Photon-counting Sensor Research Articles

First experimental evaluation of a high-resolution deep silicon photon-counting sensor.

Current photon-counting computed tomography detectors are limited to a pixel size of around 0.3 to 0.5mm due to excessive charge sharing degrading the dose efficiency and energy resolution as the pixels become smaller. In this work, we present measurements of a prototype photon-counting detector that leverages the charge sharing to reach a theoretical sub-pixel resolution in the order of . The goal of the study is to validate our Monte-Carlo simulation using measurements, enabling further development. We measure the channel response at the MAX IV Lab, in the DanMAX beamline, with a 35keV photon beam, and compare the measurements with a 2D Monte Carlo simulation combined with a charge transport model. Only a few channels on the prototype are connected to keep the number of wire bonds low. The measurements agree generally well with the simulations with the beam close to the electrodes but diverge as the beam is moved further away. The induced charge cloud signals also seem to increase linearly as the beam is moved away from the electrodes. The agreement between measurements and simulations indicates that the Monte-Carlo simulation can accurately model the channel response of the detector with the photon interactions close to the electrodes, which indicates that the unconnected electrodes introduce unwanted effects that need to be further explored. With the same Monte-Carlo simulation previously indicating a resolution of around with similar geometry, the results are promising that an ultra-high resolution detector is not far in the future.

Deriving vertical profiles of chlorophyll-a concentration in the upper layer of seawaters using ICESat-2 photon-counting lidar.

Chlorophyll-a concentration (chl-a) is a great indicator for estimating phytoplankton biomass and productivity levels and is also particularly useful for monitoring the water quality, biodiversity and species distribution, and harmful algal blooms. A great deal of studies investigated to estimate chl-a concentrations using ocean color remotely sensed data. With the development of photon-counting sensors, spaceborne photon-counting lidar can compensate for the shortcomings of passive optical remote sensing by enabling ocean vertical profiling in low-light conditions (e.g., at night). Using geolocated photons captured by the first spaceborne photon-counting lidar borne on ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2), this research reported methods for deriving vertical profiles of chl-a concentration in the upper layer of ocean waters. This study first calculates the average numbers of backscattered subaqueous photons of ICESat-2 at different water depths, and then estimates the optical parameters in water column based on a discrete theoretical model of the expected number of received signal photons. With the estimated optical parameters, vertical profiles of chl-a concentration are calculated by two different empirical algorithms. In two study areas (mostly with Type I open ocean waters and small part of Type II coastal ocean waters), the derived chl-a concentrations are generally consistent when validated by BGC-Argo (Biogeochemical Argo) data in the vertical direction (MAPEs<15%) and compared with MODIS (Moderate Resolution Imaging Spectroradiometer) data in the along-track direction (average R2>0.86). Using globally covered ICESat-2 data, this approach can be used to obtain vertical profiles of chl-a concentration and optical parameters at a larger scale, which will be helpful to analyze impact factors of climate change and human activities on subsurface phytoplankton species and their growth state.

Theoretical analysis and image reconstruction for multi-bit quanta image sensors

A new breed of photon-counting sensors, called quanta image sensor (QIS), enables the detection of light with precision represented by the number of photons arriving within a time period. However, most existing analysis results on QIS systems are formulated for the single-bit case only. Directly extending the existing single-bit analysis to the multi-bit case leads to the situation that the variance of the estimated exposure is greater than the Cramér-Rao bound (CRB). Also, the existing dynamic range analysis leads to a strange situation that the maximum exposure level is not a function of the spatiotemporal jot kernel size. This paper formally analyzes the properties of multi-bit QIS (MBQIS) systems. It derives the log likelihood function of the received photon counts in a spatiotemporal jot kernel, and introduces the concept of the probability of all jots being saturated. From the likelihood function result, we can obtain a maximum likelihood (ML) estimate for the exposure level and present an image construction algorithm, namely ML multi-bit (MLM). Since the estimate is ML based, the variance of the estimated exposure achieves the CRB asymptotically and the MLM is an asymptotically unbiased estimator. Also, based on the Fisher information concept, this paper derives the CRB on the variance of the estimated exposure. Hence the CRB given by this paper can be considered as a performance indicator for all algorithms. From the jot saturation analysis result, we can accurately formulate the relationship between dynamic range and spatiotemporal kernel size. Specifically, with our two analysis results, we can model the relationships between sensor design parameters and performance metrics (variance of the estimated exposure and the dynamic range). Since the two analysis results are independent of the construction algorithms used, they give us some guidelines to design a QIS system. In addition, this paper empirically studies the effect of the readout Gaussian noise. Finally, to demonstrate another application of the likelihood analysis result, we develop an enhanced version of MLM, namely MLM with denoising (MLMDN), based on the proposed likelihood function and the regularization concept.

A maximum bathymetric depth model to simulate satellite photon-counting lidar performance

With the development of photon-counting sensors, spaceborne photon-counting lidars have shown many advantages in mapping underwater topography. Although a space based lidar is normally a profiling system, the depth penetration and the vertical accuracy achieved with a bathymetric lidar is superior to imagery (that only provides relative depths). Therefore, many satellite derived bathymetry products use both active/passive spaceborne data to achieve spatial coverage as well as absolute depth measurements. Compared with shipborne and airborne bathymetric systems, satellite derived bathymetry data can have a global coverage, especially covering some remote areas where airborne/shipborne systems are hard or very expensive to reach. In this paper, a theoretical model is proposed to quantitatively analyze the maximum bathymetric depth of a satellite photon-counting lidar, which considers the system parameters, environmental effects, and the constraint of the SNR (signal to noise ratio). System parameters of ICESat-2 (Ice, Cloud, and land Elevation Satellite-2) as well as the MODIS (Moderate-resolution Imaging Spectroradiometer) and NCEP (National Centers for Environmental Prediction) datasets are used to provide systematic and environmental inputs to the model, and the ICESat-2 actual bathymetric data are used to verify the maximum depth estimated by the proposed model. In six different sites with different water qualities, solar angles and wind speeds, the theoretical maximum bathymetric depths agree well with the actually achieved ICESat-2 lidar depths, in terms of MAE (mean absolute error) of 0.50 m and the RMSE (root mean square error) of 0.60 m. The errors in all study areas are within 14% of the corresponding maximum depths. Additionally, the non-linear response of a photon-counting lidar, independent parameter impacts on the theoretical model, and the errors in the experimental process were quantitatively analyzed. In the future, this theoretical model can be used to evaluate the maximum bathymetric depth for a bathymetry investigation when applying system parameters and environmental parameters. In addition, it can be used to optimize the hardware parameters of a photon-counting lidar in its early design process.

Realization of a time-correlated photon counting technique for fluorescence analysis.

An ultralow level light detection module, the time-correlated photon counter, is proposed and evaluated for fluorescence analysis. The time-correlated photon counter employs a silicon photomultiplier as a photon counting sensor in conjunction with a Poisson statistics algorithm and a double time windows technique, and therefore it can accurately count the photon number. The time-correlated photon counter is compatible with the time-correlated single photon counting technique and can record the arrival time of very faint light signals. This low-cost and compact instrument was used to analyze the intensity and lifetime of fluorescein isothiocyanate; a limit of detection of 16 pg/ml with a large linear dynamic range from 2.86 pg/ml to 0.5 µg/ml was obtained, and the lifetime of fluorescein isothiocyanate was measured to be 3.758 ns, which agrees well with the results of a sophisticated commercial fluorescence analysis instrument. The time-correlated photon counter may be useful in applications such as point-of-care testing.

Extreme background-rejection techniques for the ELROI optical satellite license plate.

The extremely low-resource optical identifier (ELROI) is a concept for an autonomous, low-power optical "license plate" that can be attached to anything that goes into space. ELROI uses short, omnidirectional flashes of laser light to encode a unique ID number that can be read by a small ground telescope using a photon-counting sensor and innovative extreme background-rejection techniques. ELROI is smaller and lighter than a typical radio beacon, low-power enough to run on its own small solar cell, and can safely operate for the entire orbital lifetime of a satellite or debris object. The concept has been validated in ground tests, and orbital prototypes are scheduled for launch in 2018 and beyond. In this paper, we focus on the details of the encoding scheme and data analysis that allow a milliwatt optical signal to be read from orbit. We describe the techniques of extreme background-rejection needed to achieve this, including spectral filtering and temporal filtering using a period- and phase-recovery algorithm, and discuss the requirements for an error-correcting code to encode the ID number. Worked examples with both simulated and experimental (long-range ground test) data illustrate the methods used. We present these techniques to describe a new optical communication concept, and to encourage others to consider observing and analyzing our upcoming test flights.

Widefield High Frame Rate Single-Photon SPAD Imagers for SPIM-FCS

Photon-counting sensors based on standard complementary metal-oxide-semiconductor single-photon avalanche diodes (SPADs) represent an emerging class of imagers that enable the counting and/or timing of single photons at zero readout noise (better than high-speed electron-multiplying charge-coupling devices) and over large arrays. They have seen substantial progress over the last 15 years, increasing their spatial resolution, timing accuracy, and sensitivity while reducing spurious signals such as afterpulsing and dark counts. They are increasingly being applied for time-resolved applications with the added advantage of enabling real-time options such as autocorrelation. We report in this study on the use of such a state-of-the-art 512 × 128 SPAD array, capable of a time resolution of 10−5–10−6 s for full frames while retaining acceptable photosensitivity thanks to the use of dedicated microlenses, in a selective plane illumination-fluorescence correlation spectroscopy setup. The latter allows us to perform thousands of fluorescence-correlation spectroscopy measurements simultaneously in a two-dimensional slice of the sample. This high-speed SPAD imager enables the measurement of molecular motion of small fluorescent particles such as single chemical dye molecules. Inhomogeneities in the molecular detection efficiency were compensated for by means of a global fit of the auto- and cross-correlation curves, which also made a calibration-free measurement of various samples possible. The afterpulsing effect could also be mitigated, making the measurement of the diffusion of Alexa-488 possible, and the overall result quality was further improved by spatial binning. The particle concentrations in the focus tend to be overestimated by a factor of 1.7 compared to a confocal setup; a calibration is thus required if absolute concentrations need to be measured. The first high-speed selective plane illumination-fluorescence correlation spectroscopy in vivo measurements to our knowledge were also recorded: although two-component fit models could not be employed because of noise, the diffusion of eGFP oligomers in HeLa cells could be measured. Sensitivity and noise will be further improved in the next generation of SPAD-based widefield sensors, which are currently under testing.

Reduction of CMOS Image Sensor Read Noise to Enable Photon Counting.

Recent activity in photon counting CMOS image sensors (CIS) has been directed to reduction of read noise. Many approaches and methods have been reported. This work is focused on providing sub 1 e− read noise by design and operation of the binary and small signal readout of photon counting CIS. Compensation of transfer gate feed-through was used to provide substantially reduced CDS time and source follower (SF) bandwidth. SF read noise was reduced by a factor of 3 with this method. This method can be applied broadly to CIS devices to reduce the read noise for small signals to enable use as a photon counting sensor.

Vision without the Image

Novel image sensors transduce the stream of photons directly into asynchronous electrical pulses, rather than forming an image. Classical approaches to vision start from a good quality image and therefore it is tempting to consider image reconstruction as a first step to image analysis. We propose that, instead, one should focus on the task at hand (e.g., detection, tracking or control) and design algorithms that compute the relevant variables (class, position, velocity) directly from the stream of photons. We discuss three examples of such computer vision algorithms and test them on simulated data from photon-counting sensors. Such algorithms work just-in-time, i.e., they complete classification, search and tracking with high accuracy as soon as the information is sufficient, which is typically before there are enough photons to form a high-quality image. We argue that this is particularly useful when the photons are few or expensive, e.g., in astronomy, biological imaging, surveillance and night vision.

광계수형 센서의 포톤계수효율 평가를 위한 포톤플루엔스 측정

ABSTRACT Recently, the various digital X-ray imaging devices using CCD and TFT LCD-based flat panel digital X-ray sensor are being used. In particular, a number of studies on photon counting sensor technique have been reported. In this study, the incident X-rays fluence on the photon counting sensor material was measured to estimate photon detection efficiency which is the quantitative performance evaluation factor of photon counting sensor. The result of measuring the photon fluence by using RQA-M2 Radiation beam quality of IEC 61223-1-2 recommendations, the incident photon fluence could be defined as about 4 photons/(0.01mm) 2 ․μGy within 10 ㎛ pin-hole area, and about 50 photons/(0.03mm) 2 ․μGy within 30 ㎛ pin-hole area, and about 698 photons/(0.1mm) 2 ․μGy within 100 ㎛ pin-hole area. Consequently, with the previously setup of the incident fluence, the measuring of actual photon counting efficiency by observing the output waveform of the photon counting sensor material was considered possible.

The cadmium telluride photon counting sensor in panoramic radiology: gray value separation and its potential application for bone density evaluation

To investigate whether bone mineral density can be evaluated more accurately using a panoramic device with a new cadmium telluride photon-counting sensor and software than 2 panoramic devices with a conventional semiconductor sensor. A fiduciary test object with several known levels of hydroxy apatite mineral concentration was placed in a phantom on the mandibular occlusal plane. Panoramic images were acquired by changing the position of the test object within the dental arch. The gray value, the spectrum deformation index (SDI), and the relative attenuation index (RAI) of the test object were evaluated. The SDI and RAI represent unique energy information as acquired by the QR-Master panoramic machine and the corresponding special QR-MC analysis software. In order to compare the values expressed in the different units (gray, SDI, and RAI values), the percentage discrepancy was calculated. The cadmium telluride photon-counting fitted machine more consistently separated each of the hydroxy apatite concentrations in all of the different positioning configurations and locations. The SDI function of the QR Master machine produced more stable values than the RAI value and the gray values of the 2 conventional panoramic machines. The methodologies as developed for this study can be used to test more sophisticated analyses for the determination of bone density.

광계수형 기반의 X선 영상센서 적용을 위한 광도전체 물성 연구

Abstract Digital X-ray imaging devices using a TFT based flat panel array has been used in medical field. But, recently, lots of research on the photon counting sensor has been reported. In this study, we evaluated the physical properties of the photoconductor by suggesting the standard and testing method for quantitative performance evaluation of photon counting x-ray imaging sensor. First, we measured the leakage current and the sensitivity of photon counting x-ray imaging sensor and we evaluated the characteristic of rising time for determining the signal shaping time. In addition, the set-up study was conducted on the basis of the IEC 62220-1-2 recommendations to define the number of incident photons per unit area. And the efficiency of the charge collection was also evaluated. As a result, the characteristic was measured as 200 pA/mm 2 of the leakage current, 7 μC/cm 2 R of the X-ray sensitivity, and 0.765 μs of the rising time. Key Words : photon counting, photoconductor, x-ray sensor, charge collection efficiency

Scintillation index measurement using time-correlated single-photon counting laser radar

The scintillation index is a common measure of the effects of atmospheric turbulence. Using a photon-counting sensor, the integration time for each sample needs to be short enough to ensure that the intensity is constant during this time. Simultaneously, hardware limitations, including detector dead-time, restrict the count rate so that the number of counts in a single time segment is extremely low. The dead-time also introduces nonlinear effects. The variance calculation in the scintillation index formula is then dominated by quantization error, and the scintillation index is severely overestimated. We investigate two methods of correcting the scintillation index based on data from a time-correlated single-photon counting laser radar system. The first approach is based on the covariance calculation of the data and can be used for very low count rates and high temporal resolution. This method may also be useful in other cases where the variance of noisy, time-resolved data needs to be calculated. The second method is based on fitting the theoretical probability density function for the intensity fluctuation caused by propagation through turbulence to the experimental data. This method can take dead-time effects into account and be used for higher count rates.

Amyloid Beta Oligomer Formation Analysis by Photon Counting Histogram

Amyloid beta (Ab) is 4 kDa peptide which is thought to form aggregates such as oligomers and fibrils, and to cause Alzheimer disease (AD). Recently, it has been suggested that soluble Ab oligomers are the causative agent of AD since such oligomers are more cytotoxic than fibrils. It was also suggested that Ab oligomers affect not only cell death but also early stage of cell dysfunction and cause memory loss.However, the mechanism how soluble oligomers are produced is still unknown. In this study, we analyzed formation of Ab oligomers in vitro at a single molecule level using photon counting histogram (PCH)(Chen, Y. et al. (1999) Biophys. J. 77, 553-67; Terada, N. et al. (2007) Biophys. J. 92, 2162-71.). Using PCH method, the number of protomers in oligomers and concentrations are obtained from histograms of photons from fluorescent molecules. Combination of a conforcal optics and a photon counting sensor enables us to catch the fluorescence from molecules diffusing through the conforcal volume at PCH. The concentration distribution of oligomers can be calculated from histograms.Fluorescent intensity of fluorescein-labeled Ab monomer (FL-Ab) was evaluated using PCH. After 30 min incubation of FL-Ab in buffer solution, dimer fraction was successfully observed with PCH, assuming that fluorescent intensity is in proportion to the number of protomers in oligomers. Detailed analysis of formation of Ab oligomers such as amyloid-beta derived diffusible ligands (ADDLs) using these techniques is in progress.

Effect of a Guard-Ring on the Leakage Current in a Si-PIN X-Ray Detector for a Single Photon Counting Sensor

PIN diodes for digital X-ray detection as a single photon counting sensor were fabricated on a floating-zone (FZ) n-type (111), high resistivity (5-10 kΩcm) silicon substrates (500μm thickness). Its electrical properties such as the leakage current and the breakdown voltage were characterized. The size of pixels was 100μm×100μm. The p+ guard-ring was formed around the active area to reduce the leakage current. After the p+ active area and guard-ring were fabricated by the ion-implantation, the extrinsic-gettering on the wafer backside was performed to reduce the leakage current by n+ ion-implantation. PECVD oxide was deposited as an IMD layer on front side and then, metal lines were formed on both sides of wafers. The leakage current of detectors was significantly reduced with a guard-ring when compared with that without a guard ring. The leakage current showed the strong dependency on the gap distance between the active area and the guard ring. It was possible to achieve the leakage current lower than 0.2nA/cm2.

Evaluation of mixed-signal noise effects in photon-counting X-ray image sensor readout circuits

In readout electronics for photon-counting pixel detectors, the tight integration between analog and digital blocks causes the readout electronics to be sensitive to on-chip noise coupling. This noise coupling can result in faulty luminance values in grayscale X-ray images, or as color distortions in a color X-ray imaging system. An exploration of simulating noise coupling in readout circuits is presented which enables the discovery of sensitive blocks at as early a stage as possible, in order to avoid costly design iterations. The photon-counting readout system has been simulated for noise coupling in order to highlight the existing problems of noise coupling in X-ray imaging systems. The simulation results suggest that on-chip noise coupling should be considered and simulated in future readout electronics systems for X-ray detectors.

Photon counting threshold optimization in mammography

The influence of detection threshold on the performance of ideal photon counting sensors is investigated in mammography for several X-ray tube anode/filter combinations and various breast thicknesses. Based on linear X-ray system theory, a method is proposed to take into consideration detector contrast modulation in the domain of X-ray energy as well as effects related to scattered radiation. Breast screening procedures are modelled as radiographic tasks in which low-contrast objects (tumours or microcalcifications) have to be detected in a noisy image background. For ideal photon counting detectors, results at zero spatial frequency show that higher detection thresholds must be implemented when imaging a thick compressed breast, compared to a thin compressed breast. In practice, the photon counting threshold also affects the image noise and the spatial resolution of hybrid pixel detectors, by tuning the amount of charge-shared events. To be generalized to more realistic imaging situations, this analysis must be performed over the full range of spatial frequencies and include additional detector noise parameters.

Performance limitations of a four-channel polarimeter in the presence of detection noise

The performance limitations of an imperfectly calibrated four- channel polarimeter in the presence of photon-counting detection noise is examined for arbitrary light levels. The theory described here provides a framework for developing polarimeter designs with minimal noise sen- sitivity and to evaluate the noise performance of existing designs. The polarimeter decomposes the input field into four channels, each of which is detected by the photon-counting sensor. We theoretically describe the propagation of detection noise through the polarimeter calibration matrix. The variances of both the mutual phase delay and the orthogonal inten- sities are given for photon-counting noise following Poisson statistics, and additive Gaussian detection noise. The variances of these param- eters depend on the average number of photons incident on the polar- imeter, the detector read noise, and errors in the polarimeter calibration matrix. A particular polarimeter design, whose calibration matrix is known exactly, is examined for moderate, low, and very low light levels. Theo- retical performance curves are shown for various sensor parameters and light levels, and are compared to simulated results. Very good agree- ment between theory and simulation is shown. The simulation validates the use of the Gaussian probability density function for the parallel (in- phase) and normal components of the phase fluctuations, and provides an accurate theoretical prediction of phase delay fluctuations for arbitrary light levels. The phase-delay noise cloud is illustrated for several cases.

Low digital interference counter for photon-counting pixel detectors

Single photon-counting pixel sensors are widely used in radiation imaging because of their energy resolving capability and high dynamic range. However, the close integration of the analog and digital circuits in the small pixel area puts critical constraints on the mixed mode circuit design. This includes the design considerations regarding the noise injection from the digital circuits into the analog part due to the digital switching. In addition, the large number of components requires a pixel circuit with low power consumption. In this paper, we address these two design constraints. An event counter architecture that decreases the digital switching and power consumption in a pixel is presented. The counter has the same dynamic range for capturing events and hardware cost as the conventionally used counters. We present design rules for adopting the counter architecture to a certain application. These rules enable the trade-off between readout accuracy from the pixel vs. the current in the digital parts of the pixel. Simulation of a case study circuit shows a reduction of digital activity by a factor seven, and a significant reduction of power consumption.

A new MOS photon-counting sensor operating in the above-breakdown regime

A solid-state optical sensor based on a buried-channel charge-transfer MOS structure and operated at voltages in the above, breakdown regime is proposed. In this mode of operation the MOS photosensor performs as a photon counter with, it is suggested two significant advantages over similar sensors based on p-n junction diodes, namely: self-quenching of the avalanche discharge and possible implementation in the form of a self-scanned CCD array. In this first demonstration of the proposed device, discrete structures in silicon are investigated experimentally. It is demonstrated that internal gains of 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> electrons/photon are possible during operation at about 10 V above breakdown. It is also shown that, after accousting for dark generation and retriggering effects, the photon-induced count rate saturates with increasing bias above breakdown. The results are in excellent agreement with the theoretical predictions from a two-dimensional model and imply that, at 10-15 V above breakdown avalanche initiation probabilities for electrons in excess of 0.9 have been attained.