Chemical method to increase extreme ultraviolet microchannel-plate quantum efficiency

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.

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

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.

Photon counting detectors (PCDs) are an emerging technology with applications in spectral and low-dose radiographic and tomographic imaging. This paper develops an analytical model of PCD imaging performance, including the system gain, modulation transfer function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). A cascaded systems analysis model describing the propagation of quanta through the imaging chain was developed. The model was validated in comparison to the physical performance of a silicon-strip PCD implemented on an experimental imaging bench. The signal response, MTF, and NPS were measured and compared to theory as a function of exposure conditions (70 kVp, 1-7 mA), detector threshold, and readout mode (i.e., the option for coincidence detection). The model sheds new light on the dependence of spatial resolution, charge sharing, and additive noise effects on threshold selection and was used to investigate the factors governing PCD performance, including the fundamental advantages and limitations of PCDs in comparison to energy-integrating detectors (EIDs) in the linear regime for which pulse pileup can be ignored. The detector exhibited highly linear mean signal response across the system operating range and agreed well with theoretical prediction, as did the system MTF and NPS. The DQE analyzed as a function of kilovolt (peak), exposure, detector threshold, and readout mode revealed important considerations for system optimization. The model also demonstrated the important implications of false counts from both additive electronic noise and charge sharing and highlighted the system design and operational parameters that most affect detector performance in the presence of such factors: for example, increasing the detector threshold from 0 to 100 (arbitrary units of pulse height threshold roughly equivalent to 0.5 and 6 keV energy threshold, respectively), increased the f50 (spatial-frequency at which the MTF falls to a value of 0.50) by ∼30% with corresponding improvement in DQE. The range in exposure and additive noise for which PCDs yield intrinsically higher DQE was quantified, showing performance advantages under conditions of very low-dose, high additive noise, and high fidelity rejection of coincident photons. The model for PCD signal and noise performance agreed with measurements of detector signal, MTF, and NPS and provided a useful basis for understanding complex dependencies in PCD imaging performance and the potential advantages (and disadvantages) in comparison to EIDs as well as an important guide to task-based optimization in developing new PCD imaging systems.

The Scanning-Beam Digital X-ray (SBDX) system utilizes a scanning x-ray pencil beam and a small-area detector array for low-dose cardiac angiography with tomographic imaging capabilities. For the system to provide adequate signal-to-noise ratios, the multi-element detector must be highly efficient and capable of high photon count rates. Cadmium telluride (CdTe) is well suited to these purposes. The CdTe SBDX detector is a direct-conversion photon-counting device consisting of 2304 elements. The efficiency of the detector is a function of several factors including the incident photon energy, the fluorescence properties of CdTe, and the discriminator threshold that determines whether sufficient energy was deposited in an element to register a count. For maximum efficiency, the discriminator threshold must be set low enough to detect CdTe k-fluorescence photons (23-31 keV), but not so low as to register false counts from electronic noise. The purpose of this investigation was to evaluate the energy-dependent quantum detective efficiency (QDE) of a new lower-noise SBDX detector design and to determine whether adequately low thresholds can be achieved. Experiments were performed using metal fluorescer foils to generate quasi-monochromatic x-ray beams with energies of 17.5, 25.3, and 46.0 keV. The resulting spectral purities were high, although fluence rates were low. The measured QDE values at 17.5, 25.3, and 46.0 keV were 60%, 76%, and 86% repsectively.

The physical performance of a scanning multislit full field digital mammography system was determined using basic image quality parameters. The system employs a direct detection detector comprised of linear silicon strip sensors in an edge-on geometry connected to photon counting electronics. The pixel size is 50 microm and the field of view 24 x 26 cm2. The performance was quantified using the presampled modulation transfer function, the normalized noise power spectrum and the detective quantum efficiency (DQE). Compared to conventional DQE methods, the scanning geometry with its intrinsic scatter rejection poses additional requirements on the measurement setup, which are investigated in this work. The DQE of the photon counting system was found to be independent of the dose level to the detector in the 7.6-206 microGy range. The peak DQE was 72% and 73% in the scan and slit direction, respectively, measured with a 28 kV W-0.5 mm Al anode-filter combination with an added 2 mm Al filtration.

We report upon our continued investigation of a chemical treatment method for improving the quantum detection efficiency and gain of photon counting microchannel plate detectors for vacuum UV and soft x-radiation. The process improved MCP quantum detection efficiency for approximately 100-1200 angstrom radiation by a factor of 1.5-2. The performance results of DIP treated microchannel plates are summarized.

The emergence of Silicon based microchannel plates (MCP's) has been awaited for a number of years, with many proposed advantages over standard glass MCPs for space-based detectors. Si should have a very low inherent background ( -1 cm -2 ), as well as being a low Z element with low stopping power for x, gamma and cosmic rays. The surface is oxidized and can be baked to very high temperatures (> 800 degrees Celsius), and will not react with photocathodes deposited on the surface. This could potentially allow opaque photocathodes, with their higher resolution and efficiency, to be used in the near UV/optical bands. Since the microchannel positions are determined photolithographically, the pattern will be uniform and coherent, resulting in more uniform flat fields and less differential non-linearity in the spatial response. Microchannel spacing could decrease to the micron regime, while size formats could increase. The potential advantages of Si MCPs encompass increased gain, stability, longevity, event rate, and QE. However, glass MCPs have a strong and successful heritage in space-based detector systems and the advantages of Si MCP's must be demonstrated in the laboratory before being considered for flight applications. We have tested some newly developed silicon (Si) MCP's provided by Nanosciences Corp. Although these are still in the developmental stage we have achieved a number of significant results. The gain, pulse height, response and gain uniformity, and quantum detection efficiency are very similar to glass MCP's. However the Si MCP background is approximately 0.02 events sec -1 cm -2 without shielding, a significant improvement over even low noise MCP's. The small samples we have tested are 25 mm format with 8 micrometer pore spacing, but they are taken from a 75 mm substrate, which offers the possibility of large MCP's in the near future. More testing and process development are underway to probe other operational parameters and optimize the manufacturing process.

An experimental scheme and data processing approaches are proposed for measuring by analog photo detectors the normalized second-order correlation function of the biphoton field generated under spontaneous parametric down-conversion. Obtained results are especially important for quantum SPDC-based technologies in the long-wave spectral ranges, where it is difficult to use the single-photon detector at least in one of the two biphoton channels. The methods of discrimination of analog detection samples are developed to eliminate the negative influence of the detection noises and get quantitatively true values of both the correlation function and the detector quantum efficiency. The methods are demonstrated depending on whether two single-photon avalanche photo detectors are used in both SPDC channels, or at least one single-photon detector is replaced by a photo-multiplier tube which cannot operate in the photon counting mode.

In any quantum communication system, such as a quantum key distribution (QKD) system, data rates are mainly limited by the system clock rate and the various link losses. While the transmission clock rate is limited by the temporal resolution of the single-photon detectors, losses in a fiber-based quantum communication system can be minimized by operating in the near infrared range (NIR), at 1310 nm or 1550 nm. Commercially available InGaAs-based avalanche photo-diodes (APDs) can be operated as single-photon detectors in this wavelength range [Hadfield, 2009]. Due to the severe after-pulsing, InGaAs APDs are typically used in a gated mode and this can limit their application in high-speed quantum communications systems. Superconducting single-photon detectors (SSPDs) can work in the NIR wavelength range with good performance [Gol’tsman et al. 2001; Hadfield, 2009]. However, SSPDs require cryogenic temperatures, and are not widely available on the commercial market at present. In addition, InGaAs/InP based photomultiplier tubes (PMT) can operate in the NIR range, but its performance is limited by very low detection efficiency (1 % at 1600 nm) and large timing jitter (1.5 ns) [Hamamatsu, 2005]. Microchannel plates (MCP) are micro-capillary electron multipliers coated with an electron-emissive material and multiply photon-excited electrons from a photon cathode [Wiza, 1979]. MCPs usually have faster rise times and lower timing jitter than is achievable with PMTs. InGaAs MCPs can work in the NIR range. These MCPs, but are limited by low detection efficiency (~1 %) [Martin, J. & Hink P. 2003]. On the other hand, silicon based avalanche photo-diodes (Si APDs) are compact, relatively inexpensive, and can be operated at ambient temperatures with high detection efficiency and low noise in the visible or near-visible range. Unfortunately they do not work at wavelengths longer than 1000 nm. For those wavelengths, an up-conversion technique has been developed that uses sum-frequency generation (SFG) in a non-linear optical medium to convert the signal photons to a higher frequency (shorter wavelength) in the visible or near visible range. The up-converted photons can then be detected by a Si APD. Up-conversion detectors use commercially available components and devices, and are a practical solution for many applications in quantum communications. To date, several groups have successfully developed highly efficient up-conversion single-photon detectors in the nearinfrared range using periodically poled lithium niobate (PPLN) waveguides [Diamanti et al., 2005; Langrock et al., 2005; Thew et al., 2006; Tanzilli et al., 2005; Xu et al., 2007;] and bulk crystals [Vandevender & Kwiat, 2004].

Evaluation of Photon-event-counting Intensifiers

Imaging detectors for wavelengths between 10 nm and 105 nm generally rely on microchannel plates (MCPs) to provide photon detection (via the photo-electric effect) and charge amplification. This is because silicon-based detectors (CCD or APS) have near zero quantum detection efficiency (QDE) over this wavelength regime. Combining a MCP based intensifier tube with a silicon detector creates a detector system that can be tuned to the wavelength regime of interest for a variety of applications. Intensified detectors are used in a variety of scientific (e.g. Solar Physics) and commercial applications (spectroscopic test instrumentation, night vision goggles, low intensity cameras, etc.). Building an intensified detector requires the mastery of a variety of technologies involved in integrating and testing of these detector systems. We report on an internally funded development program within the Southwest Research Institute to architect, design, integrate, and test intensified imaging detectors for space-based applications. Through a rigorous hardware program the effort is developing and maturing the technologies necessary to build and test a large format (2k × 2k) UV intensified CCD detector. The intensified CCD is designed around a commercially available CCD that is optically coupled to a UV Intensifier Tube from Sensor Sciences, LLC. The program aims to demonstrate, through hardware validation, the ability to architect and execute the integration steps necessary to produce detector systems suitable for space-based applications.

Recent advances in photon counting detection technology have led to significant research interest in X-ray imaging. As a tutorial level review, this paper covers a wide range of aspects related to X-ray photon counting detector characterization. The tutorial begins with a detailed description of the working principle and operating modes of a pixelated X-ray photon counting detector with basic architecture and detection mechanism. Currently available methods and techniques for charactering major aspects including energy response, noise floor, energy resolution, count rate performance (detector efficiency), and charge sharing effect of photon counting detectors are comprehensively reviewed. Other characterization aspects such as point spread function (PSF), line spread function (LSF), contrast transfer function (CTF), modulation transfer function (MTF), noise power spectrum (NPS), detective quantum efficiency (DQE), bias voltage, radiation damage, and polarization effect are also remarked. A cadmium telluride (CdTe) pixelated photon counting detector is employed for part of the characterization demonstration and the results are presented. This review can serve as a tutorial for X-ray imaging researchers and investigators to understand, operate, characterize, and optimize photon counting detectors for a variety of applications.

Sealed tube MCP detectors have provided a valuable platform for high spatial resolution photon counting detectors. Specifically, electronic readout microchannel plate (MCP) based sensors have been used as photon counting, imaging, event time tagging detectors for astronomy, time resolved biological imaging, imaging LIDAR, Cherenkov (RICH), scintillation detection, and neutron imaging [1]–[10]. For future applications we have been developing novel atomic layer deposited MCPs, UV optimized photocathodes and high resolution cross strip readouts for sealed tube detectors. The Photonis Planacon is a useful sealed detector format (~50 mm square active area) to employ and demonstrate these novel technologies. The Planacon standard geometry has been used, but the standard anode pad array is replaced with a cross strip anode that demonstrates ~20 µm FWHM spatial resolution for single photon events at electron gains of the order 106. Atomic layer deposited MCPs with 10µm pores have been used in place of conventional MCPs, providing tight pulse amplitude distributions, enhanced gain stability and reduced gamma ray sensitivity. The entrance window (Sapphire or MgF2) permits the high UV sensitivity of bialkali photocathodes (>30% @180nm) to be utilized. Event encoding electronics implemented in surface mount components also provides high resolution imaging at event rates of up to 5 MHz. ASIC versions of the encoding electronics are in the prototype phase and show considerable promise for enhanced performance with lower power and smaller footprints.

Technological innovations over the past several decades have led to pixelated photon-counting x-ray detectors suitable for medical imaging applications. One component of this research effort is the development of mathematical models of detector performance. Such models provide physical insight and understanding of relationships between the basic physics of photon-counting x-ray detectors and image quality. Models also enable identifying upper limits of detector performance, establishing performance benchmarks, and optimizing detector design. Over the past several decades, the image science community developed a Fourier-based approach to the objective assessment of detector performance. Within this framework, image spatial resolution and image noise are quantified in terms of the modulation transfer function and noise power spectrum, respectively. These metrics are combined in the detective quantum efficiency, which provides an objective measure of the signal-to-noise performance of an x-ray imaging system as a function of spatial frequency. In this chapter, we summarize recent efforts to develop a framework for modelling and understanding of Fourier-based metrics of the performance of photon-counting x-ray detectors. We present a brief introduction to Fourier-based metrics of image quality, present a generic framework for modelling of Fourier-based metrics for photon-counting systems, and then present a simple model of a photon-counting detector that enables understanding relationships between detector design parameters, spatial resolution, and image noise. The development of models of the performance of photon-counting x-ray imaging systems is an ongoing effort and future research is expected to transcend the material presented in this chapter.