Summary 500 words
The NA62 GigaTracker (GTK) hybrid pixel detector comprises three stations situated early in the beam line, designed to measure the momentum, angle and traversal time of the beam particles. 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 MHz − 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 required to be less than 1%. The radiation environment is expected to be extremely harsh, with the total dose per year approximately 10^14 1 M eV n eq.: similar to that expected in 10 years of operation for a central tracking detector in an LHC experiment.
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 µm2 . Time stamping is required to the level of 150 ps (RMS) for the GigaTracker as a whole and better than 200 ps (RMS) per station. A demonstrator readout ASIC and detector were designed and fabricated and have undergone extensive combined and independent electrical, laser and beam testing. Architecturally, this design is based on an asynchronously operating pixel matrix with a single threshold discriminator per pixel. Hit time stamping is done in the end of column region by a delay locked loop based time-to-digital converter. A time over threshold measurement, coupled with an off-detector look-up is used to eﬀect the discriminator time walk correction. Discrepancies between the time resolution measured with particles during the beam test and that measured in the lab with the laser system were signiﬁcant and warranted investigation. A detailed study of the behaviour of the sensor and readout electronics was subsequently undertaken to identify and characterise these additional contributions to the timing resolution seen with particles. The demostrator system exhibits a time-stamping resolution of better than 200 ps (RMS) per station for single particles.
A significant effort is underway at present to design the readout ASIC required for the ﬁnal system, now dubbed TDCPix. This chip will instrument a tenth of the total area per station with 40 columns of 45 pixels. A description of this ongoing work will be included.