LAT Newsletter contributions
Issue 1
See here
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.
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 be interpreted as background and therefore rejected.
For this reason, the LAT ACD was designed as a segmented detector and only 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 to backsplash is relatively small.
Nevertheless, knowledge of 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 well reproduce the backsplash effect, thanks to the detailed description of 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 represents the largest source of uncertainty in the present simulation, as it was not measured for the ACD tiles installed on the CU, so the analysis was performed considering both fully efficient light collection and the worst case of a 70% efficiency as known from previous ACD surveys.
The result is shown in figures 1, and a beautiful reassuring agreement can be seen.
Details of this analysis will be shown at the collaboration meeting at GSFC next week, in a dedicated plenary session from the Beam Test Team where the state of the art of Data-MonteCarlo comparison will be presented to the collaboration.
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 through the tile edge. In the MAX light leakage 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.