Jeffrey S. Hull

High-resolution cross delay line detectors for the GALEX mission

The GALEX instrument consists of a 50cm normal incidence mirror telescope in combination with a grism, and a dichroic beamsplitter system projecting images onto two detectors simultaneously. The objective of this instrument is to provide sensitive high resolution imaging of galaxies in two bandpasses, with the option of the modest resolution spectroscopy. We are currently developing the microchannel plate, delay line, sealed tube detectors for the Galaxy Evolution Explorer mission to be launched in 2001.

Performance of the double delay line microchannel plate detectors for the Far-Ultraviolet Spectroscopic Explorer

The microchannel plate, delay line, detectors developed for the far ultraviolet spectroscopic explorer mission to be launched in 1998 are described. The two FUSE detectors have a large format (approximately equals 184 mm by 10 mm split into two 88.5 by 10 mm segments), with high spatial resolution (less than 20 micrometers by 50 micrometers FWHM, greater than 9000 by 200 resolution elements) and good linearity (plus or minus 25 micrometers), high image stability, and counting rates in excess of 4 by 104 events sec-1. KBr opaque photocathodes have been employed to provide quantum detection efficiencies of 30 - 40% in the 900 - 1200 angstrom range. Microchannel plates with 10 micrometer pores and an 80:1 pore length to diameter ratio, with a 95 mm by 20 mm format have been used in a Z stack configuration to provide the photon amplification (gain approximately equals 2 by 107). These show narrow pulse height distributions (less than 35% FWHM) even with uniform flood illumination, and good background levels (less than 0.3 event cm-2sec-1). Flat field images are demanded by the microchannel plate multifiber boundary fixed pattern noise and are stable.

Delay-line detectors for the UVCS and SUMER instruments on the SOHO Satellite

Microchannel plate based detectors with cross delay line image readout have been rapidly implemented for the SUMER and UVCS instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in July 1995. In October 1993 a fast track program to build and characterize detectors and detector control electronics was initiated. We present the detector system design for the SOHO UVCS and SUMER detector programs, and results from the detector test program. Two deliverable detectors have been built at this point, a demonstration model for UVCS, and the flight Ly (alpha) detector for UVCS, both of which are to be delivered in the next few weeks. Test results have also been obtained with one other demonstration detector system. The detector format is 26mm x 9mm, with 1024 x 360 digitized pixels,using a low resistance Z stack of microchannel plates (MCPs) and a multilayer cross delay line anode (XDL). This configuration provides gains of approximately equals 2 X 107 with good pulse height distributions (<50% FWHM) under uniform flood illumination, and background levels typical for this configuration (approximately equals 0.6 event cm-2 sec-1). Local counting rates up to approximately equals 400 event/pixel/sec have been achieved with no degradation of the MCP gain. The detector and event encoding electronics achieves approximately equals 25 micrometers FWHM with good linearity (+/- approximately equals 1 pixel) and is stable to high global counting rates (>4 X 105 events sec-1). Flat field images are dominated by MCP fixed pattern noise and are stable, but the MCP multifiber modulation usually expected is uncharacteristically absent. The detector and electronics have also successfully passed both thermal vacuum and vibration tests.

Cross strip imaging anodes for microchannel plate detectors

We have developed a novel microchannel plate readout scheme, the cross strip anode. The cross strip anode has a coarse (0.5 mm) multilayer metal and ceramic cross strip pattern that encodes event positions by direct sensing of the charge on each strip and subsequent determination of the charge cloud centroid for each event. Event position encoding is accomplished with chip level preamplifiers on the anode, subsequent analog-to-digital conversion of individual strip charge values, and a software centroid determination. We find that the spatial resolution ( 1 MHz and with low-power consumption (/spl sim/2 W) that is suitable for applications in space astrophysics.

High resolution cross strip anodes for photon counting detectors

A new photon counting, imaging readout for microchannel plate sensors, the cross strip (XS) anode, has been investigated. Charge centroiding of signals detected on two orthogonal layers of sense strip sets are used to derive photon locations. The XS anode spatial resolution (< 3 μm FWHM) exceeds the spatial resolution of most direct charge sensing anodes, and does so at low gain ( < 2 × 10 6 ). The image linearity and fidelity are high enough to resolve and map 7 μm MCP pores, offering new possibilities for astronomical and other applications.

Microchannel plates for the UVCS and SUMER instruments on the SOHO satellite

The microchannel plates for the detectors in the SUMER and UVCS instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in late 1995 are described. A low resistance Z stack of microchannel plates (MCPs) is employed in a detector format of 27 mm multiplied by 10 mm using a multilayer cross delay line anode (XDL) with 1024 by 360 digitized pixels. The MCP stacks provide gains of greater than 2 multiplied by 107 with good pulse height distributions (as low as 25% FWHM) under uniform flood illumination. Background rates of approximately equals 0.6 event cm-2 sec-1 are obtained for this configuration. Local counting rates up to approximately equals 800 events/pixel/sec have been achieved with little drop of the MCP gain. MCP preconditioning results are discussed, showing that some MCP stacks fail to have gain decreases when subjected to a high flux UV scrub. Also, although the bare MCP quantum efficiencies are close to those expected (approximately equals 10%), we found that the long wavelength response of KBr photocathodes could be substantially enhanced by the MCP scrubbing process. Flat field images are characterized by a low level of MCP fixed pattern noise and are stable. Preliminary calibration results for the instruments are shown.

Gallium Nitride Photocathode Development for Imaging Detectors

Recent progress in Gallium Nitride (GaN, AlGaN, InGaN) photocathodes show great promise for future detector applications in Astrophysical instruments. Efforts with opaque GaN photocathodes have yielded quantum efficiencies up to 70% at 120 nm and cutoffs at ~380 nm, with low out of band response, and high stability. Previous work with semitransparent GaN photocathodes produced relatively low quantum efficiencies in transmission mode (4%). We now have preliminary data showing that quantum efficiency improvements of a factor of 5 can be achieved. We have also performed two dimensional photon counting imaging with 25mm diameter semitransparent GaN photocathodes in close proximity to a microchannel plate stack and a cross delay line readout. The imaging performance achieves spatial resolution of ~50μm with low intrinsic background (below 1 event sec-1 cm-2) and reasonable image uniformity. GaN photocathodes with significant quantum efficiency have been fabricated on ceramic MCP substrates. In addition GaN has been deposited at low temperature onto quartz substrates, also achieving substantial quantum efficiency.

The FUV detector for the cosmic origins spectrograph on the Hubble Space Telescope

Abstract The Cosmic Origins Spectrograph (COS) is a high throughput spectrometer that will be placed on the Hubble Space Telescope (HST) during the last servicing mission in the year 2003. COS will be the most sensitive UV spectrograph ever flown aboard HST and will investigate such fundamental issues as the ionization and baryon content of the intergalactic medium and the origin of large-scale structure of the Universe. The driving design goal for COS is to maximize throughput at a moderate spectral resolution of >20,000 using optics with very few reflections and detectors with high quantum efficiency in two bandpass channels: FUV (1150– 1775 A ) and NUV (1750– 3200 A ). The COS FUV detector, a windowless microchannel plate (MCP) detector, consists of two segments each 85 mm ×10 mm concatenated end to end with a 9 mm gap between them. The design is based on the Far Ultraviolet Spectroscopic Explorer detectors with identical format and front surface radius of curvature that matches the grating focal plane of the spectrograph. However, enhancements have been made in the design and fabrication of the MCPs, the photocathode, the delay line anode and the readout electronics. We discuss these design enhancements and their significance.

Cross-delay-line microchannel plate detectors for the Spectrographic Imager on the IMAGE satellite

We have developed compact microchannel plate detectors utilizing a cross delay line readout system for the IMAGE- FUV Spectrographic Imager. We present a description of the detector head assembly and performance data typical for both detectors. Both detectors are nearly identical, the only different being the position of the input window on the front cover. Each detector, optimized for operation in the far UV with a KBr photocathode, provides high spatial resolution and good linearity over a 20 mm square format.

High-resolution monolithic delay-line readout techniques for two-dimensional microchannel plate detectors

Developments in high resolution double delay line (DDL) and cross delay line image readouts for applications in UV and soft X-ray imaging and spectroscopy are described. Our current DDLs achieve approximately equals 15 micrometers X 25 micrometers FWHM over 65 X 15 mm (> 4000 X 500 resolution elements) with counting rates of > 105 (10% dead time), good linearity (+/- approximately equals 1 resolution element) and high stability. We have also developed 65 mm X 15 mm multilayer cross delay line anodes with external serpentine delay lines which currently give approximately equals 20 micrometers FWHM resolution in both axes, with good linearity (approximately equals 30 micrometers ) and flat field performance. State of the art analog to digital converter and digital signal processor technology have been employed to develop novel event position encoding electronics with high count rate capability (2 X 105 events sec-1).