Scintillator Fibre Tracker, SciFi
The LHCb experiment has planned a major upgrade of its detector for the Long Shutdown 2 of the LHC. This upgrade plans to increase the total integrated luminosity from the current 3fb-1 after 2012 to 50fb-1 over 10 years, thus reducing the experimental uncertainties in the current measurements. The two main changes in the operation conditions of the detector will be the replacement of the current 1 MHz hardware trigger by a 40 MHz software trigger at the front-end electronics level and the increase of the instantaneous luminosity to 2 x 1033 cm-2s-1. The new luminosity has a significant impact on the tracking detectors downstream from the magnet, increasing the detector occupancy and ionising radiation near the beam pipe over the acceptable levels of the current system. As a replacement, the Scintillating Fibre (SciFi) Tracker is being developed.
The SciFi Tracker will use plastic scintillating fibres with a diameter of 250 μm as the active material, covering an area of 5m x 6m. These fibers also transport the optical signal to the pixelated silicon photomultipliers (SiPM) used as photodetectors. The 128-channel SiPM arrays, with a pitch of 250 μm , are a custom design that is being developed in colaboration with Hamamatsu and Ketek. The SciFi Tracker is composed of three stations, each comprised of four planes, amounting to a total 580k channels.
The readout of the detector is performed by the low-Power ASIC for the sCIntillating FIbres traCker (PACIFIC), a chip specifically designed for this purpose. PACIFIC is a 64-channel ASIC implemented in 130 nm CMOS technology, aiming at a radiation tolerant design with a low power consumption. Its current mode input allows a direct connection to the SiPM anode, allowing also a fine adjustment of the gain of these photodetectors. PACIFIC relies on a double pole-zero cancellation shaper and a dual gated integrator to assure a zero dead-time operation with a minimal spillover. Finally, PACIFIC performs a 2bit non-linear digitization, based on three configurable thresholds that define the starting point of the clusterization algorithm that provides the hit positions.
The development of PACIFIC began in 2012, following a series of increasingly complex prototype phases that reached a fully operational version on the third release. Throughout all these phases, the LHCb Group from IFIC (Valencia) has actively collaborated with the groups from the Laboratoire de Physique Corpusculaire (Clermont-Ferrand, France), the Physikalisches Institut (Heidelberg, Germany) and the Instituto de Ciencias del Cosmos (Barcelona) in this task. In terms of design, both analog and digital circuits have been provided to the collaboration, such as the slow control comunication salve and registers or the slow shaper used in an alternative solution studied in the first version of the ASIC. Regarding the characterization and test of these prototypes, the group has participated of the development of the software and firmware of the test setup, putting together an instance of this setup at the institute to provide independent measurements, as well as taking part in the testbeam campaign that have been faced.
