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Special microchannel plates (MCPs) developed by Nova Scientific Inc. incorporate high efficiency neutron conversion materials into the MCP to provide a high neutron stopping power. [sup.10]B and [sup.nat] Gd have high interaction cross sections for thermal neutrons and their incorporation into MCP glass is a convenient way to make efficient MCPs for neutron detection with high spatial resolution. We have evaluated neutron event counting 2D imaging detectors using these MCPs with a cross delay line readout, cross strip readout, and a Medipix2 readout. Tests at several reactors with the cross delay line and cross strip readouts have established spatial resolution with neutrons as good as ~30 microns FWHM over a 27 mm diameter detector, with event rates approaching 1 MHz, low fixed pattern noise, event time tagging of 25 ns and intrinsic background rates of < 0.05 events [cm.sup.-2] [sec.sup.-1] x Evaluation of neutron sensing MCP detector with Medipix2 readout (14 [mm.sup.2]) has allowed operation at high counting rates (500 MHz) with the spatial resolution limited by the 55 micron pixel size of the Medipix2 readout. We have also used the Medipix2 for centroiding of neutron events to sub pixel resolution to obtain better spatial resolution (< 15 [micro]m) for neutrons at reduced event rates (100 kHz). Initial measurements of thermal neutron detection efficiencies give values of 20% to 25% for thermal neutrons and 45 % for cold neutrons without optimization of the detection geometry. Preliminary tests with shielding and a LaBr scintillator to gate neutron detections in coincidence with gamma rays produced by neutron interactions has enabled gamma ray rejection factors of 3 x [10.sup.4] to be achieved. Further improvements in the neutron detection efficiency and gamma ray discrimination efficiency can be gained by optimization of the geometrical and electronic configurations. Index Terms--Neutron detector, neutron imaging.

A number of modern experiments require simultaneous measurement of charges on multiple channels at > MHz event rates with an accuracy of 100-1000 [e.sup.-] rms. One widely used data processing scheme relies on application of specific integrated circuits enabling multichannel analog peak detection asserted by an external trigger followed by a serial/sparsified readout. Although this configuration minimizes the back end electronics, its counting rate capability is limited by the speed of the serial readout. Recent advances in analog to digital converters and FPGA devices enable fully parallel high speed multichannel data processing with digital peak detection enhanced by finite impulse response filtering. Not only can accurate charge values be obtained at high event rates, but the timing of the event on each channel can also be determined with high accuracy. We present the concept and first experimental tests of fully parallel 128-channel charge sensitive data processing electronics capable of measuring charges with an accuracy of ~1000 e- rms. Our system does not require an external trigger and, in addition to charge values, it provides the event timing with an accuracy of ~1 ns FWHM. One of the possible applications of this system is high resolution position sensitive event counting detectors with microchannel plates combined with cross strip readout. Implementation of fast data acquisition electronics increases the counting rates of those detectors to multi-MHz level, preserving their unique capability of virtually noiseless detection of both position (with an accuracy of ~ 10 [micro]m FWHM) and timing (~1 ns FWHM) of individual particles, including photons, electrons, ions, neutrals, and neutrons. Index Terms--Digital signal filtering, high counting rate, position sensitive detectors, timing resolution.

A conceptual design of a novel type of thermal and cold neutron counting detector with one-dimensional spatial resolution of Another unique characteristic of the proposed neutron detector is its rather high neutron detection efficiency, exceeding 50%, as predicted by our extensive analytical modeling, accuracy of which was confirmed by the first experimental measurements. The active area of these detectors can be as large as ~10 x 10 [cm.sup.2] in the 2-D configuration and ~100 x 1 [mm.sup.2] in the 1-D case. One-dimensional detectors can be staggered to perform imaging in two dimensions with high timing resolution. Index Terms--Event counting, high resolution timing, neutron imaging.

High brilliance and high spectral resolution of synchrotron sources and associated optics 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 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. Index Terms--Delayed fluorescence, high resolution photon counting, time resolved event detection.

A new type of high performance and compact neutron collimator can be manufactured from Gd- or B-doped microchannel plates (MCPs). Structures only a few mm thick have very narrow rocking curves and high out-of-angle rejection ratios, as observed previously with a cold neutron beam. We present the results of measurements with a collimated (L/D ratio ~ 280 : 1) thermal neutron beam. MCP collimators doped with 3 mole % of [sup.nat][Gd.sub.2][O.sub.3] as well as doped with 10 mole % of [sup.10][B.sub.2][O.sub.3] were calibrated for transmission versus tilt angle. The MCPs used in this study were only 0.6 and 0.8 mm thick with ~ 8.5 [micro]m circular pores on 11.5 [micro]m centers. All the measured rocking curves agree well with the theoretically predicted performance. Both experimental and modeling results indicate that very efficient MCP collimators (with < 0.1[degrees] wide rocking curves and a rejection ratio exceeding [10.sup.3]) can be built with the existing technology. The possibility to manufacture collimators with very large L/D ratios exceeding 1000:1 is also discussed for the case of unetched MCPs. The peak transmission of such devices with very sharp rocking curves will be limited to ~ 40 % by the transmission of the undoped glass. Application of MCP collimators for scatter rejection in neutron radiography is also considered in terms of possible image distortions, which are shown to occur only for the systems with detector spatial resolution better than 20 [micro]m FWHM. Index Terms--Collimators, neutron optics, neutron scattering.

Recent advances in the photon counting, imaging readout for microchannel plate (MCP) detectors has led to a substantial improvement in their spatial resolution. The spatial accuracy (~7-10 [micro]m) of an MCP detector with a cross strip (XS) readout has been shown to be limited by the MCP pore size (~10/[micro]m). In this paper, we study the ultimate resolution limits of the XS readout itself. The model allows us to determine the requirements on the anode's geometry and the signal processing electronics in order to reach a particular spatial resolution. The optimal detector parameters, such as the width of the charge footprint at the anode (determined by the distance and the field between the MCP and the anode), and the gain of the detector can also be found with the help of our model. The model indicates that the optimum full-width at half-maximum of the charge footprint distribution at the anode is a factor of ~1.6 larger than the anode period. Given a noise of charge sensitive amplifiers of 350 electrons rms each we predict that the MCP gain can be as low as 2.5 x [10.sup.5] for this detector to resolve ~7/[micro]m features. Results of our modeling also indicate that the accuracy of the position obtained for center of gravity centroiding of the charge distribution is inferior to fitting a Gaussian-like analytical function, providing the geometry of the anode is accurate enough. The model predictions are compared with the experimentally measured images and reveal the critical parameters (anode's geometric accuracy and amplifier noise), which can be improved in future detectors. Index Terms--Image resolution, photon detectors, position sensitive detectors.

Detectors with microchannel plates (MCPs) are currently widely used in photon and charged particle detection with high spatial (~10/[micro]m) and temporal ( The same MCPs can be used as very efficient and compact thermal neutron collimators. In this paper, we compare the efficiency of circular- and square-pore MCP collimators with the help of our model, the validity of which has already been verified by our experimental measurements reported last year. The rocking curve of 5-mm and 2.5-mm thick MCPs doped with 3 mole % of [sup.nat][Gd.sub.2][O.sub.3] is predicted to be only [+ or -] 0.1[degrees] and [+ or -] 0.3[degree] wide, respectively, for both geometries. A very compact device with high thermal neutron detection efficiency and angular sensitivity can be built by combining an MCP neutron detector with an MCP collimator. Index Terms--High spatial resolution, neutron detection, neutron optics, position sensitive detectors.

The future of ground-based optical astronomy lies with advancements in adaptive optics (AO) to overcome the limitations that the atmosphere places on high resolution imaging. A key technology for AO systems on future very large telescopes are the wavefront sensors (WFS) which detect the optical phase error and send corrections to deformable mirrors. Telescopes with > 30 m diameters will require WFS detectors that have large pixel formats (512 x 512), low noise (< 3 [e.sup.-]/pixel) and very high frame rates (~1 kHz). These requirements have led to the idea of a bare CMOS active pixel device (the Medipix2 chip) functioning in counting mode as an anode with noiseless readout for a microchannel plate (MCP) detector and at 1 kHz continuous frame rate. First measurement results obtained with this novel detector are presented both for UV photons and beta particles. Index Terms--Adaptive optics, CMOS pixel chip, high frame rate, microchannel plate detector, photon counting, wavefront sensor.

The combination of a photocathode, microchannel plate stack and photon counting, imaging readout provides a powerful tool for high dynamic range, high spatial resolution, and high timing accuracy detectors for the X-ray to visible light spectral range. Significant improvements in spatial resolution, quantum efficiency, and background rate have made these devices attractive for many applications and are being applied to completely new research areas. We describe the latest developments in high-resolution cross-strip (XS) anode readout for photon counting imaging with microchannel plates (MCPs). We show that the spatial resolution of an MCP detector with the cross-strip readout is now limited only by the MCP pore width. For the first time, we show images of resolution test masks illustrating the exceptional spatial resolution of the cross-strip readout, which at the present time can resolve features on the scale of ~7 [micro]m with MCP gains as low as 6 x [10.sup.5]. Low-gain operation of the detector with high spatial resolution shown in this paper is very beneficial for applications with high local counting rate and long lifetime requirements. The spatial resolution of the XS anode can be increased even further with improved anode uniformity, lower noise front-end electronics, and new centroiding algorithms. This could be important when MCPs with smaller than existing 6 [micro]m pores become commercially available, thus improving the detector resolution down to the few micrometer scale. Index Terms--Position-sensitive detectors, photon counting, high spatial resolution.

In most neutron scattering experiments and in boron neutron capture therapy, the angular spread of the neutron beam is defined by the quality of the neutron collimator. A typical collimator consists of a large number of parallel plates coated with neutron absorbing material, and at present these plates are at least few centimeters in length. In order to obtain collimation in both vertical and horizontal planes, two orthogonally aligned collimators are installed in the neutron beam. We present a new type of high performance neutron collimator made with Gd-doped microchannel plates (MCPs). Such collimators are only few millimeters thick and the rocking curve is expected to be even sharper than that of conventional 0.5[degrees] collimators. While collimation is performed in two perpendicular planes simultaneously, the geometry of these new collimators can be changed so that the degree of collimation in each direction is controlled independently. The modeling of the proposed collimator indicates that for the existing MCP technology the rocking curve can be made as sharp as 0.2[degrees] FWHM, which can be further improved by current developments in the MCP technology. The preliminary experimental evaluation of our first very thin (only 0.6 mm) MCP collimators confirms the accuracy of our numerical model. Index Terms--Collimators, neutron optics.

The optical constants of thin films of CsI, KI, and KBr and the quantum efficiency (QE) of planar photocathodes made with these alkali halides in the 53.6-174.4-nm spectral range are presented. The optical constants were obtained from measurements of the reflectance as a function of incidence angle. The effect of film heating and exposure to UV irradiation on the optical properties and on the QE of the three alkali halides was investigated. KBr was found to be the most stable material for both heating and UV irradiation. KI appeared to be close to temperature stable, whereas UV exposure affected its optical constants. CsI optical constants changed after 420 K heating and after UV exposure. The changes in the optical constants were related to the QE changes, and a certain correlation between both variations was determined. However, it was also demonstrated that the QE changes cannot be explained solely by the changes in optical constants. OCIS codes: 260.7200, 260.7210, 120.4530.

The spatial resolution of position-sensitive detectors that use stacks of microchannel plates (MCP's) with high-resolution anodes can be better than 20-[[micro]meter] FWHM [Proc. SPIE 3114, 283-294 (1997)]. At this level of accuracy, channel misalignments of the MCP's in the stack can cause observable moire interference patterns. We show that the flat-field detector response can have moire beat pattern modulations of as great as ~27% with periods from as small as a few channel diameters to as great as the size of a MCP multifiber. These modulations, however, may be essentially eliminated by rotation of the MCP's or by a mismatch of the channel sizes. We also discuss how the modulation phenomena can be a useful tool for mapping the metric nonlinearities of MCP detector readout systems. Employing the optical moire effect, we demonstrate a simple, but effective, technique for evaluation of geometrical deformations simultaneously over a large MCP area. For a typical MCP, with a 60-channel-wide multifiber, we can obtain accuracies of 1.2 mrad for multifiber rotations and twists and 35/(L/p) mrad for channel-long axis distortions (where L/p is MCP thickness to interchannel distance ratio). This technique may be used for the development of MCP x-ray optics, which impose tight limitations on geometrical distortions, which in turn are not otherwise easily measurable with high accuracy.

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 ([is less than] 7 [micro]m) is sufficient to resolve 12-[micro]m microchannel plate pores. The cross strip anode can achieve this resolution while using low microchannel plate gain ([approximately equals] 5 x [10.sup.6]), thus increasing the local counting rate capacity and overall lifetime of the microchannel plate detector system. The image linearity is good enough ([is less than] 5 [micro]m) to enable distortions in the microchannel plate hexagonal multi-fiber boundaries to be seen. We also discuss plans for custom chip electronics development so that encoding may he accomplished at photon counting rates [is greater than] 1 MHz and with low-power consumption (~2W) that is suitable for applications in space astrophysics.

A detailed study of the stability of CsI and KBr photocathodes under UV irradiation is presented. UV quantum efficiency degradation was found to be more pronounced at lower illumination intensity for the same accumulated dose and illumination wavelength. For an equal number of extracted photoelectrons in-band UV exposure led to a larger sensitivity decay as compared to out-of-band illumination. The angle of radiation incidence was not important for the UV sensitivity degradation, while changes of visible light rejection (i.e., degradation of solar blindness) did depend on the incidence angle: the photocathodes illuminated at normal incidence were activated much faster than the films irradiated at grazing angle. We found that the increase of visible sensitivity can be characterized by the total accumulated dose and is independent of irradiation flux during UV activation. We also observed that heat annealing substantially improves the visible light rejection of C-sI photocathodes. Index Terms--Detectors, photocathodes, sensitivity, stability.