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Delay Line Detectors for UVCS

The detectors developed for UVCS are two dimensional photon counting, centroiding, microchannel plate sensors with electronic readout. Two virtually identical detectors are used for the Ly- and O VI channels of the spectrometer assembly. A KBr coated, low resistance Z stack of microchannel plates (MCP's) provides detection and amplification, and a multilayer cross delay line anode (XDL) accomplishes position readout. The format is , electronically digitized to pixels. This detector and its detailed performance characteristics are discussed by Siegmund et al, (1994). KBr photocathode material is deposited directly on the MCP Z stack as an opaque layer. Incoming photons interact with the photocathode, resulting in photoelectron emission (Siegmund et al, 1987; Siegmund et al, 1990) and a subsequent charge avalanche in the MCP's, giving an overall charge multiplication of ~ 2 X 10^7. This charge cloud is drifted from the MCP output to the delay line anode. Here the charge is divided equally between two orthogonal sets of charge collection fingers on the XDL in the X and Y axes. These are connected to X and Y external serpentine delay lines (Siegmund et al, 1994) so that the X and Y photon event centroid positions may be deduced from the signal arrival time differences at the two ends of each delay line. The KBr photocathode quantum detection efficiencies (QDE), which were measured prior to installation in the instrument, for both UVCS detectors is given in Table IV. The photocathodes are solar blind, cutting off at about 1600 Å.


Table IV: Quantum Efficiencies of UVCS Detectors

MCP photon event pulse amplitude distributions are narrow (<30%
FWHM for O VI) allowing amplitude discrimination of background events if desired. The overall background rate is low, with a rate of <1 event cm s uniformly distributed across the FOV. This was expected for intrinsic beta decay MCP noise (Siegmund et al., 1988; Fraser et al., 1987) for 40K in the Philips MCP glass. Single pixel spot image event counting rates in excess of 100 events/pixel/sec have been achieved with no degradation of the MCP gain. The detector is stable to better than 1 pixel for global counting rates up to events per sec, with a calibrated photometric rate/deadtime characteristic (<40% dead time @ events s). The thermal drift is <1 pixel over the expected UVCS temperature range. Flat field images are dominated by MCP fixed pattern noise and are stable, however the expected MCP multifiber modulation is at an unusually low level (a few %). The Ly- detector has an optically flat MgF entrance window, thus blocking out all thermal ions or electrons from the detector, and all wavelengths <1100 Å. The O VI detector is ``open face'' with a 90% transmission mesh, biased at +15V, for attenuation of low energy ions, placed 9mm in front of the MCP's at the entrance to the detector vacuum housing. A feature of the XDL system is a commandable artificial pulse (stimulation pulse) injected into each delay line at a rate of ~40 sec, resulting in a spot image effectively at the center of the detector aperture. The data obtained from this feature gives an in orbit evaluation of electronics health. A number of other command and diagnostic functions are provided for the XDL detectors. Specific types of command functions include high voltage control, signal threshold level adjustments, and stimulation pulser control. Housekeeping data also includes power supply voltage monitors, X and Y pulse height data, fast event counters, and temperatures.



next up previous
Next: Image Processors Up: The Spectrometer Assembly Previous: Grating mechanism



Raid Suleiman
Thu Apr 11 09:35:42 EDT 1996