An ultra fast silicon pixel detector for the NA62 experiment: the Gigatracker

The beam spectrometer of the NA62 experiment has to sustain high and non-uniform beam rate (∼1.5 MHz/mm2 in the hot center and 0.8-1.0 GHz in total, hence the name Gigatracker) and should preserve beam divergence and limit beam hadronic interactions. The Gigatracker has to provide precise momentum, time and angular measurements on every single track of the secondary 75 GeV/c hadronbeam with a timing precision of 150 ps (rms). To meet these requirements, three hybrid silicon pixel detector stations will be installed in vacuum. An adequate strategy to compensate the discriminator time-walk must be implemented and R&D investigating two different options is ongoing. Two prototypes have been designed in order to have an experimental comparison of the performances: one approach is based on the use of a constant-fraction discriminator followed by an on-pixel TDC, while the other one is based on the use of a Time-over-Threshold circuit followed by a TDC shared by a group of pixels. © Copyright...