An ASIC for fast single photon counting in the LHCb RICH upgrade

The LHCb experiment will be upgraded during the second LHC long shutdown (years 2019-2020) to operate at higher luminosity. The new triggerless architecture of LHCb requires data from the entire detector to be read out at 40 MHz. The basic element of the front-end electronics of the Ring Imaging Cherenkov (RICH) detector upgrade is the "Elementary Cell" (EC), a readout system for multianode photomultiplier tubes designed to minimise parasitic capacitance at the anodes, to obtain a fast readout with low noise and low crosstalk. At the heart of the EC is the CLARO, an 8 channel, low power and radiation hard front-end ASIC designed in 0.35 mu m CMOS technology. Each channel compares the charge signals from the photomultiplier anodes with a programmable threshold, and gives a digital pulse at the output when the threshold is exceeded. Baseline recovery occurs in less than 25 ns for typical single photon signals. In the LHCb RICH upgrade environment, the chips will have to withstand radiation up to a total ionising dose of 2 kGy (200 krad) and neutron and hadron fluences up to 3 x 10(12) cm(2) and following irradiation, the chips have been shown to tolerate such doses with a margin of safety.