The field of nuclear physics is generally concerned with the study of the structure and dynamics of nuclei. ELI-NP will be able to deliver photons with energies of a few keV up to 19.5 MeV for this kind of fundamental research. The nuclei to be studied will be excited by the photons and the subsequent decay observed in the form of gamma rays in germanium detectors, which offer the highest energy resolution up to date. During the interaction of gamma radiation with matter effects occur in the germanium detectors, which increase the background in a spectrum. These can be reduced by using a combination of scintillation crystals with semiconductor diodes around the detector, as a so-called anti-Compton shield (ACS).
For gamma quanta with high energies the pair production is the dominant interaction process in matter. In the case of germanium this energy lies at 8 MeV. The produced positrons escape from the material or annihilate with the electrons from the surrounding and two 511 keV gamma quanta are generated and emitted in opposite directions. This can lead generally to the case that one of those quanta, as well as both, can escape out of the detector and thus do not deposit the full energy of the primary gamma quantum. Thereby single-escape and double-escape peaks arise that make the recorded spectrum more complex. Another source of background is Compton scattering. Here, the incident photon is scattered out of the detector without having deposited all of its energy in it. To prevent this impurities, as well as to reduce the Compton background, often an ACS out of an efficient material (mostly BGO) to stop gamma quanta is built around a detector. Coincident events from HPGe detector and ACS will be neglected whereby the spectrum becomes cleaner.
The present work is about the development, the assembly and the characterization of a HPGe-BGO pair spectrometer, also called ELIPS (ELI-NP Pair Spectrometer), on the gamma beam of ELI-NP in Bucharest-Magurele, Romania. The ELI-NP is an unique European research facility, which should in future provide among other things brilliant gamma radiation with high intensity up to an energy of 19.5 MeV. ELIADE is there an important instrument for the gamma-spectroscopic study of photonuclear reactions and consists of 8 HPGe-clover detectors. With the help of the pair spectrometer the efficiency of one of those detectors at high energies should be increased and hence the high-resolution gamma spectroscopy extended. The use of 64 BGO crystals makes the pair spectrometer an improved version of an ACS as it allows a more accurate determination for the position of the gamma quanta. Coincident events from, for example, two 511 keV gamma rays in two opposing BGO crystals and one double-escape energy in the HPGe detector can be considered as one coherent process and thus the full energy of the primary gamma ray can be reconstructed, but at the expense of statistics. However, the higher granularity also requires additionally smaller readout electronics to ensure the mechanical compatibility with ELIADE, for which reason MPPC arrays are used for this purpose.
ELIPS can also be used as a stand-alone device. By inserting of a target instead of a HPGe detector into the spectrometer, it can be used as an intensity monitor for a gamma beam or for instance for the investigation of the pair-production cross section close to the threshold.
The first experiments with the completed pair spectrometer were performed for testing purposes at the TU Darmstadt, at the institute for nuclear physics in Cologne, as well as at the institute Laue-Langevin in Grenoble. The results of these tests are presented in this work
