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Issue 2 - Studying the ACD Backsplash with beam test Data
When high energy cosmic photons cross the LAT, electro-magnetic showers will develop so that the Calorimter can measure the sources spectra up to 300GeV with good energy resolution.
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 such that the event would would cause the gamma-ray to be interpreted as background and therefore rejected.
For this reason, the LAT ACD was designed as a segmented detector and only , so thatonly the ACD segment intersected by the backwards projected path of the particle is used to veto the event. Therefore, the ACD area that can contribute contributes to backsplash in a given event is relatively small.
Nevertheless, knowledge of the 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 verified.
A careful analysis conducted by Luis C. Reyes demonstrated that the current LAT simulations reproduce well reproduce the backsplash effect, thanks to the detailed description of the . 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.
This actually The latter currently represents the largest source of uncertainty in the present simulation, as it was has not been measured yet for the ACD tiles installed on the CU, so . The expected backsplash distribution is therefore bracketed in the analysis was performed considering both fully efficient light collection and the worst case of a 70% efficiency as known from previous ACD surveysby considering the maximum and minimum light collection efficiency measured for the LAT ACD tiles (Moiseev et al, ACD paper, in preparation).
The result is shown in figures figure 1, and a beautiful reassuring agreement can be seen.
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Figure 1 - see attachment
Figure 1 explanation: Backsplash distribution for an ACD tile as obtained from beam test data (black points) and Monte Carlo expectations. In every case, backsplash is expressed as the fraction of events for which the signal in the tile is above a
given threshold. The error bars in the data are statistical 1 sigma. Monte Carlo simulations consider two extreme scenarios of light leakage collection efficiency through the tile edge. In the MAX light leakage collection inefficiency scenario, the light collection efficiency decreases linearly from 100% (3 cm away from the edge) to 70% at the tile edge. In the MIN case, the light collection decreases linearly from 100% (1 cm away from the edge) to 90% at the tile edge. Both scenarios are shown in the backsplash distribution as bands that bracket the expected backsplash distribution. The width of each band is given by twice the statistical error 2 _sigma_obtained from the simulation.