  Samples of the monolithic silicon pixel ASIC prototype produced in 2022
within the framework of the Horizon 2020 MONOLITH ERC Advanced project were
irradiated with 70 MeV protons up to a fluence of 1 x 1016 neq/cm2, and then
tested using a beam of 120 GeV/c pions. The ASIC contains a matrix of 100 \mu m
pitch hexagonal pixels, readout out by low noise and very fast frontend
electronics produced in a 130 nm SiGe BiCMOS technology process. The dependence
on the proton fluence of the efficiency and the time resolution of this
prototype was measured with the frontend electronics operated at a power
density between 0.13 and 0.9 W/cm2. The testbeam data show that the detection
efficiency of 99.96% measured at sensor bias voltage of 200 V before
irradiation becomes 96.2% after a fluence of 1 x 1016 neq/cm2. An increase of
the sensor bias voltage to 300 V provides an efficiency to 99.7% at that proton
fluence. The timing resolution of 20 ps measured before irradiation rises for a
proton fluence of 1 x 1016 neq/cm2 to 53 and 45 ps at HV = 200 and 300 V,
respectively.
