An experiment was carried out in the Physical and Chemical Research (RIKEN), Japan, in order to study the nuclear structure of (_^66)Se, a key nucleus in the astrophysical rp-process. The excited states were populated in the neutron knock-out process, and the emitted gamma-rays were detected by 100 CsI(Na) scintillator crystals. My task was to calibrate these detectors for energy by analyzing the spectra recorded with radioactive sources. This also required to learn the usage of the ROOT framework capable of handling big data.