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The capability of the LAT to measure photon energies up to 300 GeV with good energy resolution
requires the presence of a heavy calorimeter to absorb enough of the electromagnetic cascade
produced by the incident gamma-ray. Unfortunately, a small fraction of secondary particles in the shower can travel backwards from the calorimeter into the tracker and up to the external Anti-Coincidence Detector.
This backsplash radiation consist mostly of 100-1000 keV photons and represent a potential problem since it can generate a signal in the ACD that would cause the gamma-ray to be interpreted as background and therefore rejected.
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