Performance of Si pad sensors for a RICH detector

Silicon pad sensors with 256 cells have been used to detect single photoelectrons from a CsI photocathode. These sensors are part of a Hybrid Photon Detector (HPD) to be used in a RICH detector in the LHCb experiment. The I-V and C-V measurements are presented. The pad sensors are read out with the low noise VA2 chip. Results from tests with these detectors with a $^{90}$Sr source are reported.Measurements of photoelectrons created by 160~nm wavelength photons in a CsI cathode in a test set-up are also reported. Finally, the prospects of using such sensors with fast LHC readout electronics are discussed.