Photon Counting X-ray Detectors Research Articles

MAXIPIX, a fast readout photon-counting X-ray area detector for synchrotron applications

A 2D photon-counting X-ray detector system with 1.4 kHz frame rate and 55 μm spatial resolution has been developed and commissionned on ESRF beamlines. The system called MAXIPIX (Multichip Area X-ray detector based on a photon-counting PIXel array) consists of a detector module implementing up to five MEDIPIX-2 or TIMEPIX photon-counting readout chips, a custom readout interface board and a Linux acquisition workstation. The detector module readout time is 290 microseconds, allowing the system to achieve sustained frame rates of 280 Hz to 1400 Hz depending on the number of connected chips. An effective time resolution of 60 ns was measured using the ESRF pulsed modes and a TIMEPIX module. The system architecture and characteristics are presented, as well as a summary of its applications on ESRF beamlines.

X-ray photon-counting detector based on a micro-channel plate for pulsar navigation

The pulse time of arrival (TOA) is a determining parameter for accurate timing and positioning in X-ray pulsar navigation. The pulse TOA can be calculated by comparing the measured arrival time with the predicted arrival time of the X-ray pulse for pulsar. In this study, in order to research the measurement of pulse arrival time, an experimental system is set up. The experimental system comprises a simulator of the X-ray pulsar, an X-ray detector, a time-measurement system, and a data-processing system. An X-ray detector base is proposed on the basis of the micro-channel plate (MCP), which is sensitive to soft X-ray in the 1-10 keV band. The MCP-based detector, the structure and principle of the experimental system, and results of the pulse profile are described in detail. In addition, a discussion of the effects of different X-ray pulse periods and the quantum efficiency of the detector on pulse-profile signal-to-noise ratio (SNR) is presented. Experimental results reveal that the SNR of the measured pulse profile becomes enhanced as the quantum efficiency of the detector increases. The SNR of the pulse profile is higher when the period of the pulse is smaller at the same integral.

Study of charge-sharing in MEDIPIX3 using a micro-focused synchrotron beam

X-ray photon-counting detectors consisting of a silicon pixel array sensor bump-bonded to a CMOS electronic readout chip offer several advantages over traditional X-ray detection technologies used for synchrotron applications. They offer high frame rate, dynamic range, count rate capability and signal-to-noise ratio. A survey of the requirements for future synchrotron detectors carried out at the Diamond Light Source synchrotron highlighted the needs for detectors with a pixel size of the order of 50μm. Reducing the pixel size leads to an increase of charge-sharing events between adjacent pixels and, therefore, to a degradation of the energy resolution and image quality of the detector. This effect was observed with MEDIPIX2, a photon-counting readout chip with a pixel size of 55μm. The lastest generation of the MEDIPIX family, MEDIPIX3, is designed to overcome this charge-sharing effect in an implemented readout operating mode referred to as Charge Summing Mode. MEDIPIX3 has the same pixel size as MEDIPIX2, but it is implemented in an 8-metal 0.13μm CMOS technology which enables increased functionality per pixel. The present work focuses on the study of the charge-sharing effect when the MEDIPIX3 is operated in Charge Summing Mode compared to the conventional readout mode, referred to as Single Pixel Mode. Tests of a standard silicon photodiode array bump-bonded to MEDIPIX3 were performed in beamline B16 at the Diamond Light Source synchrotron. A monochromatic micro-focused beam of 2.9μm x 2.2μm size at 15keV was used to scan a cluster of nine pixels in order to study the charge collection and X-ray count allocation process for each readout mode, Single Pixel Mode and Charge Summing Mode. The study showed that charge-shared events were eliminated when Medipix3 was operated in Charge Summing Mode.

A modular high-resolution photon-counting x-ray detector

A modular high-resolution photon-counting x-ray detector

An X-ray Detector for X-ray Pulsar Based Navigation and Timing

The pulse time of arrival (TOA) is a determinant parameter in the X-ray pulsar based navigation and timing. The measurement of TOA is implemented by super sensitive X-ray photon counting detector on the spacecraft. By comparing different kinds of detectors, an x-ray detector based MCP sensitive to X-ray energy band of 1~10 keV is proposed. The detector consists of input window, CsI photocathode, micro-channel plate (MCP) stack and a designed anode. The incident X-ray photons are detected, the pulse profile is reconstructed. By comparing the measurement pulse profile with the standard pulse profile of the X-ray pulsar source, the pulse TOA can be calculated. The experiment system, performance of the detector and results on the pulse profile are presented.

Sufficient Statistics as a Generalization of Binning in Spectral X-ray Imaging

It is well known that the energy dependence of X-ray attenuation can be used to characterize materials. Yet, even with energy discriminating photon counting X-ray detectors, it is still unclear how to best form energy dependent measurements for spectral imaging. Common ideas include binning photon counts based on their energies and detectors with both photon counting and energy integrating electronics. These approaches can be generalized to energy weighted measurements, which we prove can form a sufficient statistic for spectral X-ray imaging if the weights used, which we term μ-weights, are basis attenuation functions that can also be used for material decomposition. To study the performance of these different methods, we evaluate the Cramér-Rao lower bound (CRLB) of material estimates in the presence of quantum noise. We found that the choice of binning and weighting schemes can greatly affect the performance of material decomposition. Even with optimized thresholds, binning condenses information but incurs penalties to decomposition precision and is not robust to changes in the source spectrum or object size, although this can be mitigated by adding more bins or removing photons of certain energies from the spectrum. On the other hand, because μ-weighted measurements form a sufficient statistic for spectral imaging, the CRLB of the material decomposition estimates is identical to the quantum noise limited performance of a system with complete energy information of all photons. Finally, we show that μ-weights lead to increased conspicuity over other methods in a simulated calcium contrast experiment.

An analytical model of the effects of pulse pileup on the energy spectrum recorded by energy resolved photon counting x‐ray detectors

Recently, novel CdTe photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed. When such detectors are operated under a high x-ray flux, however, coincident pulses distort the recorded energy spectrum. These distortions are called pulse pileup effects. It is essential to compensate for these effects on the recorded energy spectrum in order to take full advantage of spectral information PCXDs provide. Such compensation can be achieved by incorporating a pileup model into the image reconstruction process for computed tomography, that is, as a part of the forward imaging process, and iteratively estimating either the imaged object or the line integrals using, e.g., a maximum likelihood approach. The aim of this study was to develop a new analytical pulse pileup model for both peak and tail pileup effects for nonparalyzable detectors. The model takes into account the following factors: The bipolar shape of the pulse, the distribution function of time intervals between random events, and the input probability density function of photon energies. The authors used Monte Carlo simulations to evaluate the model. The recorded spectra estimated by the model were in an excellent agreement with those obtained by Monte Carlo simulations for various levels of pulse pileup effects. The coefficients of variation (i.e., the root mean square difference divided by the mean of measurements) were 5.3%-10.0% for deadtime losses of 1%-50% with a polychromatic incident x-ray spectrum. The proposed pulse pileup model can predict recorded spectrum with relatively good accuracy.

Beyond single photon counting X-ray detectors

Synchrotron radiation applications require detectors with a high sensitivity and a large dynamic range in order to study both the strong and weak features of the samples. Moreover, a high spatial resolution is necessary for imaging and diffraction studies. The photon counting technique is currently the best solution to these requirements but it presents some limitations which can only be partially overcome by technological improvements i.e. the count loss at high impinging intensities, due to the readout electronics shaping time, and the restrictions on the minimum pixel size, due to the charge diffusion in the sensor. In order to overcome both these limitations a new generation of charge integrating hybrid detectors with single photon sensitivity is being developed. These preserve the advantages of counting detectors and at the same time allow a processing of the analog information in order to enhance the spatial resolution and perform some spectral analysis. A comparison between purely counting and analog readout for 50 and 25 μ m pitch microstrip detectors is presented in terms of spectral reconstruction capability and spatial resolution.

Temperature dependence of AlGaAs soft X-ray detectors

We have fabricated and characterised unpassivated 200μm diameter circular mesa Al0.8Ga0.2As p+–i–n+ diodes as soft X-ray photon counting detectors. The spectroscopic performance, as measured by the FWHM of the 5.9keV X-ray line from an 55Fe radioisotope, shows that these detectors can be used for X-ray photon counting spectroscopy with promising energy resolution (0.9–2.5keV) over a −30 to +90°C temperature range.

A new family of pixel detectors for high frame rate X-ray applications

We are developing a new family of hybrid, single photon counting X-ray detectors for very high frame rate applications. A dedicated readout chip was designed, to be used as building block for detectors up to 9 Mpixel and about 550 cm 2 area. It has an area of 19.3 × 20.0 mm 2 and contains 256 × 256 pixels of 75 × 75 μ m 2 , resulting in an active region of 19.2 × 19.2 mm 2 . Each pixel contains a 12 bit counter with double buffering for continuous image acquisition. Moreover, it allows a partial readout (4, 8 or 12 bits) with corresponding frame rates up to 24, 12 and 8 kHz. The chip is designed with Hardening By Design techniques [1], to obtain high radiation tolerance from a standard commercial 0.25 μ m CMOS technology. The chip was recently received from fabrication and it is at present under test.

Projection x-ray imaging with photon energy weighting: experimental evaluation with a prototype detector

The signal-to-noise ratio (SNR) in x-ray imaging can be increased using a photon counting detector which could allow for rejecting electronics noise and for weighting x-ray photons according to their energies. This approach, however, was not feasible for a long time because photon counting x-ray detectors with very high count rates, good energy resolution and a large number of small pixels were required. These problems have been addressed with the advent of new detector materials, fast readout electronics and powerful computers. In this work, we report on the experimental evaluation of projection x-ray imaging with a photon counting cadmium–zinc–telluride (CZT) detector with energy resolving capabilities. The detector included two rows of pixels with 128 pixels per row with 0.9 × 0.9 mm2 pixel size, and a 2 Mcount pixel−1 s−1 count rate. The x-ray tube operated at 120 kVp tube voltage with 2 mm Al-equivalent inherent filtration. The x-ray spectrum was split into five regions, and five independent x-ray images were acquired at a time. These five quasi-monochromatic x-ray images were used for x-ray energy weighting and material decomposition. A tissue-equivalent phantom was used including contrast elements simulating adipose, calcifications, iodine and air. X-ray energy weighting improved the SNR of calcifications and iodine by a factor of 1.32 and 1.36, respectively, as compared to charge integrating. Material decomposition was performed by dual energy subtraction. The low- and high-energy images were generated in the energy ranges of 25–60 keV and 60–120 keV, respectively, by combining five monochromatic image data into two. X-ray energy weighting was applied to low- and high-energy images prior to subtraction, and this improved the SNR of calcifications and iodine in dual energy subtracted images by a factor of 1.34 and 1.25, respectively, as compared to charge integrating. The detector energy resolution, spatial resolution, linearity, count rate, noise and image uniformity were investigated. The limitations of this technology were emphasized and possible solutions were discussed.

AlGaAs diodes for X-ray spectroscopy

Al 0.8Ga 0.2As p +–p −–n + diodes with low reverse leakage currents (<10 nA) were evaluated as room temperature (23 °C), soft X-ray photon-counting detectors. X-ray spectra from a 55Fe radioactive source show these diodes can be used for spectroscopy with promising energy resolution of 1.47 keV FWHM at 5.9 keV. At elevated reverse bias levels (>20 V), we observed avalanche multiplication which, while improving the signal-to-noise ratio, degraded the spectroscopic performance.

WE-C-342-03: Photon Counting X-Ray Detectors and DR and CT Imaging Methods

Two major x-ray-based techniques used for medical imaging are digital radiography (DR) and computed tomography (CT). Most of their detectors integrate the intensity of x-ray flux and output gray scale images or projections. Detectors with indirect detection type heavily weigh x-ray photons with higher energy as they generate more lights. This results in suboptimal contrast of images because the contrast of different tissues reduces in general as the energy of photons increases. In the past few years, there have been strong reviving research interests in an old concept—the use of the photon energy—to improve the quality and accuracy of diagnosis. Recently, novel photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed for x-ray DR and CT imaging. These PCXDs counts the number of x-ray photons within multiple energy windows. This allows us to improve the quality of the current gray scale images and the accuracy of the material decomposition. We will discuss the state-of-the-art detector technologies and imaging methods unique to these PCXDs.

High resolution X-ray photon-counting detector with scintillator-deposited charge-coupled device

We report on a new photon-counting detector possessing unprecedented spatial resolution and moderate spectral resolution for 0.1-100 keV X-rays. It consists of an X-ray charge-coupled device (CCD) and a scintillator. The scintillator is directly coupled to the back surface of the X-ray CCD. Low-energy X-rays below 10 keV can be directly detected by the CCD. The majority of hard X-rays above 10 keV pass through the CCD but can be absorbed by the scintillator, generating visible photons. We coupled needlelike CsI(Tl) on the front surface of the back-illuminated (BI) CCD. The high detection efficiency of BI CCDs in the visible band enables us to collect visible photons emitted from the CsI(Tl) efficiently, leading to the moderate spectral resolution of 28.4% at 22.1 keV and 25% at 59.5 keV. We also investigated the imaging capability of our device and demonstrated high resolution imaging with an accuracy of 10 /spl plusmn/3 /spl mu/m at 17.4 keV.

Dead-time in X-ray photon counting detectors

Dead-time in X-ray photon counting detectors

Silicon-based photon counting X-ray detector for synchrotron applications

We have built the second version of a Large Area Detector (LAD) with a 500 μm thick pixellated silicon detector that is bump-bonded to seven readout application-specific integrated-circuits, (ASICs) each with 64×64 pixels. The detector module has 448×64 pixels, each of 150 μm×150 μm with uniform pixel size and no dead regions. The whole module (67.2 mm×9.6 mm) of photon counting pixels can be activated to readout X-rays with low observed pixel to pixel crosstalk and a high counting rate. Each pixel on the ASIC has a novel 5-bit energy adjustment that is optimised automatically by a Labview-based routine. This cures the observed pixel non-uniformity problem of the earlier design. The detector is sensitive to X-rays from 8 to 25 keV with a programmable energy threshold. A National Instruments based readout system has been built which has been up-graded to a Xilinx FPGA system allowing 1000 frames/s to be acquired from the system. This hardware is controlled with a Labview GUI to program different runs and to display images and spectra. The system can be expanded in a modular fashion to cover 300 mm×300 mm for high-frame-rate protein-diffraction studies at third generation synchrotrons or for other X-ray imaging applications.

Silicon-based photon counting X-ray detector for synchrotron applications

We have built the second version of a Large Area Detector (LAD) with a 500 μm thick pixellated silicon detector that is bump-bonded to seven readout application-specific integrated-circuits, (ASICs) each with 64×64 pixels. The detector module has 448×64 pixels, each of 150 μm×150 μm with uniform pixel size and no dead regions. The whole module (67.2 mm×9.6 mm) of photon counting pixels can be activated to readout X-rays with low observed pixel to pixel crosstalk and a high counting rate. Each pixel on the ASIC has a novel 5-bit energy adjustment that is optimised automatically by a Labview-based routine. This cures the observed pixel non-uniformity problem of the earlier design. The detector is sensitive to X-rays from 8 to 25 keV with a programmable energy threshold. A National Instruments based readout system has been built which has been up-graded to a Xilinx FPGA system allowing 1000 frames/s to be acquired from the system. This hardware is controlled with a Labview GUI to program different runs and to display images and spectra. The system can be expanded in a modular fashion to cover 300 mm×300 mm for high-frame-rate protein-diffraction studies at third generation synchrotrons or for other X-ray imaging applications.

Novel, photon counting X-ray detectors

A novel sealed gaseous parallel plate avalanche chamber detector is described which is significantly less prone to discharges and can consequently achieve high gas gains at high counting rates. The detector has demonstrated stable gains greater than 104 at counting rates in excess of 107 counts/mm2 s.

Large-scale images taken with the Medipix1 chip

Two methods to acquire large-scale X-ray images with high spatial resolution using the Medipix1 detector chip are presented. The Medipix1 chip is a pixelated, photon counting X-ray detector which was developed within the framework of the Medipix Collaboration.11http://www.cern.ch/MEDIPIX. Since the lateral dimensions of the Medipix1 chip are only 10.88mm×10.88mm, larger images are acquired by tiling, which also helps to minimise the influence of defective pixels on the image quality.Measurements on the modulation transfer function (MTF) and the detective quantum efficiency (DQE) of the detector were performed and first results are given. The MTF of the Medipix1 chip is quite close to the theoretical limit. The DQE is lower than expected and has to be investigated further.

X-ray and optical photon counting detector using superconducting tunnel junctions

The properties of Superconducting Tunnel Junctions (STJ) make them very suitable for low light level astronomical observations. STJ detectors could be used to improve the sensitivity of such a system to be implemented on ground based telescopes. We have developed a modified version of the "SNEP" fabrication process for making Nb/Al-AlO/sub x/-Al/Nb and Ta/Al-AlO/sub x/-Al/Ta/Nb STJ's. These junctions show a very low sub-gap leakage current at 0.1 K. They use a double thin aluminum trapping layer and a Nb or Ta absorber grown epitaxially on a R-plane sapphire substrate. A new experimental set-up has been completed to achieve photon counting performances for the X-ray and for optical-near infrared wavelengths. An original, room temperature, charge sensitive preamplifier has been built for this purpose and tested with a /sup 55/Fe X-ray source. The 6 keV peak of this source can be observed in the measured spectrum. Photon counting ability in the near infrared at 0.78 /spl mu/m is also demonstrated.