The high-resolution microchannel plate detector for FUV spectroscopy in the BepiColombo mission
Abstract
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. H2 , 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 (80 µm) so that the wavelength resolution of the PHEBUS instrument should be 2 nm at the FUV range. The FUV detector consists of a Cs2 Te photocathode, microchannel plates (MCPs), and a resistive anode encoder (RAE). In a position-sensitive system with an RAE, 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-stack) followed by a gap and a clamped triplet of MCPs (Z-stack) in cascade. We have investigated the effect of the negative potential applied across the inter-stack (V-Z) gap on the PHD and the spatial resolution by means of calculation and experiments. The calculation with a simple ballistic model showed that the negative inter-stack potential reduced the size of the electron cloud by 70%. The result suggests that under such a condition the Z-stack MCP is operated in the completely saturated mode and the exhibits a more stable gain. On the other hand, we measured the performance of the test model under a variety of applied voltages. As a result, we achieved a high gain of 2×107 and the required spatial resolution (80 µm). Furthermore, we found that the negative potential applied across the V-Z gap made the electron gain more stable. In our presentation we report the specific performance of the test model of the FUV detector.
- Publication:
-
37th COSPAR Scientific Assembly
- Pub Date:
- 2008
- Bibcode:
- 2008cosp...37.2140M