Results from the NA62 gigatracker prototype: a lowmass and sub-ns time resolution silicon pixel detector

Abstract

The Gigatracker (GTK) is the upstream detector developed for NA62, the experiment that will study the rare decay of a kaon into a pion and neutrino-antineutrino pair at the CERN SuperProton-Synchrotron (SPS) accelerator. The GTK, besides performing the momentum and angular measurements of the incoming particles, will provide a time information with a precision of 100 ps (rms) in order to obtain a tight time coincidence between the kaon and the pion tracks [1]. The required time resolution can be achieved by compensating for the discriminator time-walk. For this purpose two complementary architectures have been explored. The complete separation between the analog and the digital sections is the key feature of the first solution. The discriminator output of the simple pixel cell is sent to Time-to-Digital-Converters (TDC) shared by a group of pixels in the end of column [2]. In the second option the signal processing inside the pixel is maximized, implementing a Constant-Fraction-Discriminator (CFD) followed by a TDC in each cell [3]. The two architectures have been designed and produced as small-size prototypes in 130 nm CMOS technology. The results obtained from laboratory and beam tests of the ASICs bump-bonded to 200 µm thick silicon sensors are described in this paper.