The Array Trigger subproject is one of the projects the UCLA team is responsible for on VERITAS. The UCLA team provides all necessary hardware and software development to control and operate the Array Trigger system.
VERITAS consists of an array of 4(7) telescopes that have the capability of functioning collectively or independently as sub arrays. To achieve this and to get the maximum sensitivity and lower energy threshold as a result, the trigger system is layered into 3 levels:
-The Level 1 trigger decision is made at the PMTs on the camera of each telescope.
-The Level 2 trigger is decided per telescope basis requiring 3 nearest neighbor PMTs fired for that event.
-The Level 3 trigger operates at the array level, thus it is called Array Trigger (L3). The L3 trigger requires coincident L2 triggers from a specific number of the telescopes in the array within a specific coincidence time window.
Utilizing this method helps to reduce the background rate, and drastically lowers the energy threshold of the overall system. In simulations, the optimum number of simultaneous L2 telescope triggers required to form the L3 trigger is found to be 3 out of 4(7) telescopes within a 40ns coincidence time window. On the figure on the right, it is clearly seen that at a threshold of 5 p.e. utilizing the array trigger as defined above provides a negligible background rate compared to a single telescope trigger rate L2.
Energy Threshold of an Atmospheric Cherenkov Detector is one of key quantities determining the performance of the detector. It is mainly limited by the single photoelectron (p.e.) rates on the pixels of the telescope due to the Night Sky Background (NSB). Thus it is crucial to study the single p.e. noise rates in the PMT's given the NSB rates at the experiment site. The UCLA team has prepared a note on determining the NSB rates for VERITAS collating what has been done before on this issue. A postscript file of the memo can be reached from here.
L3 Trigger Operation:
L3 uses the Level 2 (L2) signals from each telescope to determine when an overall array trigger has occurred. In the process the L3 trigger:
- sets the modes of triggering for various array configurations,
- receives and delays the L2 signals from each telescope,
- makes the array trigger decision,
- produces and latches an unique event number, and
- broadcasts the event number and L3 trigger signal to the telescopes in the array.