We present recent progress in the development of novel microchannel plates (MCPs) manufactured using standard lead glass and with borosilicate glass microcapillary arrays functionalized using Atomic Layer Deposition (ALD) technology. Standard glass MCPs have achieved high quantum efficiency (~60% @115 nm & 65 nm) using opaque alkali halide photocathodes. Enhanced performance standard glass MCPs have also been demonstrated with no fixed pattern noise due to construction defects. Novel borosilicate glass atomic layer deposited MCPs up to 20 cm format show good overall response uniformity, tight pulse height distributions and very low background levels (0.05 events cm-2). Spatial resolutions of the order of 20 μm are demonstrated with 10 μm pore atomic layer deposited MCPs, and their fixed pattern noise has been significantly reduced. Bialkali cathodes in sealed tubes show high (<30%) efficiency at ~200 nm and long wavelength cutoffs at ~360 nm have been engineered.
Microchannel plate sensors are widely used as photon counting imagers in many applications, including, astronomy, high energy physics, and remote sensing. Potential future NASA observatories with ultraviolet instruments, such as LUVOIR and HABEX, will require large area detectors (8k × 8k pixels) with large dynamic range (≥1 kHz/resel), high quantum efficiency (75% peak), and very low backgrounds (≤0.1 cts/sec/cm2 ). New microchannel plate technology combining borosilicate glass microcapillary arrays with high efficiency materials applied by atomic layer deposition are being developed with these goals in mind. Detectors with these microchannel plates can be made in large formats (up to 400 cm2 ) with focal plane matching, have high spatial resolution (<20μm), are radiation hard, and have very low background rates (<0.05 events/sec/cm2 ) with no readout noise. Typical sensors make use of high efficiency photocathodes in open faced detectors (< 110 nm range) or in ultra-high vacuum sealed tube devices (>110 nm range). New photocathodes, such as GaN and hybrid bialkali/alkali halide, have high quantum efficiencies over broadband wavelengths. Cross-strip anodes are well suited for large format detectors with high spatial resolution and high dynamic range requirements. Improvements to detector anodes and readout electronics have resulted in better spatial resolution (10×), output event rate (100×), and temporal resolution (1000×), all the while operating at lower gain (10×). Combining these developments can have a significant impact to potential future NASA sub-orbital and satellite instruments.
The GOLD mission is a NASA Explorer class ultraviolet Earth observing spectroscopy instrument that will be flown on a telecommunications satellite in geostationary orbit in 2018. Microchannel plate detectors operating in the 132 nm to 162 nm FUV bandpass with 2D imaging cross delay line readouts and electronics have been built for each of the two spectrometer channels for GOLD. The detectors are “open face” with CsI photocathodes, providing ~30% efficiency at 130.4 nm and ~15% efficiency at 160.8 nm. These detectors with their position encoding electronics provide ~600 x 500 FWHM resolution elements and are photon counting, with event handling rates of > 200 KHz. The operational details of the detectors and their performance are discussed.
The optimization and performance of opaque Galium Nitride (GaN) photocathodes deposited directly on novel Microchannel Plates (MCPs) are presented in this paper. The novel borosilicate glass MCPs, which are manufactured with the help of Atomic Layer Deposition, can withstand higher temperatures enabling direct deposition of GaN films on their surfaces. The quantum efficiency of MBE-grown GaN photocathodes of various thickness and buffer layers was studied in the spectral range of ~200-400 nm for the films grown on different surface layers (such as Al2O3 or buffer AlN layer) in order to determine the optimal opaque photocathode configuration. The MCPs with the GaN photocathodes were activated with surface cesiation in order to achieve the negative Electron Affinity for the efficient photon detection. The opaque photocathodes enable substantial broadening of the spectral sensitivity range compared to the semitransparent configuration when the photocathodes are deposited on the input window. The design of currently processed sealed tube event counting detector with an opaque GaN photocathode are also described in this paper. Our experiments demonstrate that although there is still development work required the detection quantum efficiencies exceeding 20% level should be achievable in 200-400 nm range and <50% in 100-200 nm range for the event counting MCP detectors with high spatial resolution (better than 50 μm) and timing resolution of <100 ps and very low background levels of only few events cm-2 s-1.
Gallium nitride opaque and semitransparent photocathodes provide high ultraviolet quantum efficiencies from 100 nm
to a long wavelength cutoff at ~380 nm. P (Mg) doped GaN photocathode layers ~100 nm thick with a barrier layer of
AlN (22 nm) on sapphire substrates also have low out of band response, and are highly robust. Opaque GaN photocathodes
are relatively easy to optimize, and consistently provide high quantum efficiency (70% at 120 nm) provided the
surface cleaning and activation (Cs) processes are well established. We have used two dimensional photon counting
imaging microchannel plate detectors, with an active area of 25 mm diameter, to investigate the imaging characteristics
of semitransparent GaN photocathodes. These can be produced with high (20%) efficiency, but the thickness and conductivity
of the GaN must be carefully optimized. High spatial resolution of ~50 μm with low intrinsic background (~7
events sec-1 cm-2) and good image uniformity have been achieved. Selectively patterned deposited GaN photocathodes
have also been used to allow quick diagnostics of optimization parameters. GaN photocathodes of both types show great
promise for future detector applications in ultraviolet Astrophysical instruments.
Epitaxial growth of p-type GaN-based UV photocathode by RF plasma assisted molecular beam epitaxy (MBE) on
sapphire, fused silica, and alumina substrates was investigated. The electrical measurements indicted the growth of
highly p-type GaN films as thin as 0.1 μm on c-plane sapphire with a thin AlN nucleation layer. Polycrystalline p-type
GaN was obtained for growth on fused silica and alumina. Negative electron affinity (NEA) photocathodes were
fabricated by cesium activation of the p-type GaN films in vacuum. Quantum efficiency for UV detection on different
substrates was then characterized. To study the integration of UV photocathodes with MCPs, direct deposition of p-type
GaN films on glass MCPs were done at low growth temperatures by MBE. The detection efficiency of polycrystalline p-
GaN photocathodes in reflection mode was much less than the high quality p-type GaN films on sapphire, however, it
was comparable to the detection efficiency of the latter measured in the semitransparent mode. This indicates the
potential for fabrication of improved photocathodes with higher gain and better spatial and temporal resolutions.
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.
Until recently the spatial resolution of microchannel plate based photon/particle counting sensors has generally been
limited by the accuracy of the readout technique. The accuracy of novel readouts, in particular cross strip anodes, have
now reached the 6-10μm scale (the typical size of pores in a microchannel plate) and no longer determine the ultimate
resolution limit of the detector. Although there are some issues (e.g. fixed pattern distortions seen on 5 μm scale) to be
resolved for the cross strip (XS) anodes, one of the major drawbacks of the previous generation XS readouts is the low
counting rate capability (10 KHz), determined by the processing electronics, in particular by the signal amplifier
ASICs. In this paper we describe a new signal processing technique which should allow for high counting rates
exceeding 1 MHz with the same high spatial resolution (<10 μm FWHM). The slow analog sample and hold signal
processing is replaced by a fully parallel signal amplification followed by digital peak detection in each output channel.
The charge values in each electrode are calculated from the digitized waveforms passed into a Field Programmable
Gate Array (FPGA) where the signal peak detection and event centroiding is performed continuously.
A detailed model was developed in order to optimize the digital peak detection algorithms and to determine the
acceptable parameters for the electronic elements for a given spatial resolution. The results of our Monte Carlo
modeling indicate that the spatial resolution of fast XS anode encoding electronics will still be better than 10 μm
We describe the design and development of the CHIPS microchannel plate detector. The Cosmic Hot Interstellar Plasma Spectrometer will study the diffuse radiation of the interstellar medium in the extreme ultraviolet band pass of 90Å to 260Å. Astronomical fluxes are expected to be low, so high efficiency in the band pass, good out-of-band rejection, low intrinsic background, and minimal image non-linearities are crucial detector properties. The detector utilizes three 75mm diameter microchannel plates (MCPs) in an abutted Z stack configuration. A NaBr photocathode material deposited on the MCP top surface enhances the quantum detection efficiency. The charge pulses from the MCPs are centroided in two dimensions by a crossed-delayline (XDL) anode. A four panel thin-film filter array is affixed above the MCPs to reduce sensitivity to airglow and scattered radiation, composed of aluminum, polyimide/boron, and zirconium filter panes. The detector is housed in a flight vacuum chamber to preserve the hygroscopic photocathode, the pressure sensitive thin-film filters, and to permit application of high voltage during ground test.
Significant advances in readout elements of microchannel plate based sensors have led to the development of detectors with less than 10 μm spatial resolution. We have shown that cross strip (XS) anodes have spatial resolution as small as 5 μm FWHM when a simple and fast center of gravity centroiding technique is used. In this paper we investigate the variation of XS anode spatial resolution for several types of centroiding algorithms (N-finger center of gravity, Gaussian, Lorentzian, parabolic and hyperbolic cosine centroiding) and determine the optimum algorithm in terms of spatial resolution
and image linearity. We found that for existing 32x32 mm2 cross strip anodes and associated electronics, the best resolution and linearity is achieved with center of gravity centroiding with properly chosen thresholds on the data set. The images of USAF resolution test target obtained with MCP's with 6 μm pores on 7.5 μm centers resolve 71.8 line pairs per mm (Group 6 element 2, with line width of only ~7 μm). Thus the ~7 μm spatial resolution of the detector with cross strip anode is indeed limited now by the size of the MCP pores, while the resolution of XS readout is on the order of
only few micrometers FWHM.
The SPEAR (Spectroscopy of Plasma Evolution from Astrophysical Radiation) mission to map the far ultraviolet sky uses micro-channel plate (MCP) detectors with a crossed delay line anode to record photon arrival events. SPEAR has two MCP detectors, each with a ~25mm x 25 mm active area. The unconventional anode design allows for the use of a single set of position encoding electronics for both detector fields. The centroid position of the charge cloud, generated by the photon-stimulated MCP, is determined by measuring the arrival times at both ends of the anode following amplification and external delay. The temporal response of the detector electronics system determines the readout's positional resolution for the charge centroid. High temporal resolution (< 35ps x 75ps FWHM) and low power consumption (<6W) are required for the SPEAR detector electronics system. We describe the development and performance of the detector electronics system for the SPEAR mission.
The SPEAR micro-satellite payload consists of dual imaging spectrographs optimized for detection of the faint, diffuse FUV (900-1750 Å) radiation emitted from interstellar gas. The instrument provides spectral resolution, R~750, and long slit imaging of <10' over a large (8°x5') field of view. We enhance the sensitivity by using shutters and filters for removal of background noise. Each spectrograph channel uses identically figured optics: a parabolic-cylinder entrance mirror and a constant-ruled ellipsoidal grating. Two microchannel plate photon-counting detectors share a single delay-line encoding system. A payload electronics system conditions data and controls the instrument. We will describe the design and predicted performance of the SPEAR instrument system and its elements.
The Far Ultraviolet (FUV) detector for the Cosmic Origins Spectrograph (COS), scheduled to be installed in the Hubble Space Telescope in June 2003, is currently being built by the Experimental Astrophysics Group at The University of California, Berkeley. The COS FUV detector system is based on the detectors flown on the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite with changes to take advantage of technological improvements since the development of those detectors. The COS FUV detector is a dual segmented, cylindrical input face, MCP detector with cross delay line (XDL) readouts. Each segment is a Z-stack of MCPs with an active area 85 mm by 10 mm. The segments are abutted end to end to form a total active area approximately 180 mm by 10 mm (with a gap in the middle). Detector spatial resolution in the long (spectral) dimension is better than 25 microns and in the short dimension (cross-dispersion) is better than 50 microns. The MCPs are coated with a CsI photocathode to achieve the optimal quantum detection efficiency (QDE) in the 1150 - 1750 angstrom bandpass. Improvements in the understanding of the processing required to produce higher QDE MCPs has lead to significant improvements in the FUV QDE relative to previous missions. This paper presents the basic design parameters and performance characteristics of the COS FUV detector.
We will describe the development of the design and processing of the near ultraviolet (NUV, 1800 - 3000 angstroms) and far ultraviolet (FUV, 1350 - 1800 angstroms) sealed tube microchannel plate cross delay line detectors for the NASA Galaxy Evolution Explorer Satellite. Specifications for the two detector systems for GALEX define a large 65 mm diameter circular format, with high spatial resolution (< 30 micrometers FWHM, approximately 2200 X 2200 resolution elements) and good image linearity (+/- 50 micrometers ), low diffuse background rates of < 1 event cm-1 sec-1, and event processing rates of > 2 X 105 events sec-1. We have implemented a detector design using a microchannel plate Z stack to amplify the signals detected by the photocathodes, and a cross delay line anode to provide the photon event position encoding. These detectors were produced using a new sealed tube production facility installed at the Space Sciences Laboratory, University of California, Berkeley.
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.
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.
Microchannel plate (MCP), detectors are currently being used with great success on many recent NASA and ESA missions. These include SOHO, ALEXIS, EUVE, ACE, ORFEUS and sounding rocket experiments. Similar devices are in preparation for satellites such as IMAGE, FUSE, COS-HST, and the GALEX mission. For some of these missions we have pioneered the development of planar and multilayer centroid position readout anodes in the form of delay line image readout system for high resolution, large format, photon counting MCP detectors. Derived from these concepts we have devised a new type of readout system, the cross strip anode, for future mission applications which may offer significantly performance advances. Our objective is to provide a highly adaptable sensor for sub-orbital and satellite instruments which combines very high sped photon counting with high spatial resolution, low power, low mass/volume, high sensitivity, low background and high time resolution. The multilayer crossed strip position encoding anode uses two sets of orthogonal strip arrays to collect charge from a microchannel plate stack. Event position centroids are then computed using multichannel high sped electronics. A prototype system is currently under evaluation and we ultimately expect to achieve high resolution at low gain, with low power, high counting rates and low mass for a wide range of adaptable senor formats.
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.
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.
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 (MCP's) 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.
Delay line detectors have been chosen for the Far Ultraviolet Spectroscopic Explorer1 mission to be launched in 2000. The demands of the FUSE detectors include large format (220mm x 10mm format), high spatial resolution (15im x 35im FWHM) and linearity, high image stability, low power consumption and weight, and counting rates in excess of 3 x i0 events sec1. The FUSE program builds on the previous work, which includes two delay line detectors (95mm x 27mm double delay line format) that have already been successfully employed in the ORFEUSASTROSPAS2 ultraviolet spectrometer launched by shuttle in September 1993. We present the plans for the FUSE detector program, and results from double delay line (DDL) detectors that are under investigation to meet the requirements of the FUSE program. Our current detector achieves 15im x 25p,m FWHM (<4000 x 500 resolution elements) over the 65 x 15mm format used for the FUSE demonstration detector (90% of the flight detector segment format length), with good linearity (±1 resolution element) and high stability. State of the art analog to digital converter (ADC), gated integrator, and digital signal processor (DSP) technology have been employed to develop novel event position encoding electronics with high count rate capability (<5 x104 events sec). Microchannel plates with lOj.tm pores and an 80: 1 pore length to diameter ratio, with a 70mm x 20mm format have been used in a Z stack configuration to provide the photon amplification (gain 2 x 10). These show good pulse height distributions (<35% FWHM) even with uniform flood illumination, and background levels typical for this configuration (<1 event cm 2 sec 1). Flat field images are dominated by the microchannel plate fixed pattern noise due to the multifiber boundaries, and are stable. High efficiency photocathodes, such as KBr have been extensively studied, and provide quantum detection efficiencies of 40-50% in the 900 - 1200A range for FUSE.
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 DDL's 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).