Noise performance of the ALPIDE-based ALICE Inner Tracking System

The new ALICE Inner Tracking System is the first large-scale MAPS-based tracking system. It is covering an active surface of 10 m$^{2}$, with a total of 12.5 billion pixels. Several optimisations of the pixel chip lead to a quasi-noise-free operation, with measured fake-hit rates of below 1 hit per pixel and billion events (system level numbers), numbers compatible with the order of magnitude expected from cosmic and natural background radiation.This contribution covers a detailed study of fake hits as recorded in the inner-most detector barrels, made of the highest quality chips. It reveals a localised noise pattern that could be traced down to originate from decoupling capacitors present on the detector module. It can most likely be explained by the radioactive decay of $^{210}$Pb, which is present in trace amounts in the solder that was used to mount decoupling capacitors on the detector modules. This hypothesis is substantiated with a dedicated simulation study and laboratory measurements.