One of the key questions in the field of nuclear structure concerns the emergence of collectivity, and its link with the microscopic structure of the nucleus. Atomic nuclei can exhibit so-called collective behaviours, which means that their constituents, protons and neutrons, move in a coherent way. The main modes of collective nuclear motion are vibrations and rotations. If a nucleus is not deformed, it cannot undergo rotations when excited, but vibrations around its spherical equilibrium shape are possible.
Even-even isotopes of cadmium have been considered textbook examples of vibrational behaviour. However, this interpretation has been questioned following recent experimental studies, which have, with a guidance from theoretical calculations, led to the reorganization of the level schemes of 110,112Cd in terms of rotational excitations, suggesting the presence of a variety of shapes in these nuclei. Thanks to a recent PhD work in our group, this new interpretation has been extended to the 106Cd nucleus. However, questions remain regarding the nature of certain low-lying excited states in this nucleus. In particular, we obtained indications that some excited states may result from a coupling between the so-called octupole (i.e. the nucleus deforms into a pear shape) and quadrupole (i.e. the nucleus oscillates between elongated and flattened shapes) vibrations. To test this hypothesis, a high-precision beta-decay experiment has been proposed at TRIUMF (Vancouver, Canada) using the world's most advanced spectrometer for beta-decay measurements GRIFFIN, to search for weak decay paths in the 106Cd level scheme, and to unambiguously determine the spins of the excited states through the analysis of gamma-gamma angular correlations. Thanks to this measurement it will be possible to solve multiple puzzles concerning the structure of this nucleus, in particular regarding the possible triaxiality of its ground state and the suspected coexistence of multiple shapes.
The student will be in charge of the analysis of this experiment, which will take place in 2025. Then, based on the results of this analysis, they will proceed to a re-evaluation of the population cross sections of excited levels in 106Cd, which were measured with the new generation gamma-ray spectrometer AGATA at GANIL using the Coulomb excitation technique. From this combination of measurements, we hope to obtain, for the first time in the nuclear chart, the complete set of transition probabilities between the states resulting from the coupling between octupole and quadrupole vibrations. We will then proceed to the interpretation of the results in close collaboration with experts in nuclear-structure theory.
This thesis work will make it possible for the student to follow a research project in its entirety, from the preparation of the experiment to its theoretical interpretation, and to become familiar with several experimental gamma-ray spectroscopy techniques, using the most advanced gamma-ray spectrometers in the world.