Anode Pixels Research Articles

Performance study of the Multi-purpose Time Projection Chamber (MTPC) using a four-component alpha source

The Multi-purpose Time Projection Chamber (MTPC) is a gas-filled detector designed to measure cross sections of neutron-induced light-charged particle emission and fission reactions at the Back-streaming white neutron facility (Back-n) at the China Spallation Neutron Source. A prototype MTPC has been constructed with a drift volume, a resistive Micromegas charge amplification structure, and a high-density pixelated anode readout plane with 1519 hexagonal pads. Test measurements have been performed using a four-component alpha source. Alpha-particle track reconstruction is performed to obtain the deposited energy and length information of the trajectory. The performance test and characterization of the detector have served as the basis for the optimized design of the MTPC, which is currently undergoing commissioning at Back-n.

73‐3: A Study on WAD Improvement of a large size AMOLED panel

In this work, the WAD Characteristics were improved by forming a concave‐convex scattering structure on the organic planarization layer of large size AMOLED backplane TFT. By using a slit mask of the scanner as a VIA mask on the photosensitive poly‐imide organic planarization film, a semi‐transmissive area and a contact hole were made simultaneously without adding a separate photo process to form a concave‐convex scattering structure. On top of that, ITO/Ag/ITO anode pixel, EL and cathode layers were sequentially formed to improve WAD Characteristics of the top emitting AMOLED panel. Confirming the effect of WAD improvement and reduction of frontal luminance when forming a concave‐convex structure from the existing EL Spectrum through simulation, angle of concave‐convex structure is 10~15° to satisfy the conditions of 30% improvement in WAD, 5% drop in frontal luminance, 40‐50% of Concave‐convex structure area ratio in the planarization film was derived. As a result of fabricating a large size AMOLED top emitting panel, when Concave‐convex scattering angle of 5 ~ 12° is formed, WAD 10~24% improvement and frontal luminance drop within 5% can be seen. it was possible to confirm uniformly improved WAD value within Δu'v' 0.014 ~ 0.018 at the center/edge 9 Points compared to the normal panel.

Latest Technological Advances with MCP-PMTs

This paper discusses the most recent advances with 2-inch MCP-PMTs. The status in terms of lifetime, detection efficiency, rate capability, time resolution, and gain behavior in magnetic fields are summarized. Significant progress in collection (CE) and quantum efficiency (QE) led to an excellent detection efficiency DQE = QE*CE of ∼30%. With a new DAQ setup up to several hundred anode pixels can be read out simultaneously in 3D allowing a glance “inside the PMT”. Parameters like dark count rates and ion afterpulsing are measurable as a function of the incident photon position as well as the temporal and spacial spread of recoil electrons and electronic and charge-sharing crosstalk even inside a magnetic field.

Development of a hybrid single-photon imaging detector with embedded CMOS pixelated anode

The development of a single-photon detector based on a vacuum tube, transmission photocathode, microchannel plate and CMOS pixelated read-out anode is presented. This imager will be capable of detecting up to 1 billion photons per second over an area of 7 cm2, with simultaneous measurement of position and time with resolutions of about 5 microns and few tens of picosecond, respectively. The detector has embedded pulse-processing electronics with data-driven architecture, based on the Timepix4 ASIC, producing up to 160 Gb/s data that will be handled by a high-throughput FPGA-based external electronics and data acquisition system. These performances will enable significant advances in particle physics, life sciences, quantum optics or other emerging fields where the detection of single photons with excellent timing and position resolutions are simultaneously required.

Characterization of a CdZnTe detector for a low-power CubeSat application

We report spectral and imaging performance of a pixelated CdZnTe detector custom designed for the MeVCube project: a small Compton telescope on a CubeSat platform. MeVCube is expected to cover the energy range between 200 keV and 4 MeV, with a sensitivity comparable to the one of the last generation of larger satellites. In order to achieve this goal, an energy resolution of few percent in full width at half maximum (FWHM) and a 3-D spatial resolution of few millimeters for the individual detectors are needed. The severe power constraints present in small satellites require very low power read-out electronics for the detector. Our read-out is based on the VATA450.3 ASIC developed by Ideas, with a power consumption of only 0.25 mW/channel, which exhibits good performance in terms of dynamic range, noise and linearity. A 2.0 cm× 2.0 cm× 1.5 cm CdZnTe detector, with a custom 8 × 8 pixel anode structure read-out by a VATA450.3 ASIC, has been tested. A preliminary read-out system for the cathode, based on a discrete Amptek A250F charge sensitive pre-amplifier and a DRS4 ASIC, has been implemented. An energy resolution around 3% FWHM has been measured at a gamma energy of 662 keV; at 200 keV the average energy resolution is 6.5%, decreasing to ≲ 2% at energies above 1 MeV. A 3-D spatial resolution of ≈ 2 mm is achieved in each dimension.

Gap uniformity study of a resistive Micromegas for the Multi-purpose Time Projection Chamber (MTPC) at Back-n white neutron source

A Multi-purpose Time Projection Chamber prototype based on a resistive Micromegas technique has been proposed, designed and fabricated at China Spallation Neutron Source to measure the cross sections of neutron-induced light-charged particle emission reactions and fission reactions at the Back-n white neutron source. For the requirements of working stability with high voltage and high granularity of readout, the prototype is fabricated by applying a resistive Micromegas with high-density pixelated anode plane. In order to fully understand the process of charge spreading and signal induction on the anode plane covered by a resistive layer, a simulation method is developed and the energy correction function is then obtained, which can be used in the further energy correction in the experimental data analysis. Measured with a 55Fe X-ray source, the uniformity of gain and avalanche gap are specially analyzed by combining the simulated gas avalanche parameters with fitting of experimental data in which the parallel plate avalanche model is adopted.

Development of a single-photon imaging detector with pixelated anode and integrated digital read-out

We present the development of a single-photon detector and the connected read-out electronics. This “hybrid” detector is based on a vacuum tube, transmission photocathode, microchannel plate and a pixelated CMOS read-out anode encapsulating the analog and digital-front end electronics. This assembly will be capable of detecting up to 109 photons per second with simultaneous measurement of position and time.The pixelated read-out anode used is based on the Timepix4 ASIC (65 nm CMOS technology) designed in the framework of the Medipix4 collaboration. This ASIC is an array of 512 × 448 pixels distributed on a 55 μm square pitch, with a sensitive area of ∼7 cm2. It features 50–70 e− equivalent noise charge, a maximum rate of 2.5 Ghits/s, and allows to time-stamp the leading-edge time and to measure the Time-over-Threshold (ToT) for each pixel. The pixel-cluster position combined with its ToT information will allow to reach 5–10 μm position resolution. This information can also be used to correct for the leading-edge time-walk achieving a timing resolution of the order of 10 ps.The detector will be highly compact thanks to the encapsulated front-end electronics allowing local data processing and digitization. An FPGA-based data acquisition board, placed far from the detector, will receive the detector hits using 16 electro-optical links operated at 10.24 Gbps. The data acquisition board will decode the information and store the relevant data in a server for offline analysis.These performance will allow significant advances in particle physics, life sciences, quantum optics or other emerging fields where the detection of single photons with excellent timing and position resolutions are simultaneously required.

Achieving sub-pixel position resolution and sub-millimeter depth resolution using a pixelated CdZnTe detector

The CdZnTe detector, a room-temperature semiconductor detector, has many applications for nuclear safety and nuclear medicine because of its high energy resolution and small size when compared with scintillator detector and high purity Germanium detector respectively. Radionuclide imaging is one of the most important applications that requires adequate three-dimensional position resolution. In this work, we used a CdZnTe crystal from eV Products with a pixelated anode to achieve approximately 1.0 mm position resolution on an electrode plane and 0.9 mm position resolution in depth when the CdZnTe detector’s pixel pitch is 2 mm and thickness is 5 mm. We also improved the energy resolution of a 662 keV full-energy peak in a 137Cs energy spectrum to 1.27% after running a selection of full-energy deposited events based on depth, compared with 4.06% without this operation.

Investigation of Response of the Pixelated CZT SPECT Imaging System and Comparison with the Conventional SPECT System.

The aim of this study is to investigate the factors affecting the response of the pixelated CZT SPECT imaging systems and to compare the performance of these systems with the conventional SPECT imaging systems. By using the simulation technique, the effect of applied voltage, gap size between the anode pixels, and electron cloud mobility on the response of a pixelated CZT SPECT system are investigated. Then, the response of this system is compared with the conventional SPECT system in both single- and dual-radioisotope imaging. The results of this study show that increasing the applied voltage, electron cloud mobility or decreasing the gap size, in the optimal range of these parameters obtained in this study, leads to reducing the lateral charge diffusion and consequently improving the probability of the complete charge collection by the target anode pixel. In dual-radioisotope imaging by the pixelated CZT SPECT system, although higher energy resolution results in better separation of photopeaks, the presence of a low-energy tail leads to an overestimation of counts in the low-energy photopeak. The use of the optimal values for the applied voltage, gap size, and electron cloud mobility strongly affect the performance of the pixelated CZT SPECT systems. In addition, the presence of a tail restricts the use of these systems for dual-radioisotope imaging and also, the use of the conventional methods for scatter correction.

Test of a novel 2 * 2 multi-anode MCP-PMT

Xi’an Institute of Optics and Precision Mechanics of CAS is developing a novel MCP-PMT with 2 × 2 matrix anode pixels with a time resolution of picosecond level. It is composed of a cermet shell, bi-alkali photocathode, and V-type cascade MCP layers. In order to study the method of measuring the characteristics of such MCP-PMT with picosecond time resolution, we develop a PMT test system to test its performance. A voltage divider resistor circuit for the MCP-PMT is designed to meet the requirements of both high gain and fast time response. The test system includes a picosecond laser source in the single-photon mode, a high-bandwidth oscilloscope, and another fast MCP-PMT as reference. Full waveforms of PMT signals are collected and analyzed offline. The overall system time error is 26 ps. The test result shows that the gain of the MCP-PMT achieves 1 × 107. The transit time spread (TTS) of all channels is better than 60 ps (σ). The uniformity and crosstalk among adjacent channels are also studied by scanning each channel’s response along the photocathode plane. The MCP-PMT shows excellent timing performance and the potential of achieving better spatial resolution by further pixelation.

Identification of single-pixel incomplete charge collection events by using cathode waveform in pixelated CdZnTe detectors

Common grid can steer the electron cloud toward anode pixels to improve the charge collection efficiency in pixelated CdZnTe detectors. Due to insufficient steering and the absence of pixels on the outside of the edge pixels, the whole electron cloud cannot be collected completely by anode pixels for some events, resulting in the incorrect energy reconstruction by the deficient anode signal. Since single-pixel events are mainly used to obtain an excellent spectrum, incomplete charge collection events increase incorrect low-energy counts. However, the cathode signals are intact for incomplete charge collection events, and the cathode slope is related to energy deposition. Therefore, incomplete charge collection on the anode electrode can be identified by using the cathode slope to anode amplitude ratio (S/A). The experiments were performed on a pixelated 20 × 20 × 15 mm3 CdZnTe detector manufactured by Redlen Technologies. Incorrect low-energy counts are significantly decreased by rejecting single-pixel incomplete charge collection events, and the photopeak counts are improved by reconstructing the single-pixel incomplete charge collection events.

Growth of Large-Area Cd₀.₉Zn₀.₁Te Single Crystals and Fabrication of Pixelated Guard-Ring Detector for Room-Temperature γ-Ray Detection

Large-volume detector-grade Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Te (CZT) single crystals were grown using a vertical Bridgman technique. The bulk resistivity of the as-grown crystal was found to be ~5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> Q-cm from current-voltage measurements. A 19.0 × 19.0 × 5.0 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> block of the crystal was used to fabricate a detector with 10 × 10 pixelated anode configuration integrated with interpixel guard rings. The detector was tested for its radiation detection properties using 662-keV gamma rays from a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs source. The pixels exhibited well-resolved gamma pulse-height spectra (PHS) with percentage energy resolution ~1.6% at 662 keV. A few pixels exhibited tailing of the photopeak on the lower energy side indicating the presence of hole traps. Biparametric plots (BPs) were obtained from digitally recorded preamplifier pulses. The BPs showed anomalous behavior, which was correlated with the gamma interactions in the active region of the detector in virtual Frisch grid configuration. The BPs also enabled to extract PHS free from the extensive tailing of the photopeak.

Charge Sharing in (CdZn)Te Pixel Detector Characterized by Laser-Induced Transient Currents.

Performance of the (CdZn)Te pixelated detectors heavily relies on the quality of the underlying material. Modern laser-induced transient current technique addresses this problem as a convenient tool for characterizing the associated charge distribution. In this paper, we investigated the charge sharing phenomenon in (CdZn)Te pixel detector as a function of the charge collected on adjacent pixels. The current transients were generated in the defined 4 mm2 spots using 660 nm laser illumination. Waveforms measured on the pixel of interest and its surroundings were used to build the maps of the collected charge at different biases. The detailed study of the maps allowed us to distinguish the charge sharing region, the region with a defect, and the finest part in terms of the performance part of the pixelated anode. We observed the principal inhomogeneity complicating the assignment of the illuminated spot to the nearest pixel.

Investigating microchannel plate PMTs with TOFPET2 multichannel picosecond timing electronics

TOFPET2 is the second-generation design of a high-performance multichannel picosecond timing readout electronics ASIC produced by PETsys Electronics SA, Portugal. Originally developed for time-of-flight positron emission tomography using silicon photomultipliers, in this work we describe an experimental programme to evaluate the performance of TOFPET2 with pixelated microchannel plate (MCP) photomultiplier tube (PMT) detectors.Investigations were performed using various signal input methods; injected electronic stim signals, SiPM photodetectors, a single anode MCP photomultiplier detector and finally imaging with a demountable multi-anode MCP detector using a 16 x 16 pixelated multi-layer ceramic readout. The detector utilised an active anode plus image charge configuration to provide the positive signal pulses required by the electronics.Measurements were undertaken using the PETsys Time-of-Flight ASIC evaluation kit (mark2) operating in single photon counting mode. We present performance results for time resolution, cross channel coincidence time resolution and energy resolution using internal pulse amplitude measurement and time-over-threshold signal paths.Photodetector timing performance was evaluated using a 40 ps wide pulsed laser, operating at single photon level using a temperature stabilised setup within a dark enclosure. Amplitude walk correction was applied using the in-built time-over-threshold output from the TOFPET2 system. Single photon timing resolution of better than 110 FWHM was demonstrated. Preliminary results obtained using signal spread over adjacent pixels of a pixellated anode yield a coincidence timing resolution of ∼87 ps rms.

High-resolution pixelated CdZnTe detector prototype system for solar hard X-ray imager

A multichannel low-noise electronic prototype system was designed for a pixelated CdZnTe detector. This system is the result of preliminary work on a solar hard X-ray imager, which is one of the three payloads for future solar observations satellite- Advanced Space-based Solar Observatory (ASO-S). A new charge-sensitive amplifier application-specific integrated circuit, VATA450.3, with an on-chip analog-to-digital converter, is used to read out $$8 \times 8$$ anode pixel signals. Two CdZnTe detectors with a thickness of 2 mm and 5 mm were tested. The 2-mm-thick detector achieved energy resolution better than 5% (full-width at half-maximum, FWHM) at 59.5 keV, and the 5-mm-thick detector had better resolution than 1.2% (FWHM) at 662 keV. The design and test results of the prototype system are discussed in this paper.

Recent progress with microchannel-plate PMTs

Abstract Microchannel-plate (MCP) PMTs were identified as the only suitable photon sensors for the two DIRC detectors of the PANDA experiment at FAIR. As the long-standing aging problem of MCP-PMTs was recently overcome by coating the MCP pores with an atomic layer deposition (ALD) technique, further improved 2 ″ MCP-PMTs were investigated. The best PHOTONIS device has reached a lifetime of > 20 C/cm2 integrated anode charge without any sign of aging. Also the newly developed 2 ″ MCP-PMTs of Hamamatsu are maturing and are usable in high rate environments. The status of our long-term lifetime measurements and the performance parameters of the currently most advanced ALD-coated MCP-PMTs from PHOTONIS and Hamamatsu are presented. In addition, first results obtained with a new quality assurance setup for MCP-PMTs are discussed. This setup consists of a high performance DAQ system to measure the response of ≥ 64 anode pixels simultaneously. The system allows to study and quantify background parameters like position dependent dark count rates and ion afterpulsing as well as temporal and spacial distributions of recoil electrons and the effects of electronic and charge-sharing crosstalk among the anode pixels.

Conception and characterization of a virtual coplanar grid for a 11×11 pixelated CZT detector

Due to the low mobility of holes in CZT, commercially available detectors with a relatively large volume typically use a pixelated anode structure. They are mostly used in imaging applications and often require a dense electronic readout scheme. These large volume detectors are also interesting for high-sensitivity applications and a CZT-based blood gamma counter was developed from a 20×20×15 mm3 crystal available commercially and having a 11×11 pixelated readout scheme. A method is proposed here to reduce the number of channels required to use the crystal in a high-sensitivity counting application, dedicated to pharmacokinetic modelling in PET and SPECT. Inspired by a classic coplanar anode, an implementation of a virtual coplanar grid was done by connecting the 121 pixels of the detector to form intercalated bands. The layout, the front-end electronics and the characterization of the detector in this 2-channel anode geometry is presented. The coefficients required to compensate for electron trapping in CZT were determined experimentally to improve the performance. The resulting virtual coplanar detector has an intrinsic efficiency of 34% and an energy resolution of 8% at 662keV. The detector's response was linear between 80keV and 1372keV. This suggests that large CZT crystals offer an excellent alternative to scintillation detectors for some applications, especially those where high-sensitivity and compactness are required.

Hybrid Pixel-Waveform (HPWF) Enabled CdTe Detectors for Small Animal Gamma-Ray Imaging Applications.

This paper presents the design and preliminary evaluation of small-pixel CdTe gamma ray detectors equipped with a hybrid pixel-waveform (HPWF) readout system for gamma ray imaging applications with additional discussion on CZT due to its similarity. The HPWF readout system utilizes a pixelated anode readout circuitry which is designed to only provide the pixel address. This readout circuitry works in coincidence with a high-speed digitizer to sample the cathode waveform which provides the energy, timing, and depth-of-interaction (DOI) information. This work focuses on the developed and experimentally evaluated prototype HPWF-CdTe detectors with a custom CMOS pixel-ASIC to readout small anode pixels of 350 μm in size, and a discrete waveform sampling circuitry to digitize the signal waveform induced on the large cathode. The intrinsic timing, energy, and spatial resolution were experimentally evaluated in this paper in conjunction with methods for depth of interaction (DOI) partitioning of the CdTe crystal. While the experimental studies discussed in this paper are primarily for evaluating HPWF detectors for small animal PET imaging, these detectors could find their applications for ultrahigh-resolution SPECT and other imaging modalities.

Preliminary Research of CZT Based PET System Development in KAERI

Background: For positron emission tomography (PET) application, cadmium zinc telluride (CZT) has been investigated by several institutes to replace detectors from a conventional system using photomultipliers or Silicon-photomultipliers (SiPMs). The spatial and energy resolution in using CZT can be superior to current scintillator-based state-of-the-art PET detectors. CZT has been under development for several years at the Korea Atomic Energy Research Institute (KAERI) to provide a high performance gamma ray detection, which needs a single crystallinity, a good uniformity, a high stopping power, and a wide band gap. Materials and Methods: Before applying our own grown CZT detectors in the prototype PET system, we investigated preliminary research with a developed discrete type data acquisition (DAQ) system for coincident events at 128 anode pixels and two common cathodes of two CZT detectors from Redlen. Each detector has a 19.4×19.4×6 mm3 volume size with a 2.2 mm anode pixel pitch. Discrete amplifiers consist of a preamplifier with a gain of 8 mV·fC-1 and noise of 55 equivalent noise charge (ENC), a CR-RC4 shaping amplifier with a 5 μs peak time, and an analog-to-digital converter (ADC) driver. The DAQ system has 65 mega-sample per second flash ADC, a self and external trigger, and a USB 3.0 interface. Results and Discussion: Characteristics such as the current-to-voltage curve, energy resolution, and electron mobility life-time products for CZT detectors are investigated. In addition, preliminary results of gamma ray imaging using 511 keV of a 22Na gamma ray source were obtained. Conclusion: In this study, the DAQ system with a CZT radiation sensor was successfully developed and a PET image was acquired by two sets of the developed DAQ system.

Performance of common-grid pixelated CZT detector with different array geometries

A 4 × 4 common-grid pixelated CZT detector with four different array geometries has been designed and fabricated. The impact of small pixel effect, guide effect of the steering grid and edge effect on detector performance has been investigated. Both the weighting potential and the real electric field distributions have been calculated for better understanding the charge induction and collection. Results show that with constant pixel pitch, smaller anode pixel suffers from serious charge loss in volume and surface layer between anode pixels and steering grid. The small pixel effect is not strong enough to remove hole trailing for lager anode pixel. A relative high potential difference between anode pixels and steering grid leads to sufficient charge collection resulting in a better detector performance, especially for smaller anode pixels. In addition to edge and corner effects due to faulty or imperfect fabrication of the detector, the weighting potential distribution difference is an inherent physical effect that alter the profile of the induced signals in edge and corner pixels.