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For this reason, the LAT ACD was designed as a segmented detector, so thatonly that only the ACD segment intersected by the backwards projected path of the particle is used to veto the event. ThereforeIn this way, the ACD area that contributes to backsplash in a given event is relatively small. The ACD hit probability per unit area as a function of energy and distance backwards from the shower has been studied with past beam tests (Moiseev, A. A., et al. 2004, Astroparticle Physics, 22, 275) and used to optimize the level of segmentation in the ACD design.
The backsplash probability was measured with the as-built detector in the Calibration Unit Beam Test campaign in summer 2006, and the capability of the LAT Monte Carlo simulations to reproduce backsplash effect was has been verified.
A careful analysis conducted by Luis C. Reyes demonstrated that the current LAT simulations reproduce well the backsplash effect. The LAT simulations take into consideration the energy loss fluctuations in the ACD tile, the Poisson fluctuations in the number of photoelectrons created in the readout photo-multiplier, and the corrections due to the non-uniform light collection at the edge of each tile. The latter currently represents the largest source of uncertainty in the present simulation, as it has not been measured yet for the ACD tiles installed on the CULAT calibration unit. The expected backsplash distribution is therefore bracketed in the analysis by considering the maximum and minimum light collection efficiency measured for the tiles in the LAT ACD tiles (Moiseev et al, ACD paper, in preparation).
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