Evaluation of Photon-event-counting Intensifiers

We report the results obtained with a novel readout system for scintillating fibres and X-ray imaging based on the combination of a silicon pixel detector and one stage of light amplification using a 2nd generation image intensifier. The pixel detector is a hybrid assembly of a high resistivity silicon detector bump-bonded to a CMOS binary readout chip and can be read out at a speed of 15 MHz. Each pixel element measures 75 /spl mu/m/spl times/500 /spl mu/m. Scintillating fibre ribbons and scintillating crystals are coupled to the detector array using one stage of light amplification provided by a micro-channel plate (MCP) image intensifier with fiberoptic faceplates. We have used the system to image electron tracks in a bundle of scintillating fibre ribbons. Further we have shown how the system can be used for imaging 122 keV X-rays such as those used in nuclear medicine. The system provides the possibility of single photon counting, high spatial resolution and Compton rejection.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spatially resolved images of particle distributions are measured in steady-state plasmas in a GEC (gaseous electronics conference) plasma reactor. Images are obtained by monochromatic imaging of scattered laser light using a microchannel plate (MCP) image intensifier and a high-speed video camera. The observed distributions of particulates generated by adding small quantities of CHF/sub 3/ to an argon plasma are extremely complex and diverse. The patterns observed are temporally varying, and rarely as simple as domes and rings observed in other reactors. The forces acting on the particles are sufficiently complex that reproducing specific spatial patterns by controlling processing parameters is often impossible.

Gain and image shift measurements are presented for a proximity-focussed microchannel plate (MCP) image intensifier as a function of magnetic field (to 0.12 T) transverse to the optical axis. A simple model involving the angle of electron deflection, end-spoiled channel length and effective open area is described which agrees with the experiment.

A novel energy-resolved four-frame x-ray framing camera was developed for Qiang-guang I high power krypton gas-puff Z-pinch generator. The camera consisted of a pinhole array, two kinds of filters, a micro-channel plate (MCP) image intensifier, and a charge coupled device (CCD) camera. There are four microstrips on the input surface of the MCP. Two images are formed on each microstrip by two pinholes, one is filtered by 2 &mu;m (micron) aluminized mylar and sensitive to 150 - 300 eV photons and those above 400 eV, another is filtered by 1 &mu;m (micron) mylar plus 0.25 &mu;m (micron) aluminum and sensitive to photons greater than 300 eV. Finally, the experimental results are discussed.

The EUV grating spectrograph (EGS) as part of the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) solar EUV experiment (SEE) presents a number of performance challenges for its microchannel plate (MCP)- based detector. The EGS measures the solar spectral irradiance from 25 to 200 nm. The intensities of the line and continuum emissions over this spectral range vary by up to five orders of magnitude. While a variety of transmission filters are strategically used to bring the signal over the entire spectrum into the dynamic range of a single chevron pair of MCPs, a number of concerns regarding the characteristic behavior of MCPs remain. The nominal TIMED mission is two years while the extended mission potentially doubles to four years. Over this period, the MCP response is expected to change to some extent. Detailed changes in MCP performance are notoriously application dependent,however. In order to better anticipate and accommodate these changes over the duration of the mission as well as to potentially enhance detector performance, we are performing a series of life-tests on a variety of MCPs. In these tests we characterize MCP gain, pulse height distribution, quantum detection efficiency, and linearity as function of accumulated charge and exposure to atmosphere. Here we report on the result of these test.s

The effects of magnetic fields on the microchannel plate (MCP) image intensifiers to be used in a novel biplanar fluoroscope are studied along with the system's overall contrast as a function of beam energy. For a second-generation device with wrap-around power supply, B-3dB values for the gain roll-off were found to be approximately 0.08 T (axial field) and 0.06 T (transverse field). The maximum image shift resuiting from a 0.0035-T transverse field is found to be 0.065 mm, limited by centroid location error resulting from low-dose x-ray noise. The results of x-ray contrast studies suggests that the presently estimated 0.1-rad dose delivered to the patient (2-h magnetic stereotaxis procedure; 12% fluoro-on duty cycle) might be reduced by increasing the x-ray energy.

Time-resolved ratio method is described for multicomponent pattern analysis by using a fluorescence lifetime imaging system and a ratio method calculation. Nanosecond-level time- resolved fluorescence images of a sample under a pulsed light excitation were detected directly by using a gated microchannel plate (MCP) image intensifier coupled to a CCD camera. Differences in fluorescence lifetimes of components in which each component has similar fluorescence wavelength characteristics were positively utilized to estimate every component spatial distribution. Spatial deconvolution of multicomponent fluorescence data was rapidly carried out from a set of time-resolved, two-dimensional fluorescence data. An application of a multigate time-resolved ratio method for fluorescence pattern analysis was provided. From the results, it is shown that the new method can be successfully used for multicomponent fluorescence pattern analysis at a reasonable signal-to-noise ratio.

Image intensifier tubes using microchannel plate amplification stages and phosphor output stages are being increasingly used in various detection applications. In this paper, measurements of the decay times of what are attributed to be the P20 phosphors in various image intensifiers are reported. It is found that the long tail on the decay curve of the phosphor following illumination can be a limitation for certain observations. In addition, the background level of phosphor glow (which is seen by the subsequent detection system as a light signal) continues to build with continuing illumination.

The research on key techniques of wide field extreme ultraviolet( EUV) spherical micro-channel plate( MCP) photon-counting imaging detector which will be used in space probe in China have been done.Firstly,the secondary electron yield model of photocathode material was built,and the secondary yield of alkali halide and MCP in soft X-ray and EUV band were calculated using the model. The measurement equipment of MCP's quantum detection efficiency also was set up. The calculated fomula of MCP' s quantum efficiency was introduced,and MCP's quantum efficiency vs. wavelength and incidence angle in soft X-ray and EUV region were measured. secondly,the instrument of solid core spherical MCP fabrication was established,a set ofspherical MCP with 150 mm radius were made using the instrument. The 48 mm induced charge wedge and strip anode in effective diameter was made by the use of traditional UV lithography,and the photon-counting imaging detector was integrated based on the anode and spherical MCP stack in Z configuration. Thirdly,the position readout electronics including analog frent-end and digital processing circuit and the software with image distortion rectification and real-time collection were developed. Finally,the measurement instrument of spherical MCP detector's spatial resolution and image linearity was built,and the specifications of the detector were measured using the instrument,showing that all technical specificaions of the detector satisfy the design requirements.

Current Generation II Gallium Arsenide (GaAs) image intensifier tube technology requires that the tube microchannel plate (MCP) component have a thin dielectric coating on the side facing the tube's photocathode component. This protective coating substantially reduces the release from the MCP of ions and neutral species, particularly when the image intensifier is operated. The prevention of MCP outgassing is necessary in order ot prevent the poisoning of the Cs:O surface on the GaAs photocathode. Many authors have experimented with omitting the MCP coating. Such experiments have consistently led to an intensifier with a significantly reduced lifetime, due to contamination of the Cs:O layer on the photocathode. Unfortunately the MCP film acts as a scattering cneter to electron transport within the intensifier and effectively reduces the photoelectron detection efficiency. Substantial enhancement of the image intensifier operating parameters is the motivation for the removal of the MCP film. Removal of the MCP film promises to simplify MCP fabrication and enhance the intensifier parameters related to Electro-Optical performance and image quality. This paper presents results showing for the first time that it is possible to fabricate a long lifetime image intensifier with a single unfilmed MCP and achieve improved imaging and performance characteristics.

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.

Lens coupled scintillating screen-CCD X-ray area detector with a high detective quantum efficiency

Some fundamental properties of 18-mm-diam gated proximity-focussed microchannel-plate (MCP) image intensifiers used as fast image shutters in the 1 to 10 ns range have been identified and studied. Light pulses (&asymp; 5-ps-wide) from a modelocked dye laser optically sample the gated MCP. Shuttering is achieved by applying a forward-biasing electrical gate pulse to the quiescently reverse-biased photocathode-MCP interface. Variable delay (&asymp; 30-ps jitter) between the gate pulse and the laser pulse permit tracing the MCP&apos;s optical response. Gating speeds, turn-on and turn-off patterns, the asymmetric spatial dependence of the MCP optical response, and resolution effects as functions of gate pulse width and photocathode-MCP bias have been characterized. Shutter times of &ge; 750 ps and &le; 5 1p/mm resolution with the MCP fully on were observed. Variations in the intensity profiles of the phosphorl.s spatial response for uniform photocathode illumination are measured with a calibrated silicon-intensified-target (SIT) focus projection, scan (FPS) television camera and a high-speed video digitizer while photomultipliers (PMTs) monitor the laser pulse and the phosphor&apos;s spatially integrated output intensities. The characterization system, gating and biasing circuits, and experimental results will be presented.

The microchannel plate (MCP) is a semiconducting glass plate containing an array of open channels that can provide photo-electron amplification for imaging applications. Conventional MCP channel geometries are designed to inhibit ion-feedback that occurs in straight channel plates at high applied potentials. Ion-feedback must be reduced as much as possible since its presence degrades the performance of the plate by adding noise to the plate's output. The performance characteristics of a new geometrical design for a high-gain MCP, that employs channels that spiral downward in a helical fashion to inhibit ion-feedback, have been evaluated in the laboratory. A Multi-Anode Microchannel Array (MAMA) detector was used to test the new MCP. The "helical-channel" MCPs tested to date have channel diameters of 50 microns and have demonstrated stable operation at gains above 106 electrons/pulse. The goal of the program is to develop "helical-channel" MCPs with channel diameters as small as 12 microns within two years. Detailed performance data including the dynamic range, pulse-height distribution, and gain characteristics of this new MCP will be presented.

A fundamental method to determine the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) for a photon counting pixel detector