NCN-SONATA-BIS13
Search for high-rank symmetries in medium-mass to super-heavy nuclei using highresolution in-beam gamma spectroscopy at HIL UW
Principal Investigator: Dr. Magdalena Matejska-Minda (Instytut Fizyki Jądrowej im. Henryka Niewodniczańskiego PAN)
Consortium Project:
- Instytut Fizyki Jądrowej im. Henryka Niewodniczańskiego PAN (leader)
- Heavy Ion Laboratory at Uniwersyty of Warsaw
The primary aim of the present project is to improve our understanding of the nuclear forces and the quantum mechanical description of properties of nuclei at the limits of their angular momentum and excitation energy. This will be done by studying discrete and continuum excited states in medium mass and heavy nuclei, which exhibit a large degree of quadrupole and octupole shape deformation. Additionally, a search for higher-rank symmetries in these nuclei is envisaged.
The proposed research program is focused on the extension of the knowledge of the excited structures in the nuclei from the A~40 mass region especially for 42Ca, 44Ti up to, or beyond the rotational band terminating states interpreted so far as particle-hole excitations. Furthermore, it includes a search for possible feeding of the high spin states (close to the band termination) with high-energy discrete transitions and determination of the electromagnetic character of this radiation. And, on top of that, establishing a link between the structures at high and low temperatures by coincident measurement of continuum and discrete gamma-rays. In these light nuclei, the so-called superdeformed (SD) bands are composed of very short living states connected by strong E2 transitions, therefore they are interpreted as collective. We aim to experimentally check if the SD bands continue beyond the Shell Model spin limit (as expected) or terminate. The advantage of the proposed experimental set-up will be also the possibility to determine the lifetimes of the high spin states, for the nuclei of interest. Most of the conclusions regarding the deformation of high spin structures excited in A~40 nuclei were drawn based on theoretical premises. Mainly, the similarities between the experimental and calculated energy sequences of gamma-band transitions were taken into account. Much more rigorous verification of the correctness of the calculations, and specifically the values of the deformations predicted by them, will be possible by measuring the lifetimes of high-spin states in the excited bands – such measurements provide information about the electric quadrupole moment of the nucleus, and thus about the nuclear deformation.
On the other hand, nuclei from the Ac-Th region are the best-known examples of the reflection asymmetric nuclear shapes. The presence of collective bands of alternative parity states is a typical feature of these nuclei. However, such collective structures, known in the actinides, extended to a relatively low angular momentum of I~20 ħ. Therefore it is still not clear if at the high spin regime the octupole – quadrupole shape transition takes place, as predicted by the theory. Furthermore, similarly as in the lighter nuclei, the connection between the octupole bands and excited structures at high nuclear temperatures reminds a puzzle.
The incomplete knowledge of the high spin structure of the octupole deformed actinides is due to a very limited number of possible beam-target combinations leading to the nuclei of interest, very low cross-section, and dominant fission background. Coupling of the Recoil Filter Detector to the EAGLE gamma-ray spectrometer at the Heavy Ion Laboratory University of Warsaw gives a chance to study deformed A~40 nuclei, and to look for the signatures of shape transitions in octupole-deformed Th nuclei, where gamma-ray spectra are normally impaired by a large background from competing processes, particle evaporation, and prompt fission.
To provide a detailed interpretation of the obtained data the advanced large-scale nuclear mean-field calculations will be performed for the light and heavy nuclear systems of interest. Based on the state-of-the-art phenomenological
Hamiltonian with the newest universal parameterization, and including high-order geometrical symmetries, which take into account the actual status of the experimental information, will enable us to push our understanding of nuclear structure in the nuclei of interest. The discussed approach allows for estimating the uncertainties of the theory predictions (analogs of the experimental error bars) opening the way for state-of-the-art cooperation between experiment and theory.