We present the concept of a single-photon imager capable of detecting up to 10$^9$ photons per second with simultaneous measurement of position ($<$10$\mu$m resolution) and time (few tens of picosecond resolution) for each individual photon over an active area of 7 cm$^2$. The detector is based on a “hybrid” concept: a vacuum tube, with a transparent input window on which a suitable photocathode material is deposited, a micro-channel plate (MCP) and a pixelated read-out anode based on the Timepix4 ASIC (65nm CMOS technology) designed in the frame of the Medipix4 collaboration. A MCP with $<$10 $\mu$m pore diameter will be used, operated at low gain (a few 10$^4$) and treated with atomic layer deposition, allowing a lifetime increase to >10 C/cm$^2$ accumulated charge. This detector will allow to fully exploit all the excellent intrinsic characteristics of a MCP, using a front-end electronics ASIC encapsulated in the tube with unprecedented performance. Timepix4 is an array of 512x448 pixels, 55$\mu$mx55$\mu$m each, with an active area of 28mmx25mm. It features 50-70 e- equivalent noise charge, a maximum rate of 1.2 Ghits/s, and allows to time-stamp leading-edge time and measure Time-over-Threshold (ToT) for each individual pixel. A weighted average of the cluster pixels position can be calculated using their ToT information, which allows to reach 5-10 μm position resolution. The ToT information can also be used to correct for the leading-edge time-walk in each pixel, and a timing resolution of few tens of picosecond is expected. The detector will be highly compact: the front-end electronics is encapsulated in the vacuum tube and allows local processing of the detector information, which are sent out of the tube in digital form. The Timepix4 architecture is data driven, producing 64 bits for each pixel hit, corresponding to a maximum data rate of 80 Gbps for 1.2 Ghits/s. A flexible design is conceived, with electro-optical transceivers linking the ASIC to a FPGA-based board, placed far from the detector, for the exchange of configuration and the collection of event data. The FPGA will perform serial decoding and send the data directly to a PC for storage using fast serial data links.
A Ring Imaging Cherenkov detector equipped with such a device could deliver unprecedented information and allow efficient particle identification in high particle multiplicity environments: the high granularity and rate capabilities are crucial in applications with large detector occupancies; few microns resolution allows to reduce the pixel size contribution to the Cherenkov angle resolution to a negligible level; few tens of picosecond timing resolution per single photon will greatly simplify pattern recognition exploiting time-association of the individual photons; small dark count rate at room temperature (~10$^2$ Hz/cm$^2$) allows to have negligible detector-related background; MCP and tube geometry guarantees robustness against magnetic fields.