Evaluation of an imaging phototube using microchannel plates with delay line readout

An imaging phototube using a bialkali photocathode, 40-mm microchannel plates (MCPs), and a crossed delay line readout has been constructed. The photocathode is found to exhibit good sensitivity, although overall detection efficiency is lowered by the MCP ion-film barrier. The MCP gain characteristics are evaluated over the sensitive area, and the reduction of gain at high count rate is measured. Several overall detector characteristics, including dark count, spatial resolution, and geometric linearity, are described. It is shown that the chevron pair of MCPs provide a highly peaked pulse height distribution at a modal gain sufficiently low to allow local count rates of up to ~5500 s^-1mm^-2, yet sufficiently high to produce a spatial resolution, of ⩽80 μm full width at half maximum (FWHM), making them suitable for use in a photon counting detector for X-ray crystallography. A factor of 1.7 variation in the modal gain points to the need for uniformity of MCP burn-in during tube construction