"Atomic nuclei are finite systems composed of fermions, the nucleons, and essentially governed by the strong force and quantum mechanical laws. Their structure is characterized by single-particle orbitals grouped in energy shells, separated by energy gaps. The numbers of nucleons that correspond to fully filled shells are called magic and represent the backbone of nuclear structure. In this proposal, we propose a new approach to investigate the most neutron-rich systems ever reached and establish the shell structure in new regions of the nuclear chart where new magic numbers or strong shell reordering are expected or controversial. This will open new horizons in the terra incognita of the nuclear landscape. Beyond the fundamental question of the nuclear force, the assessment of new shell closures in the nuclear landscape is of primary importance to better understand the stellar nucleosynthesis in the Universe.In-flight gamma spectroscopy of rare isotopes at intermediate energy is one of the most efficient tools to populate and measure excited states in exotic nuclei. We propose to develop a new method that will increase the sensitivity of prompt-gamma spectroscopy by more than one order of magnitude compared to existing setups. Experiments will be performed at the most competitive fragmentation radioactive-beam facilities worldwide. In the future, this program will take advantage of the European FAIR facility, Germany, coupled to the European new-generation gamma array AGATA spectrometer. When coupled to AGATA, the improvement will reach a factor of several hundreds. This new experimental technique will be strengthened by original developments in the theory of reaction mechanisms, which are also included in this proposal."