Imaging photon counting sensors, with high timing resolution, using GaAs and Super-GenII photocathodes have been developed. These 18 mm active area format sensors with microchannel plates and cross strip anode readouts in combination with high speed event processing electronics can process high event rates (5 MHz with 82% livetime), and support local area counting rates of ~40 kHz (100 μm spot). For GaAs the peak quantum efficiency is ~30% (@ 560 nm) with spatial resolution of ~40 μm FWHM (~ 10 gain) and event timing resolution of ~260 ps (FWHM). For Super-GenII the peak quantum efficiency is ~20% with spatial resolution of <30 μm FWHM (~6 x10 gain) and event timing resolution of <100 ps (FWHM). We have also used novel microchannel plates that have been constructed with borosilicate microcapillary arrays and activated by atomic layer deposition. Examples have been incorporated into sealed tubes for evaluation. One trial uses an opaque GaN photocathode deposited onto these microchannel plates in a tube with a cross delay line readout. This device achieves ~40 μm spatial resolution, and its gain increased by an order of magnitude during the tube processing to achieve a uniform stable level. A 50mm Planacon was also constructed using atomic layer deposited microchannel plates, and this achieves standard bialkali quantum efficiency levels and has been used to detect fast laser pulse signals.

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 similar to 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 mu m) 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 um 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, AIGaN, 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 25min 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 -50 mu m 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.

We describe the design and report on the experimental results of a novel thermal and cold neutron imaging detector utilizing neutron sensitive (B-10-doped) microchannel plates (MCPs). In this detector, the incoming neutron interaction products produce secondary electrons at the pores adjacent to the absorption point within the MCP glass. This electron signal is then multiplied by a stack of conventional MCPs within those adjacent pores limiting the spread of the signal to less than two pore diameters (currently 6-10 mu m pores on 8-12 mu m centers). The event position then can be encoded by a number of readout techniques already developed for photon/charged particle counting applications.This paper presents the results of experimental evaluation of neutron sensing MCP detector with Medipix2 readout allowing operation at high counting rate mode (>100 MHz level) at a spatial resolution limited by the 55 mu m pixel size of the Medipix2 readout. Other attractive features of MCP neutron detectors are their high detection efficiency (approaching 50% levels) for thermal and cold neutrons and the absence of readout noise.

A compact far ultraviolet (FUV) spectrograph has been developed and applied to space observation on a micro-satellite. The dual channel imaging spectrograph utilized two micro-channel plate (MCP) detectors with a single crossed delay line (XDL) anode to record photon arrival events. The unconventional anode design allows for the use of a single set of position encoding electronics for both detector fields, thereby reducing the size, weight, and power of the associated electronics. The ground and on-orbit performance tests verified the successful application of the system for astrophysical observations. In this note, we report the design, the development, and the test results of the system, focusing on the XDL anode system.

High brilliance and high spectral resolution of synchrotron sources enable a large number of soft-X-ray spectroscopic studies such as inelastic X-ray scattering, which is becoming a technique of choice for the investigation of the electronic properties of complex materials. The resolution of the detection system in such experiments has to match the accuracy of the probe beam in order to take the full advantage of the performance of modern synchrotron sources. In this paper we describe our advanced photon counting detection system capable of simultaneously registering both position and time of individual photons with 2-dimensional spatial accuracy of <50 mum and timing accuracy of <130 ps FWHM. The open face, 25 mm active area detector consists of a Z-stack of microchannel plates and a cross delay line readout and has dark count of only a few counts per second. The associated electronics allows event counting rates up to ~400 KHz with 10% dead time for randomly distributed events. We present the results of our first measurements of delayed fluorescence from different materials performed at the Advanced Light Source. Time and angular resolved fluorescence measurements allowed us to separate images for the prompt elastically scattered and the delayed photons. The detector can also distinguish registration of electrons, ions or photons by variation of the potential on its input mesh. These results demonstrate the capabilities of our detection system, which is currently being integrated into an advanced time resolved X-ray emission spectroscopy system.

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 6-10 mu m 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 mu m 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 mu m 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 mu m FWHM.

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).© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.