Optimization of the detection of very inclined showers using a spectral DCT trigger in arrays of surface detectors

The DCT trigger allows recognition of ADC traces with a very short rise time and fast exponential attenuation related to a narrow, flat muon component of very inclined extensive air showers generated by hadrons and starting their development early in the atmosphere. Showers, crossing a surface detector composed of water Cherenkow tanks, such as in the Pierre Auger Observatory, generate Cherenkov light, which may hit PMTs directly or after several reflections. The geometry of the water Cherenkov tank composes a type of the filter prefering a detection of very inclined showers (with very large zenith angle), where mainly two PMTs can be hit by direct light. The 3rd PMT is next hit by reflected light, but with some delay. By fast sampling (80 MHz) this delay gives signal in the next time bin. Two-fold coincidences of DCT coefficients allow triggering signals currently being ignored due to either too high amplitude threshold or due to their de-synchronization in time causing a tank geometry. Three DCT engines implemented into EP3C40F324I7 FPGA used all DSP blocks generate the spectral trigger, when in at least 2 channels 8 DCT coefficients simultaneously are inside the acceptance lane. Additional veto signal (analyzing the amplitude) controls a trigger rate to avoid a saturation of a transmission channel. Both lab and long-term field measurements on the test tank confirm a high efficiency of the recognition of expected patterns of ADC traces.