The structure of unstable nuclei is studied at the international facility FAIR, Darmstadt,
Germany. One experimental setup at FAIR is called R3B where radioactive beams at relativistic energies impinge on a specific target which allows to collect data on the reactions taking place. For these experiments, different detectors have been built and CEPA is one of them. CEPA is the detector that will be characterized in this thesis. This detector consists of 24 sectors, where each sector has four tightly packed scintillator detectors, each a combination of LaBr3 and LaCl3, making up a phoswich crystal unit. Each phoswich crystal unit is made out of 7 cm LaBr3 and 8 cm LaCl3, respectively. Previous CEPA prototypes have been characterized at Chalmers, but the latest CEPA crystals have a new geometrical shape, the shape of a frustum. The three characteristics of CEPA that are investigated are their energy calibration, their energy resolution and the dependence of the detected energy on the position of the interaction. It was found that the energy resolution for the four LaBr3 parts of the tested sector did not meet the requirements [5]. Crystal one was the closest to meet the requirements, but still did not met the requirements with a factor 1.73 times higher (resolution) compared with the prototype, the other crystals were approximately a factor 2.5 higher. On the LaCl3 part none of the crystals met the requirements. The calibration measurements were also not successful since the characterized peak positions for different -sources did not end up on the expected place for all the sources. Unfortunately the sector that was investigated exhibited a significant position dependence
