The new Fast Interaction Trigger (FIT) detector has been designed to serve as the main luminometer, collision time, multiplicity, centrality, and reaction plane detector for the upgraded ALICE experiment during Run 3 and 4 at LHC, CERN. FIT will consist of a large scintillator ring and 52 Cherenkov modules combined in two arrays. Each module is based on Cherenkov radiators with four-fold segmentation, optically coupled to a photosensor. Traditional vacuum or solid-state photosensors were not able to fulfil the stringent requirements imposed by the limited space, 0.5 T magnetic field, hadron fluence in excess of 3x10^11 1-MeV-neq per cm2 and expected performance during more than 6 years of service life.
The sensor chosen for FIT is Planacon XP85012/A1-Q microchannel plate-based PMT, which was modified by our group and transformed to the XP85002/FIT-Q-version. With the modified Planacons we were able to achieve time resolution below 20 ps, eliminate the crosstalk problem, and reduce the total length of the sensor. This presentation gives an overview of the extensive in-beam measurements and the outcome of dedicated ageing and magnetic field tests, as well as the description of the implemented PMT modifications