Spatial time resolution of MCP-PMTs as a t0-reference

Abstract

Abstract A hyper-fast time reference is needed for characterisation measurements of fast-timing detector prototype. The time resolution of the reference detector, normally placed together with a device under test (DUT) in a beam telescope, should be considerably better than the time resolution of the DUT. Measurements with MCP-PMTs of the type R3809U-50 by Hamamatsu have been performed in order to characterise their spatial distribution of the time resolution and to validate them as a proper reference detector. These measurements were conducted during the tests of PICOSEC fast-timing Micromegas prototypes at the CERN/SPS-H4 beam line using a beam telescope consisting of three triple-GEM tracker and various trigger detectors. A time resolution of up to 3.8 ps has been obtained in the inner part of the active area. In the outer parts, the time resolution declines up to several ten picoseconds due to decreasing pulse amplitudes. In this region, the Cherenkov light cone is not fully projected onto the photocathode anymore. Finally, only diffuse reflected light is reaching the photocathode in the outermost parts. The pulse amplitude correlates with the number of generated photo electrons and therefore with the amount of light reaching the photocathode. This explains the strong spatial differences of the MCP-PMTs time resolution.