The NA62 GigaTracker (GTK) hybrid pixel detector comprises three stations
situated early in the decay line, designed to measure the momentum, angle and
traversal time of the incident particles entering the decay line from the target.
The requirements on these parameters are driven by the demanding background
rejection level necessary for the rare kaon decay measurement. In total, the beam
rate incident on the GigaTracker is expected to be around 800 M Hz − 1 GHz,
which equates to approximately 140 kHz per pixel in the centre, where the
intensity is the highest. The total dead time of the detector is expected to be
less than 1%.
Each detector instruments an area of 60 mm x 27 mm and consists of an
array of 18000 pixels, each one nominally 300 x 300 µm^2 . Time stamping is
required to the level of 150 ps (RMS) for the GigaTracker as a whole. The front
end must accommodate a range of input charges from 5000 e− to 60000 e− .
A demonstrator ASIC has been designed and fabricated in 130 nm CMOS
technology. The architecture of the implemented design employs a pixel array
operating asynchronously with a single-threshold discriminator output, driving
an End Of Column (EOC) Delay Locked Loop (DLL) based Time-to-Digital
Converter (TDC) via calibrated transmission lines. This proof of concept ASIC
has a full column of 45 pixels alongside test structures to validate and characterise individual components of the design. The time bin of the TDC is
nominally 97 ps, achieved by feeding a low jitter 320 MHz clock into a delay line
with 32 contributing delay cells. Time-walk compensation of the discriminated
detector signal is foreseen using a Time-Over-Threshold (TOT) approach, where timestamps from the leading and trailing edges of the discriminated pixel out-
put contribute to an off-detector look-up of the corrected time. In-depth testing
of this ASIC is underway and the performance, as measured so far, is consistent
with the design specifications. Detailed results on the performance of both the
analogue front end and the TDC will be presented.