Counting Pixel Research Articles

Readout electronics for low dark count pixel detectors based on Geiger mode avalanche photodiodes fabricated in conventional CMOS technologies for future linear colliders

High sensitivity and excellent timing accuracy of the Geiger mode avalanche photodiodes make them ideal sensors as pixel detectors for particle tracking in high energy physics experiments to be performed in future linear colliders. Nevertheless, it is well known that these sensors suffer from dark counts and afterpulsing noise, which induce false hits (indistinguishable from event detection) as well as an increase in the necessary area of the readout system. In this work, we present a comparison between APDs fabricated in a high voltage 0.35 mm and a high integration 0.13 mm commercially available CMOS technologies that has been performed to determine which of them best fits the particle collider requirements. In addition, a readout circuit that allows low noise operation is introduced. Experimental characterization of the proposed pixel is also presented in this work.

On the energy response function of a CdTe Medipix2 Hexa detector

X-ray imaging based on photon counting pixel detectors has received increased interest during the past years. Attached to a semiconductor of choice, some of these devices enable to resolve the spectral components of an image. This work presents the results from measuring the energy response function of a Medipix2 MXR Hexa detector, where six individual Medipix detectors were bump bonded to a 1 mm thick cadmium telluride sensor in order to form a 3×2 array of 4.2×2.8 cm 2 size. The average FWHM of the photo peak of an 241Am source was found to be 2.2 and 2.1 keV for single pixels and bias voltages of 200 and 350 V, respectively, across the whole Hexa detector. This corresponds to a relative energy resolution of less than 4%. Adding up all pixel spectra of individual chips lead to an only small deterioration of energy resolution, with line widths of 2.7 and 2.5 keV. In general, a lower detection efficiency was observed for the lower voltage setting, along with a shift of the peak position towards lower energies.

The scope of detector Medipix2 in micro-radiography of biological samples

We present our experimental setup devoted to high resolution X-ray micro-radiography that is suitable for imaging of small biological samples. The photon source is a FeinFocus micro-focus X-ray tube. The single photon counting pixel device Medipix2 serves as imaging area. Recently used imaging detectors as radiography films or scintillator detectors, cannot visualize required information about inner structure of scanned sample. Detectors Medipix2 do not suffer from so-called dark current noise and work in unlimited dynamic range. These features of detectors confer high quality and high contrast of final images. The radiographic imaging with detectors Medipix2 represents non-invasive and non-destructive method of investigation. Hereby, we demonstrate results of micro-radiographic study of internal structures of tiny biological samples. In addition to morphological and anatomical studies, we would like to present preliminary study of dynamic processes inside of organisms using micro-radiographic video-capturing.

10 years of the Medipix2 Collaboration

The Medipix2 Collaboration was started officially in September 1999 with the aim of disseminating hybrid pixel detector technology from High Energy Physics to other fields. The Collaboration was initially composed of 13 European research institutes. Over the ensuing 10 years the Collaboration expanded to reach a peak of 17 member institutes. Although our main scientific focus has been the development of the Medipix2 and Timepix single photon counting pixel detector readout chips the Collaboration members have expanded the range of applications for the technology to many more scientific fields than initially foreseen. We have signed a number of Technology Transfer Agreements during that time, most notably with PANalytical, whose commercially available PIXcel detector is based on the second version of the Medipix2 chip. This paper will review the history of the Collaboration covering as much as possible the main technical highlights. The success of the Collaboration is testimony to the willingness of a large number of groups and individuals to pool efforts for a common purpose. The paper will also cover some of those aspects and summarize the lessons learnt.

Si and CdTe pixel detector developments at SPring-8

Single X-ray photon counting pixel detectors have become the most advanced detector technology in synchrotron radiation experiments recently. In particular, the PILATUS detector based on a silicon sensor has reached a very mature state and represents the world’s largest detector in this field. This paper first reports on threshold energy calibrations and the capability of applying an energy-resolved X-ray imaging with PILATUS. Second the design of a cadmium telluride (CdTe) pixel detector is described. A high density and high-atomic number sensor material is required in high energy X-ray applications available at SPring-8. For this purpose we are developing a CdTe pixel detector with the SP8-01 readout ASIC covering a wide dynamic range between 10 and 100 keV and containing lower and upper discriminators.

Sequential monitoring of beach litter using webcams

This study attempts to establish a system for the sequential monitoring of beach litter using webcams placed at the Ookushi beach, Goto Islands, Japan, to establish the temporal variability in the quantities of beach litter every 90min over a one and a half year period. The time series of the quantities of beach litter, computed by counting pixels with a greater lightness than a threshold value in photographs, shows that litter does not increase monotonically on the beach, but fluctuates mainly on a monthly time scale or less. To investigate what factors influence this variability, the time derivative of the quantity of beach litter is compared with satellite-derived wind speeds. It is found that the beach litter quantities vary largely with winds, but there may be other influencing factors.

Single photon counting pixel detectors for synchrotron radiation experiments

At the Paul Scherrer Institute PSI an X-ray single photon counting pixel detector (PILATUS) based on the hybrid-pixel detector technology was developed in collaboration with SPring-8. The detection element is a 320 or 450μm thick silicon sensor forming pixelated pn-diodes with a pitch of 172μm×172μm. An array of 2×8 custom CMOS readout chips are indium bump-bonded to the sensor, which leads to 33.5mm×83.8mm detective area. Each pixel contains a charge-sensitive amplifier, a single level discriminator and a 20bit counter. This design realizes a high dynamic range, short readout time of less than 3ms, a high framing rate of over 200 images per second and an excellent point-spread function. The maximum counting rate achieves more than 2×106X-rays/s/pixel.

James Cameron's Avatar the Game Pandora's Bots

Journal Article James Cameron's Avatar the Game Pandora's Bots Get access Count Pixel Count Pixel Search for other works by this author on: Oxford Academic Google Scholar ITNOW, Volume 52, Issue 2, March 2010, Page 26, https://doi.org/10.1093/itnow/bwq150 Published: 25 February 2010

Spectroscopic X-ray imaging with photon counting pixel detectors

Single particle counting hybrid pixel detectors simultaneously provide low noise, high granularity and high readout speed and make it possible to build detector systems offering high spatial resolution paired with good energy resolution. A limiting factor for the spectroscopic performance of such detector systems is charge sharing between neighbouring pixels in the sensor part of the detector. The signal spectrum at the collection electrodes of the readout electronics deviates significantly from the photonic spectrum when planar segmented sensor geometries are used. The Medipix3 implements a novel, distributed signal processing architecture linking neighbouring pixels and aims at eliminating the spectral distortion produced in the sensor by charge sharing and at reducing the impact of fluorescence photons generated in the sensor itself. Preliminary results from the very first Medipix3 readouts bump bonded to 300 μ m Si sensor are presented. Material reconstruction is a possible future application of spectroscopic imaging detectors and an example of material reconstruction in small animal imaging is given.

Exploiting the MEDIPIX2 detector for the reconstruction of X-ray spectra

The single photon counting pixel detector MEDIPIX2 has been used in a variety of applications. Because of its spectral capability, the detector can be used as spectrometer. However, the measured spectrum is not the real incident spectrum as the MEDIPIX2 suffers from charge sharing because of its small pixel size.This implies that the energy information is smeared out heavily. Taking the measured spectrum and the responses to monoenergetic exposure we applied some deconvolution methods to determine the incoming spectrum.

Low Energy Dosimetry With Photon Counting Pixel Detectors Such as Medipix

Hybrid semiconductor photon counting pixel detectors like the Medipix detector have several advantages for an use in X-ray dosimetry. The noiseless photon counting principle allows to monitor low photon energies down to 3.5 keV. Due to the small pixel size (55 mum in case of Medipix2) dosimetry at very high dose rates is possible still processing each photon individually. The large amount of pixels in combination with the possible thickness of the sensor layer enables dosimetry at very low dose rates. A method has been developed to determine personal dose equivalents from the number of counts in energy deposition intervals measured with a semiconductor photon counting pixel detector, despite the strong influence of charge sharing effects among pixels. We tested the method experimentally by reconstructing the air kerma free in air for different qualities of X-radiation in the energy range below 150 keV with an accuracy better than 4%. We show that the response of a dosimeter based on a hybrid photon counting pixel detector can fulfill the IEC type testing requirements. The statistical precision is high due to the thickness and the large area of the sensor layer. We estimate that a dosimeter based on the Medipix detector will be able to cope with dose rates of more than approximately 57 Sv/h for mathdot Hp (0.07) or 19 Sv/h for mathdot Hp(10) . We outline the advantages and perspectives of using this kind of detector in a dosimeter in comparison to standard active personal dosimeters.

PILATUS: A single photon counting pixel detector for X-ray applications

The hybrid pixel technology combines silicon sensors with CMOS-processing chips by a 2D micro bump-bonding interconnection technology developed at Paul Scherrer Institute [C. Broennimann, E.F. Eikenberry, B. Henrich, R. Horisberger, G. Huelsen, E. Pohl, B. Schmitt, C. Schulze-Briese, M. Suzuki, T. Tomizaki, H. Toyokawa, A. Wagner. J. Synchrotron Rad. 13 (2005) 120 [1]; T. Rohe, C. Broennimann, F. Glaus, J. Gobrecht, S. Heising, M. Horisberger, R. Horisberger, H.C. Kaestl, J. Lehmann, S. Streuli, Nucl. Instr. and Meth. Phys. Res. A 565 (2006) 303 [2]]. PILATUS hybrid pixel detectors like other instruments [X. Llopart, M. Campell, R. Dinapoli, D. San Segundo, E. Pernigotti. IEEE Trans. Nucl. Sci. 49 (2002) 2279 [3]; N. Boudet, J.-F. Berar, L. Blanquart, P. Breugon, B. Caillot, J.-C. Clemens, I. Koudobine, P. Delpierre, C. Mouget, R. Potheau, I. Valin, Nucl. Instr. and Meth. Phys. Res. A 510 (2003) 41 [4]] are operating in the so-called “single photon counting mode”: Every X-ray quantum is directly converted into an electrical signal and counted by the detector system. Several prototype detectors in various geometries were produced, tested and established at different synchrotron beamlines worldwide. We explain the technology and present some recent highlights from various fields of applications.

Characterization of a mammographic system based on single photon counting pixel arrays coupled to GaAs x‐ray detectors

The authors report on the imaging capabilities of a mammographic system demonstrator based on GaAs pixel detectors operating in single photon counting (SPC) mode. The system imaging performances have been assessed by means of the transfer functions: The modulation transfer function (MTF), the normalized noise power spectrum, and the detective quantum efficiency (DQE) have been measured following the guidelines of the IEC 62220-1-2 protocol. The transfer function analysis has shown the high spatial resolution capabilities of the GaAs detectors. The MTF calculated at the Nyquist frequency (2.94 cycles/mm) is indeed 60%. The DQE, measured with a standard mammographic beam setup (Mo/Mo, 28 kVp, with 4 mm Al added filter) and calculated at zero frequency, is 46%. Aiming to further improve the system's image quality, the authors investigate the DQE limiting factors and show that they are mainly related to system engineering. For example, the authors show that optimization of the image equalization procedure increases the DQE(0) up to 74%, which is better than the DQE(0) of most clinical mammographic systems. The authors show how the high detection efficiency of GaAs detectors and the noise discrimination associated with the SPC technology allow optimizing the image quality in mammography. In conclusion, the authors propose technological solutions to exploit to the utmost the potentiality of GaAs detectors coupled to SPC electronics.

X-ray polarimetry by means of Compton scattering in the sensor of a hybrid photon counting pixel detector

For the first time a hybrid semiconductor photon counting pixel detector has been used for measurements of linear X-ray polarization by exploiting the Compton effect in the silicon sensor layer. The time-to-shutter mode of the X-ray imaging detector Timepix was used to identify Compton-scattering events in the sensor layer by the analysis of coincidences. For irradiation with polarized X-ray photons of energies between 27 and 84 keV we were able to measure a large modulation factor of μ meas = ( 68.1 ± 16.4 ) % for this type of X-ray polarimeter. Degree and orientation of linear polarization can be determined. This publication describes the experimental setup, data analysis method, measurement and simulation results, and gives first estimations on the polarimetric performance for an application in X-ray astronomy.

Influence of Microsilica Content on the Slag Resistance of Castables Containing Porous Corundum–Spinel Aggregates

Corrosion of five corundum–spinel castables containing the same porous aggregates and different matrices by converter slag (C/S=3) was conducted using the static crucible test through the counting pixels method, XRD, SEM, EDAX, and so on. It was found that the corrosion and penetration resistance depend on the microsilica content. Microsilica content strongly affects liquid phase formation in the matrix, changes the porosity, pore size distribution of matrix, and viscosity of slag penetrated, and thus affects its corrosion and penetration resistance. Increasing microsilica, decreasing the porosity, or increasing the viscosity of the penetrating slag improves, whereas increasing the pore size or liquid content reduces the penetration and corrosion resistance. The penetration resistance increased abruptly with an increase of the microsilica content from 0.65 to 1.95 wt%, but changed slightly with increase of the microsilica content from 1.95 to 3.90 wt%. The corrosion resistance increased with an increase of the microsilica content from 0 to 2.60 wt%, but slightly decreased with a further increase of the microsilica content to 3.90 wt%. 1.95–2.60 wt% microsilica leads to a compromise between low corrosion and low penetration.

Evaluation of a hybrid photon counting pixel detector for X-ray polarimetry

It has already been shown in literature that X-ray sensitive CCDs can be used to measure the degree of linear polarization of X-rays using the effect that photoelectrons are emitted with a non-isotropic angular distribution in respect to the orientation of the electric field vector of impinging photons. Up to now hybrid semiconductor pixel detectors like the Timepix-detector have never been used for X-ray polarimetry. The main reason for this is that the pixel pitch is large compared to CCDs which results in a much smaller analyzing power. On the other hand, the active thickness of the sensor layer can be larger than in CCDs leading to an increased efficiency. Therefore hybrid photon counting pixel detectors may be used for imaging and polarimetry at higher photon energies. For irradiation with polarized X-ray photons we were able to measure an asymmetry between vertical and horizontal double hit events in neighboring pixels of the hybrid photon counting Timepix-detector at room temperature. For the specific spectrum used in our experiment an average polarization asymmetry of ( 0.96 ± 0.02 ) % was measured. Additionally, the Timepix-detector with its spectroscopic time-over-threshold-mode was used to measure the dependence of the polarization asymmetry on energy deposition in the detector. Polarization asymmetries between 0.2% at 29 keV and 3.4% at 78 keV energy deposition were determined. The results can be reproduced with our EGS4-based Monte-Carlo simulation.

Pixel detectors for imaging with heavy charged particles

State-of-the-art single quantum counting pixel detectors offer a large potential for different imaging applications. The TimePix pixel device can provide information about position and energy of the detected radiation allowing radiography with charged particles. Heavy charged particles of known initial energy lose their energy partially by going through a specimen material. If the resulting energies of particles passing the specimen are measured, then specimen structure can be revealed. This article shows experimental results of this technique acquired with alpha particles and the TimePix detector. The spatial resolution in detector plane depends on particle energy and can reach submicrometer level. The specimen thickness can be determined with precision up to 320 nm for organic materials if energy loss of individual alpha particle is measured.

Glucose Modulates Basement Membrane Fibroblast Growth Factor-2 via Alterations in Endothelial Cell Permeability

The effects of glucose extremes on vascular physiology and endothelial cell function have been examined across a range of time scales. Not unexpectedly, chronic glucose exposure induces long term tissue effects. Yet short term exposure can also impose lasting consequences. The persistence of vascular pathology after euglycemic restoration further suggests a glucose exposure memory. Slow turnover reservoirs such as basement membrane are candidates for prolongation of acute events. We hypothesized that glucose-induced vascular dysfunction is related to altered vasoactive compound handling within the endothelial cell-basement membrane co-regulatory unit. Endothelial cell basement membrane-associated fibroblast growth factor-2 increased linearly with culture glucose within days of elevated glucose exposure. Surprisingly, basement membrane fibroblast growth factor-2 binding kinetics remained unchanged. The glucose-induced increase in basement membrane fibroblast growth factor-2 was instead related to enhanced endothelial cell fibroblast growth factor-2 release and permeability. Cellular fibroblast growth factor-2 release occurred concomitant with apoptosis but was not blocked by caspase inhibitors. These data suggest that release was associated with sub-lethal early apoptotic cell membrane damage, perhaps related to reactive oxygen species formation. High glucose basement membrane in turn enhanced endothelial cell proliferation in a fibroblast growth factor-2-dependent manner. We now show that glucose-induced alterations in endothelial cell function promote changes in basement membrane composition, and these changes further affect endothelial cell function. These data highlight the interrelationship of cell and basement membrane in pathological conditions such as hyperglycemia. These phenomena may explain long term effects on the endothelium of short term exposure to glucose extremes.

Charge sharing suppression using pixel-to-pixel communication in photon counting X-ray imaging systems

In planar silicon detector structures, the charge sharing between pixels is one limiting factor for colour X-ray imaging using integrated photon counting pixel detectors. 3D detector structures have been proposed as one solution to this problem. However, there are also readout system solutions to the problem, i.e., introducing pixel-to-pixel communication and distributed charge summing in the readout electronics. In this work, different charge summing schemes are evaluated using Monte Carlo simulation techniques. The increase in electronic noise introduced by the charge summing is one of the most severe problems. A proper selection of summing scheme is necessary to obtain an efficient system.

Discriminator threshold dependency of the zero-frequency DQE of photon-counting pixel detectors

In this paper, we apply the method of calculating the zero-frequency DQE using the concept of an average multiplicity to the Medipix2 detector. This photon counting pixel detector sets a discriminator threshold for the minimum energy needed to trigger a count. The average multiplicity of counts per detected photon and the detection efficiency depend on the threshold setting. Therefore, the DQE depends on this setting. We investigated the impact of physical effects on the DQE and present the according results generated with the Monte Carlo simulation ROSI for monoenergetic and for spectral X-ray sources. An optimum threshold setting can be derived.