1. The VERITAS Trigger System A. Weinstein 1 for the VERITAS Collaboration 2 CFD x 499 (1 per pixel ) Pattern Trigger Shower Delay ( 1 PDM channel per trigger signal) Array Trigger Coincidence Logic (SAT Board) FIFO Buffer Compensating Delay ( 1 PDM channel per trigger signal) Event Decision Event Information Harvester Process Serialized Event Information L3 Trigger FADC Modules (1 channel per pixel) (x4 : 1 per telescope) Readout instructions Assembled telescope data L3 Trigger signal 64μs circular buffer Telescope Data Acquisition System L1 L2 L3 BUSY level Event Logic Inhibitor Abstract: The VERITAS gamma-ray observatory, situated in southern Arizona, is an array of four 12m-diameter imaging Cherenkov telescopes, each with a 499-pixel photomultiplier-tube camera. The instrument is designed to detect astrophysical gamma rays at energies above 100 GeV. At the low end of the VERITAS energy range, fluctuations in the night-sky background light and single muons from cosmic-ray showers constitute significant backgrounds. VERITAS employs a three-level trigger system to reduce the rate of these background events. The Level Two (L2, Pattern) Trigger system acts on the relative timing and distribution of L1 triggers within a given telescope camera, preferentially selecting compact Cherenkov light images and reducing the rate of triggers due to NSB fluctuations. It divides the telescope camera into overlapping patches of 19 pixels, and fires if a programmed number of L1 triggers within a patch overlap. The standard pixel coincidence requirement is three adjacent pixels within a patch; the required overlap time between adjacent CFD signals is ~6 ns. Each L2 system (four in all) produces a single logical signal, the L2 trigger, which is sent to the array trigger system (L3). Figure 1: A block diagram of the VERITAS Trigger System operation, including its interface with the data acquisition systems. The Level One (L1) Trigger system acts at the single pixel level. It consists of custom-built Constant Fraction Discriminators (CFDs), one for each photomultplier-tube (PMT) pixel in a telescope camera. The CFD triggers (produces an output pulse) if the sum of the voltages from the original PMT pulse and a time- delayed copy crosses a threshold. The VERITAS CFDs are equipped with a rate feed-back (RFB) loop, which automatically increases the effective threshold when the noise level (and thus CFD trigger rate) rises. The full VERITAS array of four telescopes was completed in Spring 2007. Most of the preliminary studies characterizing trigger performance, however, were done during the commissioning process, with a three- telescope subset of the array (T1-T3). Figure 2 illustrates the effectiveness of the array trigger in suppressing events due to night-sky background, well below the operating threshold where a single telescope trigger would be dominated by NSB. The 50 mV operating threshold for the L1 trigger (which corresponds to 4-5 photoelectrons) was conservatively chosen to ensure both stable operation and reasonable dead time performance for a wide range of weather conditions and array configurations. Figure 3 shows the array trigger rate as a function of coincidence window width. The rate is stable for window sizes between 25-100ns, which is consistent with the observed spread in L2 trigger arrival times after shower delays have been applied. The dead time of the array is determined, to first order, by the array trigger rate and the average telescope readout time (~400μs). The observed correlation of array trigger rate and fractional array dead time is shown in Figure 4. The array dead time ranges from ~6-7% for array trigger rates around 150Hz and 10-11% in the vicinity of 230Hz. The Level Three (L3, Array) Trigger system requires simultaneous observation of an air-shower event in multiple telescopes. This requirement significantly reduces the rate of background events, particularly those due to single muons. L3 uses the relative timing of the L2 trigger signals to identify a shower event. First, the system uses programmable “shower” delays to compensate for the differences in the arrival times of the Cherenkov light front at the different telescopes, as well as the differences in L2 signal propagation times. The core coincidence logic continually monitors the delayed L2 signals for a pattern that lies within a coincidence window. The width of the coincidence window compensates for the residual variation in L2 signal arrival times due to the width and curvature of the Cherenkov wavefront, variation in L2 response with respect to image size, and timing jitter in the various electronics components. Both the allowed patterns of L2 signals (typically implemented as a simple multiplicity requirement) and the coincidence window width are programmable. Trigger System Performance: Figure 5: L2 and L3 rates for a typical three-telescope run (right) and a run taken on the same night, under partial moonlight (left). It is clear the L3 rate is stable with respect to significant fluctuations in the pattern trigger rates. This, and the fact that the array dead-time is not influenced by L2 trigger rates, allows for stable running under a variety of conditions, including partial moonlight. Figure 2: The dependence of L2 and L3 trigger rates on L1 (CFD) threshold and array trigger multiplicity, for a three-telescope array with a 50ns coincidence window. This data was taken while pointing at a dark patch near zenith, under moderate weather conditions. Figure 3: Array trigger rate (for a three-telescope array with 2/3 multiplicity requirement) as a func- tion of coincidence window width. Figure 4: Fractional array dead time vs. array trigger rate for a representative sample of good runs. Custom-built 500MS/s flash-ADC (FADC) modules (one FADC channel per pixel) continuously digitize the PMT pulses with a memory buffer depth of 64μs. The telescope data acquisition systems read out a portion of this buffer (24 samples) for every channel as directed by the array trigger system. Compensating delays applied to the L3 trigger signals ensure that every L3 trigger signal is received at the telescopes at a fixed time relative to the start of the PMT pulse, allowing the data acquisition system to “look back” to the appropriate starting point in the buffer before reading out. Each data acquisition system also raises an ECL level (BUSY) while occupied with buffer readout; the L3 system inhibits the coincidence logic as long as one of these levels is raised. (x4 : 1 per telescope) 1.University of California Los Angeles, Los Angeles, CA USA amandaw@astro.ucla.eduamandaw@astro.ucla.edu 2. See G. Maier, “VERITAS: Status and Latest Results”, for a complete listing of the VERITAS collaboration and ICRC contributions. Single-tel. operating threshold (6-7 photoelectrons) (Standard operation) This research is supported by grants from the U.S. Department of Energy, the U.S. National Science Foundation, and the Smithsonian Institution, by NSERC in Canada, by PPARC in the UK, and by Science Foundation Ireland.