ray quanta, which lifts not only the excess energy created but also the angular momentum (i.e. inhibit rotation).
For over 40 years, the conditions for the appearance of high angular momentum in a fission system, where the angular momentum was practically zero, remained experimentally unexplored. In particular, it was not clear whether this high angular momentum arises before or after the atomic nucleus splits into smaller fragments. A decisive conclusion in this matter was reached thanks to a series of experiments performed at Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (IJC) in Orsay, France.
The results, published in the journal “Nature”, are the effect of cooperation of physicists from 37 institutions (from 16 countries), including the Faculty of Physics, University of Warsaw, forming Nu-ball collaboration. The leading role in research was due to IJC, where, since 2018, over 1200 hours of measurements with a collimated beam of fast neutrons were performed in the ALTO facility. Neutrons impinged on a target with fissionable materials, 238U or 232Th, and induced nuclear fission. An additional measurement was performed with spontaneously fissioning 252Cf nuclei. Gamma radiation, accompanying the fission, was recorded by over 200 detectors, and cascades of gamma transitions in over 30 nuclei were reconstructed.
Separate sources of angular momentum
As a result of the analysis, it was concluded that there is no correlation in the emitted radiation between the fission fragments. This means that, contrary to the most of the currently used nuclear fission models, the sources of angular momentum are separate, and the fragments do not exchange information. A new model was proposed, in which the fissioning nuclei form a neck, and after it breaks, two separate fragments with very elongated shapes appear. The new systems tend towards a spherical shape, and the energy associated with the deformation is converted into excitation of the newly formed atomic nuclei. The proposed process explains the statistical nature of excitations, independent for each fission fragment.
The data obtained by the physicist from the Nu-ball group are relevant to nuclear reactor modelling, where gamma radiation and its multiplicity are important components of the heat transport calculations. They are also important in the production of new superheavy elements and exotic nuclides with large neutron excess.
The article “Angular momentum generation in nuclear fission” was published in “Nature” on 24th February.