The continuous strive for more efficient and reliable nuclear power plants is leading to increasing complexity in the design and operations of reactors. As a consequence, comprehensive analyses are required to assess the reactor's safety during normal and accidental conditions. Validated simulation tools with high-fidelity capabilities, i.e., at the pin or sub-pin scale, are necessary to predict local neutronics effects in the core. However, the scarcity of high-resolution in-core experimental data imposes limitations to the validation of such high-fidelity pin-resolved neutronics codes. The challenges in performing online high-resolution in-core experiments are numerous: from the accessibility of in-core locations to the availability of neutron detection technologies with adequate dimensions and online capabilities. In the present thesis, the development of a novel miniature and minimalistic (MiMi) neutron detection technology is presented, allowing unprecedented spatial resolution for the online study of the neutron flux in zero-power research reactors. The MiMi neutron detectors are tested in the EPFL zero-power reactor CROCUS and used to build a data set of high-resolution neutronics experiments in CROCUS, including measurements of local gradients and directionality of neutron flux, and the local impact of a fuel rod displacement. As the next level of development, a three-dimensional (3D) full-core mapping system named SAFFRON, consisting of 149 MiMi neutron detectors distributed in-core, is designed and installed in CROCUS. Static thermal neutron flux maps are measured in absolute terms and between different core configurations, e.g., water level vs. control rod operation at criticality. The obtained results open up the investigation of a variety of space-dependent neutronics phenomena in CROCUS.