Abstract.A selection of the properties of large volume, cylindrical 3.5" x 8" LaBr 3 :Ce scintillation detectors coupled to a 3.5" PMT (model R10233-1000SEL from HAMAMATSU) and a special designed Voltage Divider (LABRVD) will be discussed. A number of 10 of such detectors constitute the HECTOR + array which, in fall 2012, measured at GSI coupled to the AGATA DEMOSTRATOR at the PRESPEC experimental setup. These crystals are among the largest ever produced and needed to be characterized. We have performed several tests and here we discuss, in particular, the energy resolution measured using monochromatic γ−ray sources and in-beam reactions producing γ−rays up to 22.6 MeV. As already measured in two previous works a saturation in the energy resolution was observed in case of high energy gamma rays. Crystal non-homogeneities and PMT gain drifts can affect the resolution of measurements especially in case of high energy γ−rays.
A Compton camera prototype for ion beam range monitoring via prompt (< 1 ns) gamma detection in hadron therapy is being developed and characterized at the Medical Physics Department of LMU Munich. Co photon sources were used for 2D irradiation scans (step size 0.5 mm) as prerequisite for studying the performance of the "k-Nearest-Neighbors" algorithm developed at TU Delft [2] (together with its variant "Categorical Average Pattern", CAP) and extending its applicability into the energy range beyond the original 511 keV. In this paper we present our most recent interaction position analysis in the absorbing scintillator, leading to a considerably improved value for the spatial resolution: systematic studies were performed as a function of the k-NN parameters and the PMT segmentation. A trend of improving spatial resolution with increasing photon energy was confirmed, resulting in the realization of the presently optimum spatial resolution of 2.9(1) mm @1.3 MeV, thus reaching the design specifications of the Compton camera absorber. The specification goal was reached also for a reduced PMT segmentation of 8x8 anode segments (each with 6x6 mm 2 active area), thus allowing to reduce the complexity of the signal processing while preserving the performance.
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