A Tunable Delay Line for Fast Analog Pulses as Key Element of a New Sum-Trigger for Cherenkov Telescopes

Abstract

In 2007 a prototype of an analog Sum-Trigger for air Cherenkov telescopes was installed in the MAGIC I telescope lowering the trigger threshold from 55 GeV to 25 GeV and led to the detection of pulsed gamma-radiation from the Crab pulsar. However, that first prototype requires intensive maintenance and manual adjustment of each trigger channel and thus strongly limits its applicability for new high-resolution cameras comprising a larger number of pixels in the trigger area. Hence, a new setup with fully automatic setting of the trigger parameters has recently been designed. To minimize system costs and complexity, the improved version utilizes a sequential rate counting procedure to determine and compensate the gain and delay shifts of all channels caused by aging and changing operation conditions of the camera's photomultipliers (PMTs). The key element of the new system is a continuously tunable, passive delay line for fast analog signals enabling precise equalization of the PMT's signal transit times. When only a few small signals form the trigger, as in low energy air showers, precise temporal overlap in the analog sum is particularly essential for efficient detection. Unlike current commercial devices with bandwidths below 200 MHz and discrete delay setting, this new delay line allows smooth variation of each channel's delay within a range of 6 ns while maintaining a bandwidth of above 300 MHz, thus preserving the system's minimal pulse width of 2.6 ns. In 2010 a fully working new Sum-Trigger test setup comprising eight pixels has been built and temporarily installed at the MAGIC I telescope. It has been shown that the new design meets or even surpasses the requirements of a new analog trigger system with an anticipated energy threshold below 25 GeV.