Photon Counting Chip Research Articles

Characterization of the Photon Counting CHASE Jr., Chip Built in a 40-nm CMOS Process With a Charge Sharing Correction Algorithm Using a Collimated X-Ray Beam

This paper presents the detailed characterization of a single photon counting chip, named CHASE Jr., built in a CMOS 40-nm process, operating with synchrotron radiation. The chip utilizes an on-chip implementation of the C8P1 algorithm. The algorithm eliminates the charge sharing related uncertainties, namely, the dependence of the number of registered photons on the discriminator’s threshold, set for monochromatic irradiation, and errors in the assignment of an event to a certain pixel. The article presents a short description of the algorithm as well as the architecture of the CHASE Jr., chip. The analog and digital functionalities, allowing for proper operation of the C8P1 algorithm are described, namely, an offset correction for two discriminators independently, two-stage gain correction, and different operation modes of the digital blocks. The results of tests of the C8P1 operation are presented for the chip bump bonded to a silicon sensor and exposed to the 3.5- $\mu \text{m}$ -wide pencil beam of 8-keV photons of synchrotron radiation. It was studied how sensitive the algorithm performance is to the chip settings, as well as the uniformity of parameters of the analog front-end blocks. Presented results prove that the C8P1 algorithm enables counting all photons hitting the detector in between readout channels and retrieving the actual photon energy.

Measurements of Ultra-Fast single photon counting chip with energy window and 75 μm pixel pitch with Si and CdTe detectors

Single photon counting pixel detectors become increasingly popular in various 2-D X-ray imaging techniques and scientific experiments mainly in solid state physics, material science and medicine. This paper presents architecture and measurement results of the UFXC32k chip designed in a CMOS 130 nm process. The chip consists of about 50 million transistors and has an area of 9.64 mm × 20.15 mm. The core of the IC is a matrix of 128 × 256 pixels of 75 μm pitch. Each pixel contains a CSA, a shaper with tunable gain, two discriminators with correction circuits and two 14-bit ripple counters operating in a normal mode (with energy window), a long counter mode (one 28-bit counter) and a zero-dead time mode. Gain and noise performance were verified with X-ray radiation and with the chip connected to Si (320 μm thick) and CdTe (750 μ m thick) sensors.

Spectral Hounsfield units: a new radiological concept

Computed tomography (CT) uses radiographical density to depict different materials; although different elements have different absorption fingerprints across the range of diagnostic X-ray energies, this spectral absorption information is lost in conventional CT. The recent development of dual energy CT (DECT) allows extraction of this information to a useful but limited extent. However, the advent of new photon counting chips that have energy resolution capabilities has put multi-energy or spectral CT (SCT) on the clinical horizon. This paper uses a prototype SCT system to demonstrate how CT density measurements vary with kilovoltage. While radiologists learn about linear attenuation curves during radiology training, they do not usually need a detailed understanding of this phenomenon in their clinical practice. However SCT requires a paradigm shift in how radiologists think about CT density. Because radiologists are already familiar with the Hounsfield Unit (HU), it is proposed that a modified HU be used that includes the mean energy used to obtain the image, as a conceptual bridge between conventional CT and SCT. A suggested format would be: HU(keV). • Spectral computed tomography uses K-edge and slope effects to identify element signatures. • New visualisation tools will be required to efficiently display spectral CT information. • This paper demonstrates HU variation with keV using the Medipix3 chip. • HU ( keV ) is a suggested format when stating spectral HU measurements.

Performance and Applications of the CdTe- and Si-XPAD3 photon counting 2D detector

The XPAD3 is the third generation of a single photon counting chip developed in collaboration by SOLEIL Synchrotron, the Institut Néel and the Centre de Physique de Particules de Marseille (CPPM). The chip contains 9600 pixels of 130 μm side and a counting electronic chain with an adjustable low level threshold in each pixel. Imaging and detection performance (detective quantum efficiency, modulation transfer function and energy resolution) of the XPAD3 detectors hybridized with Si and CdTe sensors have been evaluated and compared using monochromatic synchrotron X-rays beam. A second version of the chip, optimized for pump-probe experiments, has been realized and successfully tested. Three 7.3 cm x 12.5 cm Si-XPAD3 imagers, composed of 8 silicon modules (7 chips per module) and one 2.1 cm x 3.1 cm CdTe-XPAD3 imager (4 chips) have been constructed and successfully used for synchrotron diffraction experiments and biomedical imaging.

Detector commissioning and development for PETRA-III

The new synchrotron light source PETRA-III produced its first beam last year. The extremely high brilliance of PETRA-III and the large energy range of many of its beamlines make it useful for a wide range of experiments, particularly in materials science. The detectors at PETRA-III will need to meet several requirements, such as operation across a wide dynamic range, high-speed readout and good quantum efficiency even at high photon energies. PETRA-III beamlines with lower photon energies will typically be equipped with photon-counting silicon detectors for two-dimensional detection and silicon drift detectors for spectroscopy and higher-energy beamlines will use scintillators coupled to cameras or photomultiplier tubes. Longer-term developments include ‘high-Z’ semiconductors for detecting high-energy X-rays, photon-counting readout chips with smaller pixels and higher frame rates and pixellated avalanche photodiodes for time-resolved experiments.

Correction of the charge sharing in photon-counting pixel detector data

Although photon-counting pixel detector arrays provide energy discrimination capabilities, their energy resolution is limited by the diffusion of the photoelectric charge between neighboring pixels. This effect often referred to as charge sharing induces a low-energy tail in the measured spectra and is particularly significant for small pixel sizes. In order to correct this effect, an iterative deconvolution method has been developed and applied to data collected with the Medipix2 photon-counting chip. The obtained spectra are corrected from charge sharing and moreover show a better energy resolution than the raw spectra. For instance the Kα (22.1 keV) and Kβ (25 keV) emission lines of a 109Cd source are reconstructed as two well-separated peaks. The principle of the method will be exposed. Various results obtained with synchrotron beams, X-ray tubes and radioactive sources will be shown.

A 20 kpixels CdTe photon-counting imager using XPAD chip

A 20kpixels CdTe sensor has been hybridized on XPAD3S CMOS photon-counting chips, forming a 19,200pixels imaging device. P-type CdTe with rectifying contact has been employed. This sensor works in hole collection mode with a pulse shaping time of about 150ns. Detector construction and operation are described, and first results obtained with 241Am source as well as diffraction images using an X-ray synchrotron beam are presented. Polarization effects are present, but remain at a very manageable level.

Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements

A novel approach for the readout of a TPC at the future linear collider is to use a CMOS pixel detector combined with some kind of gas gain grid. A first test using the photon counting chip Medipix2 with GEM or Micromegas demonstrated the feasibility of such an approach. Although this experiment demonstrated that single primary electrons could be detected the chip did not provide information on the arrival time of the electron in the sensitive gas volume nor did it give any indication of the quantity of charge detected. The Timepix chip uses an external clock with a frequency of up to 100 MHz as a time reference. Each pixel contains a preamplifier, a discriminator with hysteresis and 4-bit DAC for threshold adjustment, synchronization logic and a 14-bit counter with overflow control. Moreover, each pixel can be independently configured in one of four different modes: masked mode: pixel is off, counting mode: 1-count for each signal over threshold, TOT mode: the counter is incremented continuously as long as the signal is above threshold, and arrival time mode: the counter is incremented continuously from the time the first hit arrives until the end of the shutter. The chip resembles very much the Medipix2 chip physically and can be readout using slightly modified versions of the various existing systems. This paper presents the main features of the new design, electrical measurements and some first images.

XPAD3: A new photon counting chip for X-ray CT-scanner

The X-ray pixel chip with adaptable dynamics (XPAD3) circuit is the next generation of 2D X-ray photon counting imaging chip to be connected to a pixel sensor using the bump and flip-chip technologies. This circuit, designed in IBM 0.25 μm technology, contains 9600 pixels (130 μm×130 μm) distributed into 80 columns of 120 elements each. Its features have been improved to provide high-counting rate over 10 9 ph/pixel/mm 2, high-dynamic range over 60 keV, very low-noise detection level of 100e − rms, energy window selection and fast image readout less than 2 ms/frame. An innovative architecture has been designed in order to prevent the digital circuits from bothering the very sensitive analogue parts placed in their neighbourhood. This allows to read the chip during acquisition while conserving the precise setting of the thresholds over the pixel array. Finally, the aim of this development is to combine several XPAD3 to form the PIXSCAN detector.

PIXSCAN: Pixel detector CT-scanner for small animal imaging

The PIXSCAN is a small animal CT-scanner based on hybrid pixel detectors. These detectors provide very large dynamic range of photons counting at very low detector noise. They also provide high counting rates with fast image readout. Detection efficiency can be optimized by selecting the sensor medium according to the working energy range. Indeed, the use of CdTe allows a detection efficiency of 100% up to 50 keV. Altogether these characteristics are expected to improve the contrast of the CT-scanner, especially for soft tissues, and to reduce both the scan duration and the absorbed dose. A proof of principle has been performed by assembling into a PIXSCAN-XPAD2 prototype the photon counting pixel detector initially built for detection of X-ray synchrotron radiations. Despite the relatively large pixel size of this detector (330×330 μm 2), we can present three-dimensional tomographic reconstruction of mice at good contrast and spatial resolution. A new photon counting chip (XPAD3) is designed in sub-micronique technology to achieve 130×130 μm 2 pixels. This improved circuit has been equipped with an energy selection circuit to act as a band-pass emission filter. Furthermore, the PIXSCAN-XPAD3 hybrid pixel detectors will be combined with the Lausanne ClearPET scanner demonstrator. CT image reconstruction in this non-conventional geometry is under study for this purpose.

USB interface for Medipix2 pixel device enabling energy and position-sensitive detection of heavy charged particles

Abstract We have designed an interface board between the Medipix2 single photon counting chip and Universal Serial Bus (USB), which is presently the most widespread PC interface. All necessary detector support is integrated into one compact system ( 80 × 50 × 20 mm 3 ) including the detector bias source (up to 100 V). Power supply is internally derived from the voltage provided by the USB connection, so no external device is required. This solution allows to achieve maximum portability of the measurement setup. One of the most significant advantages is the support of back-side pulse processing. The charge generated by an ionizing particle is measured in the bias circuit and can be used for spectroscopic purposes or for triggering. With the present design, the energy resolution of such spectrometry is about 44 keV.The interface board has been designed for connection with present chip-boards carrying one or four Medipix2 chips.

Charge-sharing observations with a CdTe pixel detector irradiated with a57Co source

Abstract Charge sharing is a limiting factor of detector spatial resolution and contrast in photon counting imaging devices because multiple counts can be induced in adjacent pixels as a result of the spread of the charge cloud generated from a single X-ray photon of high energy in the detector bulk. Although this topic has been debated for a long time, the full impact of charge sharing has not been completely assessed. In this work, we look at the importance of charge sharing in CdTe pixel detectors by exposing such a device to a low-activity (37 kBq)57Co source, whose main emission line is at 122 keV.The detectors used are 1 mm thick with a pixel pitch of 55 μm. These detectors are bump-bonded to Medipix2 photon-counting chips. This study gives an insight of the impact on the design and operation of pixel detectors coupled to photon-counting devices for imaging applications.

Performance limits of a 55-/spl mu/m pixel CdTe detector

In this work, the results from simulation of a CdTe pixel detector with a pixel size of 45 mum and a pitch of 55 mum are presented. Simulation of charge sharing between neighboring pixels has been compared with experimental results obtained by the use of the Medipix2 photon counting chip by means of a unique read-out system developed for the Dear-Mama project. Electrical simulations have been performed using the commercial software package DESSIS (ISETCAD) and combined with the Monte Carlo code GEANT4 to simulate the process of interaction and subsequent charge transport of charges induced by radiation. This allowed the analysis of charge sharing between pixel elements, an important limiting factor in imaging applications. Simulation was compared with experimental results obtained by means of a 1 mm thick CdTe detector. Each of these pixel electrodes was connected to the corresponding readout pixel on the Medipix2 chip, via an indium bump-bond. The backside contact was biased at -100 V, so that the pixel electrodes were collecting electrons. The results obtained imply that using a detector 1 mm thick with a pixel size smaller than 55 mum in single photon counting mode is unrealistic, because such fine spatial resolution cannot be attained in the corresponding image due to charge sharing

Performance of an imaging system based on silicon pixel detectors of different thickness

To compare the characteristics of imaging systems based on pixel detectors of different thickness, we have measured the respective imaging capabilities, spatial resolution and spectroscopic characteristics. Each system consists of a single photon counting chip, developed in the framework of the Medipix Collaboration, bump bonded to a silicon detector. The detector is a matrix of 64 /spl times/ 64 square pixels, with 170 /spl mu/m pitch and thickness ranging from 300 to 800 /spl mu/m. As expected, the intrinsic detection efficiency increases with the detector thickness; nevertheless the spatial resolution can be affected by a charge sharing mechanism between adjacent pixels due to charge diffusion. The aim of the work reported in this paper is to demonstrate that, optimizing the settings of the image systems, we can increase the detection efficiency without losing in spatial resolution.

First detective quantum efficiency measurement of 500 μm silicon hybrid pixel sensor with photon counting readout for X-ray medical imaging

We report the performances of a 500 μm pixellated silicon sensor bonded to the photon counting chip Medipix2 [1]. In order to perform an absolute characterization of our detector, we measured both the pre-sampling MTF and NPS with respect to the International standard IEC-62220-1. From those data we have been able to extract the Detective Quantum Efficiency (DQE) and hence to assess the suitability of our detector for X-ray medical imaging purpose. Due to poor absorption of the Si at 70 kV the DQE peaked at 0.06 for null frequency. Nevertheless, these results are very promising since thicker Si or more absorbing material such as GaAs will soon be available.

Fabrication and characterization of n-on-n silicon pixel detectors compatible with the Medipix2 readout chip

Pixel detectors for mammographic applications have been fabricated at ITC-irst on 800 μm thick silicon wafers adopting a double side n +-on-n fabrication technology. The activity aims at increasing the X-ray detection efficiency in the energy range of interest minimizing the risk of electrical discharges in hybrid systems operating at high voltages. The detectors, having a layout compatible with the Medipix2 photon counting chip, feature two different design solutions for the p-isolation between neighboring n +-pixels. We report on the characterization of the fabrication process and on preliminary results of electrical measurements on full detectors and pixel test structures. In particular, we found that the detectors can be reliably operated above the full depletion voltage regardless of the isolation design, that however, impacts the performances in terms of current–voltage characteristics, single pixel currents, inter-pixel resistances and inter-pixel capacitances.

Design and test of data acquisition systems for the Medipix2 chip based on PC standard interfaces

We describe two readout systems for hybrid detectors using the Medipix2 single photon counting chip, developed within the Medipix Collaboration. The Medipix2 chip (256×256 pixels, 55 μm pitch) has an active area of about 2 cm 2 and is bump-bonded to a pixel semiconductor array of silicon or other semiconductor material. The readout systems we are developing are based on two widespread standard PC interfaces: parallel port and USB (Universal Serial Bus) version 1.1. The parallel port is the simplest PC interface even if slow and the USB is a serial bus interface present nowadays on all PCs and offering good performances.

Digital autoradiography with a Medipix2 hybrid silicon pixel detector

A digital imaging system for beta and gamma autoradiography has been realized. It is a hybrid pixel detector based on a Medipix2 circuit (a single photon counting chip), developed at CERN in the framework of the Medipix2 European collaboration, as a successor of the Medipix1 chip. Medipix2 is realized in 0.25-/spl mu/m CMOS technology; it has a sensitive area of 14 /spl times/ 14 mm/sup 2/ and a pixel pitch of 55-/spl mu/m and is bump bonded to a 300-/spl mu/m-thick silicon pixel detector. A test of this imaging system for digital autoradiography, with a detection threshold of about 6 keV, shows a noise in the order of 10/sup -3/ cps/mm/sup 2/, a minimum detectable activity of 0.32 Bq in 14 h for /sup 3/H, and 0.012 Bq in 10 h for /sup 14/C. A preliminary spatial resolution test for /sup 14/C gives a full-width at half-maximum resolution of about 80 /spl mu/m. Real-time images of /sup 3/H, /sup 14/C, /sup 125/I autoradiographic microscales are shown, and a comparison with existing experimental devices is presented.

Photon-counting chip for avalanche detectors

We present a microelectronic chip that can replace bulky and expensive single-photon-counting boards in many applications. The chip can be assembled with the detector into a microscopic detection head and communicates straight to a host computer. The chip includes an active quenching and active reset circuit for driving a single-photon avalanche diode, acquisition electronics for collecting the photon counts, control logic for timing the measurement windows, and serial communication for remote control and data uploading. The circuit is designed to operate avalanche photodiodes from 6 up to 50 V above breakdown, in order to detect single photons. After each ignition, the chip quenches and resets the detector with fast well-defined transitions of few nanoseconds and holds it off for an adjustable duration up to 1 /spl mu/s. Measurement time slots are internally generated and are adjustable from 1 to 20 ms, with steps of 70 /spl mu/s. Free-running and gated-detector operation are provided, with a saturated photon-counting rate of 20 Mcounts/s.

X-ray energy selected imaging with Medipix II

Two different X-ray tube accelerating voltages (60 and 70kV) are used for diagnosis of front teeth and molars. Different energy ranges are necessary as function of tooth thickness to obtain similar contrast for imaging. This technique drives the costs for the X-ray tube up and allows for just two optimized settings. Energy range selection for the detection of the penetrating X-rays would overcome these severe setbacks. The single photon counting chip MEDIPIX2 http://www.cern.ch/medipix exhibits exactly this feature.First simulations and measurements have been carried out using a dental X-ray source. As a demonstrator a real tooth has been used with different cavities and filling materials. Simulations showed in general larger improvements as compared to measurements regarding SNR and contrast: A beneficial factor of 4% wrt SNR and 25% for contrast, measurements showed factors of 2.5 and up to 10%, respectively.