A Characterization System for the Monitoring of ELI-NP Gamma Beam

The ELI-NP (Extreme Light Infrastructure-Nuclear Physics) facility, currently under construction near Bucharest (Romania), is the pillar of the project ELI dedicated to the generation of high-brilliance gamma beams and high-power laser pulses that will be used for frontier research in nuclear physics. To develop an experimental program at the frontiers of the present-day knowledge, two pieces of equipment will be deployed at ELI-NP: a high power laser system consisting of two 10 PW lasers and a high brilliance gamma beam system. The ELI-NP Gamma beam system will deliver an intense gamma beam with unprecedented specifications in terms of photon flux, brilliance and energy bandwidth in an energy range from 0.2 to 20 MeV. Such a gamma beam requires special devices and techniques to measure and monitor the beam parameters during the commissioning and the operational phase. To accomplish this task, the Gamma Beam Characterization System, equipped with four elements, was developed: a Compton spectrometer (CSPEC), to measure and monitor the photon energy spectrum; a nuclear resonant scattering system (NRSS), for absolute beam energy calibration and inter-calibration of the other detectors; a beam profile imager (GPI) to be used for alignment and diagnostics purposes; and finally a sampling calorimeter (GCAL), for a fast combined measurement of the beam average energy and intensity. The combination of the measurements performed by GCAL and CSPEC allows fully characterizing the gamma beam energy distribution and intensity with a precision at the level of few per mill, enough to demonstrate the fulfillment of the required parameters. This article presents an overview of the gamma beam characterization system with focus on these two detectors, which were designed, assembled and are currently under test at INFN-Firenze. The layout and the working principle of the four devices is described, as well as some of the main results of detector tests.