A Compton camera prototype for a position-sensitive detection of prompt γ rays from proton-induced nuclear reactions is being developed in Garching. The detector system allows to track the Comptonscattered electrons. The camera consists of a monolithic LaBr 3 :Ce scintillation absorber crystal, read out by a multi-anode PMT, preceded by a stacked array of 6 double-sided silicon strip detectors acting as scatterers. The LaBr 3 :Ce crystal has been characterized with radioactive sources. Online commissioning measurements were performed with a pulsed deuteron beam at the Garching Tandem accelerator and with a clinical proton beam at the OncoRay facility in Dresden. The determination of the interaction point of the photons in the monolithic crystal was investigated. 201, 05005 (2016) EPJ Web of Conferences

Purpose:Development of a photon detection system designed for online range verification of laser‐accelerated proton beams via prompt‐gamma imaging of nuclear reactions.Methods:We develop a Compton camera for the position‐sensitive detection of prompt photons emitted from nuclear reactions between the proton beam and biological samples. The detector is designed to be capable to reconstruct the photon source origin not only from the Compton scattering kinematics of the primary photon, but also to allow for tracking of the Compton‐scattered electrons.Results:Simulation studies resulted in the design of the Compton camera based on a LaBr3(Ce) scintillation crystal acting as absorber, preceded by a stacked array of 6 double‐sided silicon strip detectors as scatterers. From the design simulations, an angular resolution of ≤ 2° and an image reconstruction efficiency of 10−3 −10−5 (at 2–6 MeV) can be expected. The LaBr3 crystal has been characterized with calibration sources, resulting in a time resolution of 273 ps (FWHM) and an energy resolution of about 3.8% (FWHM). Using a collimated (1 mm diameter) 137Cs calibration source, the light distribution was measured for each of 64 pixels (6×6 mm2). Data were also taken with 0.5 mm collimation and 0.5 mm step size to generate a reference library of light distributions that allows for reconstructing the interaction position of the initial photon using a k‐nearest neighbor (k‐NN) algorithm developed by the Delft group.Conclusion:The Compton‐camera approach for prompt‐gamma detection offers promising perspectives for ion beam range verification. A Compton camera prototype is presently being developed and characterized in Garching. Furthermore, an arrangement of, e.g., 4 camera modules could even be used in a ‘gamma‐PET’ mode to detect delayed annihilation radiation from positron emitters in the irradiation interrupts (with improved performance in the presence of an additional third (prompt) photon (as in 10C and 14O).This work was supported by the DFG Cluster of Excellence MAP (Munich‐Centre for Advanced Photonics)

The properties of a 50 mm × 50 mm × 30 mm monolithic LaBr3:Ce scintillator crystal coupled to a position-sensitive multi-anode photomultiplier (PMT, Hamamatsu H9500), representing the absorbing detector of a Compton camera under study for online ion (proton) beam range verification in hadron therapy, was evaluated in combination with either absorptive or reflective crystal surface coating. This study covered an assessment of the energy and position-dependent energy resolution, exhibiting a factor of 2.5–3.5 improvement for the reflectively wrapped crystal at 662 keV. The spatial dependency was investigated using a collimated 137Cs source, showing a steep degradation of the energy resolution at the edges and corners of the absorptively wrapped crystal. Furthermore, the time resolution was determined to be 273 ps (FWHM) and 536 ps (FWHM) with reflective and absorptive coating, respectively, using a 60Co source. In contrast, the light spread function (LSF) of the light amplitude distribution on the PMT segments improved for the absorptively wrapped detector. Both wrapping modalities showed almost no differences in the energy-dependent photopeak detection efficiency.