  Hundreds of concurrent collisions per bunch crossing are expected at future
hadron colliders. Precision timing calorimetry has been advocated as a way to
mitigate the pileup effects and, thanks to their excellent time resolution,
microchannel plates (MCPs) are good candidate detectors for this goal. We
report on the response of MCPs, used as secondary emission detectors, to single
relativistic particles and to electromagnetic showers. Several prototypes, with
different geometries and characteristics, were exposed to particle beams at the
INFN-LNF Beam Test Facility and at CERN. Their time resolution and efficiency
are measured for single particles and as a function of the multiplicity of
particles. Efficiencies between 50% and 90% to single relativistic particles
are reached, and up to 100% in presence of a large number of particles. Time
resolutions between 20ps and 30ps are obtained.
