Development of the high-resolution FUV detector for the BepiColombo mission

Mariner-10 UV measurements and telescopic spectroscopy from the Earth identified six elements (Ca, Na, K, H, He, and O) in the Mercury's exosphere. Other species are expected, e.g. H_2, OH, and some noble gasses (Ar, Ne, and Xe). All species representative of the surface composition, directly produced by impact vaporization driven by micrometeoroids, physical sputtering, photo-stimulated desorption, and thermal desorption from the regolith, should also be present. To determine the composition of the Mercury's exosphere, the PHEBUS (Probing of Hermean Exosphere By Ultraviolet Spectroscopy) instrument on Mercury Planetary Orbiter (MPO) will measure the emission lines of the exosphere. PHEBUS is a dual FUV-EUV spectrometer working in the wavelength range from 55 to 315 nm. We are now developing the compact detector system sensitive to FUV airglow emissions of the Mercury. The FUV detector is required to have high spatial resolution (80μm) so that the wavelength resolution of the PHEBUS instrument should be 1.5 nm at the FUV range. The FUV detector consists of a Cs_2>Te photocathode, microchannel plates (MCPs), and a resistive anode encoder. In a position-sensitive system with a resistive anode encoder, the spatial resolution is determined by the signal-to-noise ratios at the anode terminals. Therefore, a high and stable electron gain of MCPs allows the position determination of each photoelectron event with high spatial resolution. We studied a method for achieving a high and stable electron gain. We fabricated a test model of the FUV detector incorporating a clamped pair of MCPs (V-MCPs) followed by a gap and a clamped triplet of MCPs (Z-MCPs) in cascade. We measured the performance of the test model under a variety of applied voltages. As a result, we achieved a high gain of 2 × 10^7 and the required spatial resolution (80μm). Furthermore, we found that the reverse voltage applied across the V-Z gap made the electron gain more stable. In this paper we report the specific performance of the test model of the FUV detector.