<title>FPGA based triggers in the Pierre Auger Observatory</title>

Abstract

The Pierre Auger Observatory is an international project for research on ultra-high energy cosmic rays (UHECR) above 10¹⁹ eV and currently the world largest "hybrid" experiment. The hybrid technique uses both the detection of particles at the ground in water Cherenkov detectors (WCD) and measurements of fluorescence light generated in the atmosphere by the extensive air showers. This technique offers unique possibilities for cross-calibration and reduction of measurement uncertainties. The surface array will contain 1600 surface detector stations distributed over 3000 km² in the Southern Hemisphere (under construction in Argentina) and 4000 WCD distributed over 10000 km² at a Northern Hemisphere site in Colorado (deployment planned for 2008). Four fluorescence stations each containing 6 wide-angle Schmidt telescopes and each equipped with 440 PMTs with 1.5° angle resolution and 30 • 30° of field of view are located on the border of the surface array in the Southern Site. The paper presents implementations of the FPGA used for triggering. The use is illustrated with currently working schemes as well as other possible ideas especially promising for the surface detector in which currently used triggers seems to be not effective enough for very inclined and horizontal showers. Proposed new triggers enlarge a simple 1-bit threshold detection to the online analysis of a FADC trace shape. This corresponds to a charge from PMTs or signal power estimation. The next category is spectral triggers analyzing sequential samples in the frequency domain. Digital sigma-delta filters immunize the trigger circuitry on the pedestal variation as results of huge temperature swings. Presented DCT trigger provides native pedestal independence.