CAESAR—A high-efficiency CsI(Na) scintillator array for in-beam γ‐rayγ‐ray spectroscopy with fast rare-isotope beams

We report on the construction and commissioning of the high-efficiency CAESium-iodide scintillator ARray CAESAR, a device designed for in-beam γ‐rayγ‐ray spectroscopy experiments utilizing fast beams of rare isotopes at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). CAESAR consists of 192 CsI(Na) crystals, totaling 290 kg of active scintillator material. For 1 MeV γγ rays, a full-energy-peak efficiency of 35% is achieved at an in-beam energy resolution of better than 10% FWHM after event-by-event Doppler reconstruction of the γγ rays emitted by nuclei moving with velocities of v/c∼0.3–0.4v/c∼0.3–0.4. The spectral quality of the array allows for the identification of γ‐rayγ‐ray transitions with intensities of several 10 counts in the full-energy peak and thus opens new avenues for the study of the most exotic nuclei available at the NSCL for in-beam spectroscopy.