Characterization and Simulation of an Adaptable Fan-Beam Collimator for Fast Calibration of Radiation Detectors for PET

Monolithic scintillators for positron emission tomography systems perform best when calibrated individually. We present a fan-beam collimator with which a crystal can be calibrated within less than 1 h when suitable positioning algorithms are applied. The collimator is manufactured from lead, features an easily adaptable slit to tune the beam width and can be operated together with a coincidence detector to select a clean sample of 511-keV annihilation photons. We evaluated the performance of the collimator with a Geant4 simulation for slit widths of 0.25 mm, 0.4 mm, and 1mm and validated the shape of the beam profile experimentally by step-wisely moving a detector into the beam. This shows a clear narrow and box-shaped beam profile even if the collimator is operated without the coincidence set-up. In the latter configuration, the fraction of gammas in the beam region on a $50\times 50$ mm
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large detector is between 48% and 79% which is improved significantly to more than 94% by using only coincidence events. Analyzing the energy distribution shows that the fraction of 511-keV photons is increased from less than 50% to more than 96% by selecting coincidences. This demonstrates that our collimator produces a very defined and clean beam and provides optimal conditions for calibration.