UV MCP Detectors for WSO-UV: Cross Strip Anode and Readout Electronics

The main instrument of the WSO-UV satellite covers the wavelength range of 102-176 nm and 174-310 nm with two high resolution echelle spectrographs. The essential requirements for the associated detectors are high quantum efficiency, solar blindness, and single photon detection. To achieve this, we are developing microchannel plate (MCP) detectors in sealed tubes. It is planned to use cesium activated gallium nitride as semitransparent photocathode, a stack of two microchannel plates in chevron configuration, and a 33 mm × 44 mm cross strip anode with 64 horizontal and 64 vertical electrodes. This type of anode requires a lower gain of the MCPs ( ≈ 10⁶) compared to other types of anodes. Therefore, it extends the expected lifetime of the detectors to about five to ten years. The challenge for the use of a cross strip anode onboard the WSO-UV satellite is the combination of contradictory constraints on the readout electronics: On the one hand it should be able to handle a maximum count rate of 3·10⁵ s^-1 with a spatial resolution better than 15 μm . On the other hand the power consumption is limited to about 8 W. One feasible solution is the so-called Beetle chip. This chip provides 128 input channels with charge-sensitive preamplifiers and shapers. It stores the sampled data temporarily in a ring buffer and multiplexes it to four analogue readout channels. The output is then digitized by four ADCs and processed in a radiation hard FPGA, which also contains the space-wire interface to the satellite bus.