Sum Mode Research Articles

Characterisation of the Medipix3 detector for 60 and 80 keV electrons

In this paper we report quantitative measurements of the imaging performance for the current generation of hybrid pixel detector, Medipix3, used as a direct electron detector. We have measured the modulation transfer function and detective quantum efficiency at beam energies of 60 and 80keV. In single pixel mode, energy threshold values can be chosen to maximize either the modulation transfer function or the detective quantum efficiency, obtaining values near to, or exceeding those for a theoretical detector with square pixels. The Medipix3 charge summing mode delivers simultaneous, high values of both modulation transfer function and detective quantum efficiency. We have also characterized the detector response to single electron events and describe an empirical model that predicts the detector modulation transfer function and detective quantum efficiency based on energy threshold. Exemplifying our findings we demonstrate the Medipix3 imaging performance recording a fully exposed electron diffraction pattern at 24-bit depth together with images in single pixel and charge summing modes. Our findings highlight that for transmission electron microscopy performed at low energies (energies <100keV) thick hybrid pixel detectors provide an advantageous architecture for direct electron imaging.

Study of gamma detection capabilities of the REWARD mobile spectroscopic system

REWARD is a novel mobile spectroscopic radiation detector system for Homeland Security applications. The system integrates gamma and neutron detection equipped with wireless communication. A comprehensive simulation study on its gamma detection capabilities in different radioactive scenarios is presented in this work. The gamma detection unit consists of a precise energy resolution system based on two stacked (Cd,Zn)Te sensors working in coincidence sum mode. The volume of each of these CZT sensors is 1cm3. The investigated energy windows used to determine the detection capabilities of the detector correspond to the gamma emissions from 137Cs and 60Co radioactive sources (662keV and 1173/1333keV respectively). Monte Carlo and Technology Computer-Aided Design (TCAD) simulations are combined to determine its sensing capabilities for different radiation sources and estimate the limits of detection of the sensing unit as a function of source activity for several shielding materials.

Validation of Geant4 Pixel Detector Simulation Framework by Measurements With the Medipix Family Detectors

Monte Carlo simulations are an extensively used tool for developing and understanding radiation detector systems. In this work, we used results of several chips and readout modes of the Medipix detector family to validate a Geant4 based pixel detector framework, developed in our group, that is capable of simulating particle tracking, charge transport in the sensor material and different readout schemes. We experimentally verified the simulation with different detector geometries in terms of pixel pitch and size as well as sensor material and sensor thickness. The single pixel mode (SPM) and charge summing mode (CSM) in Medipix3 were evaluated with fluorescence and synchrotron radiation. The integration of the charge sensitive amplifier functionality in the simulation framework allowed to simulate the time-over-threshold mode of the Timepix chip. Simulation and measurement have been compared in terms of spectral resolution using threshold scans in photon counting mode (Medipix3) and time over threshold mode (Timepix). Further comparisons were done using X-ray tube spectra and beta decay to cover a broad energy range. Additionally, TCAD simulations are performed as a comparison to a well-established simulation method. The results show good agreement between simulation and measurement.

Medipix3RX: Characterizing the Medipix3 Redesign With Synchrotron Radiation

The Medipix3RX is the latest version of the Medipix3 photon counting ASICs, which implements two new operational modes, with respect to the Medipix2 ASIC, aimed at eliminating charge shared events (referred to as Charge Summing Mode (CSM)) and at providing spectroscopic information (referred to as Colour Mode (CM)). The Medipix3RX is a redesign of the Medipix3v0 ASIC and corrects for the underperformance of CSM features observed in the previous version. This paper presents the results from synchrotron X-rays tests to evaluate the Medipix3RX ASIC performance. The newly implemented CSM algorithm eliminates the charge sharing effect at the same time as allocating the event to the readout pixel corresponding to the sensor pixel where the X-ray photon impinged. The new pixel trimming circuit led to a reduced dispersion between pixels. Further results of the linearity for all the gain modes, energy resolution and pixel uniformity are also presented.

How spectroscopic x-ray imaging benefits from inter-pixel communication

Spectroscopic x-ray imaging based on pixellated semiconductor detectors can be sensitive to charge sharing and K-fluorescence, depending on the sensor material used, its thickness and the pixel pitch employed. As a consequence, spectroscopic resolution is partially lost. In this paper, we study a new detector ASIC, the Medipix3RX, that offers a novel feature called charge summing, which is established by making adjacent pixels communicate with each other. Consequently, single photon interactions resulting in multiple hits are almost completely avoided. We investigate this charge summing mode with respect to those of its imaging properties that are of interest in medical physics and benchmark them against the case without charge summing. In particular, we review its influence on spectroscopic resolution and find that the low energy bias normally present when recording energy spectra is dramatically reduced. Furthermore, we show that charge summing provides a modulation transfer function which is almost independent of the energy threshold setting, which is in contrast to approaches common so far. We demonstrate that this property is directly linked to the detective quantum efficiency, which is found to increase by a factor of three or more when the energy threshold approaches the photon energy and when using charge summing. As a consequence, the contrast-to-noise ratio is found to double at elevated threshold levels and the dynamic range increases for a given counter depth. All these effects are shown to lead to an improved ability to perform material discrimination in spectroscopic CT, using iodine and gadolinium contrast agents. Hence, when compared to conventional photon counting detectors, these benefits carry the potential of substantially reducing the imaging dose a patient is exposed to during diagnostic CT examinations.

A new clocking method for a charge coupled device.

We propose and demonstrate a new clocking method for a charge-coupled device (CCD). When a CCD is used for a photon counting detector of X-rays, its weak point is a limitation of its counting rate, because high counting rate makes non-negligible pile-up of photons. In astronomical usage, this pile-up is especially severe for an observation of a bright point-like object. One typical idea to reduce the pile-up is a parallel sum (P-sum) mode. This mode completely loses one-dimensional information. Our new clocking method, panning mode, provides complementary properties between the normal mode and the P-sum mode. We performed a simple simulation in order to investigate a pile-up probability and compared the simulated result and actual obtained event rates. Using this simulation and the experimental results, we compared the pile-up tolerance of various clocking modes including our new method and also compared their other characteristics.

Performance of a Medipix3RX spectroscopic pixel detector with a high resistivity gallium arsenide sensor.

High resistivity gallium arsenide is considered a suitable sensor material for spectroscopic X-ray imaging detectors. These sensors typically have thicknesses between a few hundred μm and 1 mm to ensure a high photon detection efficiency. However, for small pixel sizes down to several tens of μm, an effect called charge sharing reduces a detector's spectroscopic performance. The recently developed Medipix3RX readout chip overcomes this limitation by implementing a charge summing circuit, which allows the reconstruction of the full energy information of a photon interaction in a single pixel. In this work, we present the characterization of the first Medipix3RX detector assembly with a 500 μm thick high resistivity, chromium compensated gallium arsenide sensor. We analyze its properties and demonstrate the functionality of the charge summing mode by means of energy response functions recorded at a synchrotron. Furthermore, the imaging properties of the detector, in terms of its modulation transfer functions and signal-to-noise ratios, are investigated. After more than one decade of attempts to establish gallium arsenide as a sensor material for photon counting detectors, our results represent a breakthrough in obtaining detector-grade material. The sensor we introduce is therefore suitable for high resolution X-ray imaging applications.

Reducing beam hardening effects and metal artefacts in spectral CT using Medipix3RX

This paper discusses methods for reducing beam hardening effects and metal artefacts using spectral x-ray information in biomaterial samples. A small-animal spectral scanner was operated in the 15 to 80 keV x-ray energy range for this study. We use the photon-processing features of a CdTe-Medipix3RX ASIC in charge summing mode to reduce beam hardening and associated artefacts. We present spectral data collected for metal alloy samples, its analysis using algebraic 3D reconstruction software and volume visualisation using a custom volume rendering software. The cupping effect and streak artefacts are quantified in the spectral datasets. The results show reduction in beam hardening effects and metal artefacts in the narrow high energy range acquired using the spectroscopic detector. A post-reconstruction comparison between CdTe-Medipix3RX and Si-Medipix3.1 is discussed. The raw data and processed data are made available (http://hdl.handle.net/10092/8851) for testing with other software routines.

Ultra Wide Band LPDA Design using HFSS

This paper describes the development of LPDA using HFSS. Log Periodic Antennas are antennas with high gain, large bandwidth, large front to back ratio and low cost. These antennas are quite convenient for commercial broadcast and radio signal detection. The UWB LPDA antenna is slightly modified to cover 1GHz to 4GHz bandwidth. The radiation patterns of sum and differential modes show good gain and null depth in the end-fire direction, respectively. The performance of the designed antenna is verified by finite integration and finite element method comparisons. The design of LPDA is reduced to attain better impedance matching.

Charge Summing in Spectroscopic X-Ray Detectors With High-Z Sensors

The spectroscopic performance of photon counting detectors is limited by the effects of charge sharing between neighboring pixels and the emission of characteristic X-rays. For these reasons, an event can be either missed or counted more than once. These effects become more and more of a concern when pixel pitches are reduced, and for the technology available so far, this meant that there would always be a trade-off between a high spatial and a high spectral resolution. In this work, we present first measurements obtained with the new Medipix3RX ASIC, which features a network of charge summing circuits establishing a communication between pixels which helps to mitigate these effects. Combined with cadmium telluride sensors, we show that this new technology is successful at improving a detector's spectroscopic capabilities even at pixel pitches as small as 55 μm. At this pitch, we measure an energy response function similar to that observed for a pixel pitch of 165 μm in the absence of a charge summing circuitry. This amounts to an effective reduction of the pixel area by at least one order of magnitude at a comparable energy response. Additionally, we present synchrotron measurements at high X-ray fluxes, where significant pulse pile-up occurs, and provide first experimental evidence for a net benefit when balancing spectroscopic performance and high flux tolerance in charge summing mode.

On the Frequency-Independent Modes of a Four-Arm Modulated Arm Width Spiral

This paper shows that among frequency independent (FI) conformal antennas, the modulated arm width (MAW) spiral has the simplest architecture for measuring both polarization and angle-of-arrival (AOA) instantaneously if frequency is known. The theory, design and performance of equiangular four-arm MAW spiral antennas when utilizing both sum modes (± 1) and difference mode ( +2) for direction-finding applications are discussed. It is shown that simultaneous dual-polarization and multi-mode capability is not achievable in more commonly-used four-arm spiral and sinuous antennas. While the co-polarized modes 1, 2 and 3 are well-researched with single-polarized four-arm equiangular spirals, they have not been utilized or controlled on a four-arm MAW spiral. Effects of modulation period on cross-polarization, pattern symmetry, gain, and impedance for all three pattern modes are investigated. The studies are carried out using full-wave modeling and a prototype MAW geometry is built and evaluated with a beamformer to demonstrate its wideband, dual-polarized, multi-mode capability.

Reconfigurable sum–difference pattern by means of parasitic elements for forward‐looking monopulse radar

This study describes the design of forward-looking monopulse arrays able to reconfigure the radiation pattern from the sum mode to the difference one by electronically switching a set of parasitic dipoles placed in front of a driven array of radiating dipoles. The antenna architecture is synthesised by optimising the geometric parameters of the passive elements, namely their positions and lengths. The generation of the difference beam is yielded by imposing a phase displacement of π to the excitations of half active array and activating the parasitic array by turning-on the switches that partition their lengths. As for the sum pattern, the effect of the parasitic dipoles is made negligible by turning-off the switches. A set of representative results is reported and discussed to show the effectiveness of the proposed approach.

Resonance-Enhanced Piezoelectric Microphone Array for Broadband or Prefiltered Acoustic Sensing

We report an array of piezoelectric monocrystalline silicon microphones for audio-range acoustic sensing. Thirteen cantilever-type diaphragm transducers make up the array, each having a closely spaced and precisely controlled resonant frequency. These overlapping resonances serve to greatly boost the sensitivity of the array when the signals are added; if the signals are individually taken, the array acts as a physical filter bank with a quality factor over 40. Such filtering would enhance the performance and the efficiency of speech-recognition systems. In the “summing mode,” the array demonstrates high response over a large bandwidth, with unamplified sensitivity greater than 2.5 mV/Pa from 240 to 6.5 kHz. Both modes of operation rely on the precise control of resonant frequencies, often a challenge with large compliant microelectromechanical-system (MEMS) structures, where residual stress causes deformation. We mitigate these ill effects through the use of stress-compensating layer thicknesses and a stress-free monocrystalline diaphragm. For determining device geometry, we develop a simple analytical method that yields excellent agreement between designed and measured resonant frequency; all devices are within 4.5%, and four are within 0.5% (just several hertz). The technique could be useful not only for microphones but also for other low-frequency MEMS transducers designed for resonance operation at a specific frequency.

The Medipix3RX: a high resolution, zero dead-time pixel detectorreadout chip allowing spectroscopic imaging

The Medipix3 chips have been designed to permit spectroscopic imaging in highly segmented hybrid pixel detectors. Spectral degradation due to charge sharing in the sensor has been addressed by means of an architecture in which adjacent pixels communicate in the analog and digital domains on an event-by-event basis to reconstruct the deposited charge in a neighbourhood prior to the assignation of the hit to a single pixel. The Medipix3RX chip architecture is presented. The first results for the characterization of the chip with 300 μm thick Si sensors are given. ∼ 72e− r.m.s. noise and ∼ 40e− r.m.s. of threshold dispersion after chip equalization have been measured in Single Pixel Mode of operation. The homogeneity of the image in Charge Summing mode is comparable to the Single Pixel Mode image. This demonstrates both modes are suitable for X-ray imaging applications.

Spectrum measurement using Medipix3 in Charge Summing Mode

We have obtained first spectrum measurements on a Medipix3 detector with a cadmium telluride (CdTe) sensor using Charge Summing Mode (CSM). It will be shown that CSM in Medipix3 is capable of reducing theadverse effects of charge sharing and fluorescent x-rays of CdTe onthe spectra recorded. The development of the Medipix All ResolutionSystem (MARS) x-ray camera has allowed us to explore this novel pixelcommunication feature in Medipix3. Spectrum measurements in this workwere carried out using a MARS camera consisting of a Medipix3 chipbump-bonded to a 1mm thick CdTe sensor layer. The characteristicpeaks of the Am-241 source as well as the spectroscopic properties ofthe CdTe sensor material were depicted at a spatial resolution of 55μm. Furthermore, a "connected component" algorithm shows a siliconbased Medipix3 is effective in reallocating spread charge into asingle pixel.

The detector simulation toolkit HORUS

In recent years, X-ray detectors used and developed at synchrotron sources and Free Electron Lasers (FELs) have become increasing powerful and versatile. However, as the capabilities of modern X-ray cameras grew so did their complexity and therefore their response functions are far from trivial. Since understanding the detecting system and its behavior is vital for any physical experiment, the need for dedicated powerful simulation tools arose.The HPAD Output Response fUnction Simulator (HORUS) was originally developed to analyze the performance implications of certain design choices for the Adaptive Gain Integrating Pixel Detector (AGIPD) and over the years grew to a more universal detector simulation toolkit covering the relevant physics in the energy range from below 1 keV to a few hundred keV. HORUS has already been used to study possible improvements of the AGIPD for X-ray Photon Correlation Spectroscopy (XPCS) at the European XFEL and its performance at low beam energies. It is currently being used to study the optimum detector layout for Coherent Diffration Imaging (CDI) at the European XFEL. Simulations of the charge summing mode of the Medipix3 chip have been essential for the improvements of the charge summing mode in the Medipix3 RX chip. HORUS is universal enough to support arbitrary hybrid pixel detector systems (within limitations). To date, the following detector systems are predefined within HORUS: The AGIPD, the Large Pixel Detector (LPD), the Cornell-Stanford Pixel Array Detector (CSPAD), the Mixed-Mode (MMPAD) and KEKPAD, and the Medipix2, Medipix3 and Medipix3 RX chips.

CMOS APD array for multi-function of both sequential and summing mode operations

A sensor is fabricated by a two poly-four metal 0.35 μm CMOS process in order to operate in the Geiger mode. The chip consists of an 8 × 8 array of 67 × 67 μm2 pixels that can be readout in either a summing mode or sequential mode. The features of the CMOS single photon avalanche photodiode (CMOS SPAD) were analyzed. The pixels were then arranged in a 8 × 8 array and the possibility of the summing mode and the sequential mode was verified. The CMOS SPAD pixel has a fill factor of 17.4%. The breakdown voltage of the pixel was at 18.9 V, and the photon detection efficiency was 20% at the 550 nm wavelength. The dead time was 20 nsec, and the dark count rate was 10 kHz at and below the single photon level. The operation of both summing mode and sequential mode function was sucessful in the 8 × 8 array sensor.

Generation of three-mode light entanglement via double-channel resonant four-wave mixing

We present a scheme to generate tripartite continuous-variable entanglement at the steady state by using an ensemble of N a five-level double- Λ − V atoms embedded in a three-mode cavity. The scheme is based on the two-channel resonant four-wave mixing, which leads to the parametric interaction between the sum mode of two fields from the upper double- Λ system and the mode from the lower V system. It is for such reason that three cavity modes are correlated and entangled. The present scheme has the following two striking features. One is that the steady-state tripartite entanglement is achieved under the full-resonance condition. Another feature is the fact that all the fields are coupled to the different atomic transitions, respectively. In particular, the tripartite entanglement occurs among the fields with the large frequency differences, which may be necessary for the progress of quantum information.

Characterization of Medipix3 With Synchrotron Radiation

Medipix3 is the latest generation of photon counting readout chips of the Medipix family. With the same dimensions as Medipix2 (256 × 256 pixels of 55 μm × 55 μm pitch each), Medipix3 is however implemented in an 8-layer metallization 0.13 μm CMOS technology which leads to an increase in the functionality associated with each pixel over Medipix2. One of the new operational modes implemented in the front-end architecture is the Charge Summing Mode (CSM). This mode consists of a charge reconstruction and hit allocation algorithm which eliminates event-by-event the low energy counts produced by charge-shared events between adjacent pixels. The present work focuses on the study of the CSM mode and compares it to the Single Pixel Mode (SPM) which is the conventional readout method for these kind of detectors and it is also implemented in Medipix3. Tests of a Medipix3 chip bump-bonded to a 300 μm thick silicon photodiode sensor were performed at the Diamond Light Source synchrotron to evaluate the performance of the new Medipix chip. Studies showed that when Medipix3 is operated in CSM mode, it generates a single count per detected event and consequently the charge sharing effect between adjacent pixels is eliminated. However in CSM mode, it was also observed that an incorrect allocation of X-rays counts in the pixels occurred due to an unexpectedly high pixel-to-pixel threshold variation. The present experiment helped to better understand the CSM operating mode and to redesign the Medipix3 to overcome this pixel-to-pixel mismatch.

Characterization of the Medipix3 pixel readout chip

The Medipix3 chip is a hybrid pixel detector readout chip working in Single Photon Counting Mode. It has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture charge deposited in overlapping clusters of four pixels is summed event-by-event and the incoming quantum is assigned as a single hit to the summing circuit with the biggest charge deposit (this mode of operation is called Charge Summing Mode (CSM)). In Single Pixel Mode (SPM) the charge reconstruction and the communication between neighbouring pixels is disabled. This is the operating mode in traditional detector systems.This paper presents the results of the characterization of the chip with electrical stimuli and radioactive sources.