Sharon R. Jelinsky

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.

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.

A Minisurvey of Interstellar Titanium from the Southern Hemisphere

We describe the results of a minisurvey of interstellar Ti II and Ca II absorption toward 42 early-type stars observed from the southern hemisphere at a spectral resolution of 4.5 km s-1. Results are also presented for the Na I ultraviolet line (3302 A) detected toward nine of these targets. We examine the dependence of the integrated column densities of N(Ti II), N(Ca II), and N(Na I) on distance, reddening, neutral hydrogen column density, and their Galactic elemental abundance. Our findings support the proposition that Ti II and Ca II absorption originates in the same regions of the pervasive, warm, and neutral intercloud gas of the interstellar medium. We have observed a correlation of decreasing Ti and Ca abundance with increasing line-of-sight gas density. The Ti II/Ca II abundance ratio has been found to be essentially constant under all the interstellar density conditions we have sampled. Thus, we conclude that the general absorption properties of titanium (and calcium) are similar throughout the entire disk of our Galaxy.

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.

Low reflectance EUV materials: a comparative study

A study has been undertaken of the performance characteristics of a variety of surfaces and surface treatments for use on baffle materials in Extreme UV Explorer instrumentation, in order to ascertain which materials performed with the least overall scattering between 300 and 1200 A and thereby minimize this important background source. Seven scattering samples were measured: bead-blasted aluminum treated with alodine 600, bead-blasted aluminum, Kel-f, etched molybdenum, nickel-plated etched molybdenum, bead-blasted 304 stainless steel, and bead-blasted aluminum painted with Chemglaze Z-306. Attention is given to total reflectance of each material plotted against the graze angle for 304, 584, and 1216 A.

Performance results of the GALEX cross delay line detectors

We describe the performance results for the Galaxy Evolution Explorer (GALEX) far ultraviolet (FUV) and near ultraviolet (NUV) detectors. The detectors were delivered to JPL/Caltech starting in the fall of 2000 and have undergone approximately 1000 hours of pre-flight system-level testing to date. The GALEX detectors are sealed tube micro-channel plate (MCP) delay line readout detectors. They have a 65 mm diameter active area, which will be the largest format on orbit. The FUV detector has a spectral bandpass from 115 - 180 nm and the NUV detector has a bandpass from 165 - 300 nm. We report here on the performance of the detectors before and after integration into the instrument. Characteristics measured include the background count rate and distribution, gain vs. applied high voltage, spatial resolution and linearity, flat fields, and quantum efficiency.

Large Area Microchannel Plate Imaging Event Counting Detectors With Sub-Nanosecond Timing

Progress towards the development of a 20 cm sealed tube optical detector with imaging and photon event time stamping is presented. Novel microchannel plates employing borosilicate micro-capillary arrays have been tested. These provide many performance characteristics typical of conventional MCPs, but have been made in sizes up to 20 cm, have low intrinsic background (0.08 events cm-2s-1) and very stable gain behavior for at least 7 C cm-2 of charge extraction. Bialkali (Na2KSb) photocathodes with >20% quantum efficiency have also been made on 20 cm borofloat-33 windows compatible with a large sealed tube device.

Progress in soft x-ray and UV photocathodes

We present investigations on the improvements in quantum detection efficiency (QDE) of microchannel plate (MCP) detectors resulting from the application of various photocathode materials. Nine different photocathode materials were deposited and their QDE measured in the soft x-ray and UV region from 12 angstrom to 1850 angstrom. Four of these materials (CsCl, RbCl, RbI, BaCl) significantly enhance the QDE performance of bare MCPs, and five materials were proven unsatisfactory (AgCl, LiCl, LiI, LiF, MgBr). CsCl has very high (greater than 90%) short wavelength QDE and both CsCl and RbI have UV QDE in the region of 40%. Our studies also include life testing of a KBr photocathode for a period of over 5 years. This shows good stability, and the angular response and photoemissive characteristics over time are described. The effects of long wavelength QDE activation of KBr by exposure to 2537 angstrom photons are discussed.

Advances in microchannel plates and photocathodes for ultraviolet photon counting detectors

A new method of fabricating microchannel plates has been investigated, employing microcapillary arrays of borosilicate glass that are deposited with resistive and secondary emissive layers using atomic layer deposition. Microchannel plates of this kind have been made in sizes from 33 mm to 200 mm, with pore sizes of 40 μm and 20 μm, pore length to diameter ratios of 60:1, bias angles of 8°, and open areas from 60% to 83%. Tests with single MCPs and MCP pairs have been done and show good imaging quality, gain comparable to conventional MCPs, low background rates (~ 0.085 events sec-1 cm-2), fast pulse response, and good ageing characteristics. The quantum efficiency for bare and alkali halide coated MCPs is similar to conventional MCPs, and we have also been able to deposit opaque GaN(Mg) cathodes directly onto these MCPs.

Performance of Small Pore Microchannel Plates

Small pore size microchannel plates (MCPs) are needed to satisfy the requirements for future high resolution small and large format detectors for astronomy. MCPs with pore sizes in the range 5 micrometer to 8 micrometer are now being manufactured, but they are of limited availability and are of small size. We have obtained sets of Galileo 8 micrometer and 6.5 micrometer MCPs, and Philips 6 micrometer and 7 micrometer pore MCPs, and compared them to our larger pore MCP Z stacks. We have tested back to back MCP stacks of four of these MCPs and achieved gains greater than 2 multiplied by 107 with pulse height distributions of less than 40 percent FWHM, and background rates of less than 0.3 events sec-1 cm-2. Local counting rates up to approximately equals 100 events/pore/sec have been attained with little drop of the MCP gain. The bare MCP quantum efficiencies are somewhat lower than those expected, however. Flat field images are characterized by an absence of MCP fixed pattern noise.