The ELI-NP facility, currently being built in Bucharest, Romania, will deliver an intense and almost monochromatic γ beam with tunable energy between 0.2 MeV and 19.5 MeV in two different beamlines. An articulated beam characterization system will be installed downstream of the collimator of each line. The system will use, as calibration candles, a few selected nuclear levels whose fluorescence condition will be monitored by a Nuclear Resonance Scattering System (NRSS). The NRSS will use a peculiar double-readout approach in order to detect resonant events overwhelming background: both scintillation and Cherenkov photons produced inside the same crystals will be separately read. •The NRS system will play a crucial role in the characterization of the Eli-NP beam.•It will be able to give a precise absolute energy calibration of the gamma beam.•The determination of the resonance will be achieved using a matrix of BaF/LYSO crystals.•A novel double readout technique shows a very good background rejection power.