Probing the potential of CdZnTe for high-energy high-flux 2D X-ray detection using the XIDer incremental digital integrating readout

The latest synchrotron radiation sources have the capability to produce X-ray beams with a photon flux that can be up to three orders of magnitude higher than previous-generation facilities, and that are not manageable by the currently available 2D photon-counting pixel detectors. The construction of new detectors that exceed the limitations of existing devices is a critical strategic need. Developing such detectors is a challenge in terms of readout electronics as well as sensor material, particularly in the case of devices intended to operate at X-ray energies above 30 keV. The approach adopted at the ESRF to deal with this major difficulty is twofold: the use of a novel semiconductor material with improved electrical properties, high-flux CdZnTe, and the investigation of a specific readout scheme, incremental digital integration, via the XIDer project in collaboration with the University of Heidelberg. Incremental digital integration is a method intended to be less sensitive to variations of the dark current than the conventional charge integration readout. However, this readout scheme requires that the leakage current from the sensor material stays below a certain threshold to reduce the leakage contributions. This paper introduces the ESRF strategy and few examples of the methods employed to evaluate the performance and leakage current behavior of high-flux CdZnTe pixelated sensors. These examples illustrate the first results obtained with this material under moderate to very high X-ray irradiation fluxes of up to 1012 photons/mm2/s

Characterization of CdZnTe semiconductor for applications in advanced computed tomography

Cadmium zinc telluride (CdZnTe) is currently the only single crystalline semiconductor used in direct conversion gamma-cameras since CdZnTe can operate at room temperature, unlike other materials (e.g., germanium) that require liquid nitrogen cooling. Currently, CdZnTe-based single photon emission computed tomography (SPECT) cameras are successfully used in oncology and cardiology where they demonstrate diagnostic capabilities not achievable with indirect conversion SPECT technology. The extremely high energy resolution of CdZnTe is very promising for other spectroscopy applications including spectral CT (Computed Tomography). It has been shown that use of CdZnTe detectors in CT systems has the potential to improve image quality and diagnostic capabilities while reducing the radiation dose to a patient for a wide range of imaging tasks. However, state-of-the-art CT imaging systems operate at much larger x-ray flux rates than used in gamma cameras (up to 100 Mcps/mm2 in CT vs. 20-40 kcps/mm2 in nuclear medicine systems). High flux rate operation puts a stringent requirement on detector properties like electron and hole drift mobilities and uniformity of the internal electric field. [...

Cadmium Zinc Telluride (CZT) photon counting detector Characterisation for soft tissue imaging

The use of photon counting detection technology has resulted in significant X-ray imaging research interest in recent years. Computed Tomography (CT) scanners can benefit from photon-counting detectors, which are new technology with the potential to overcome key limitations of conventional CT detectors. Researchers are still studying the effectiveness and sensitivity of semiconductor detector materials in photon counting detectors for detecting soft tissue contrasts. This study aimed to characterize the performance of the Cadmium Zinc Telluride photon counting detector in identifying various tissues. An optimal frame rate per second (FPS) of CZT detector was evaluated by setting the X-ray tube voltage and current at 25 keV, 35 keV and 0.5 mA, 1.0 mA respectively by keeping the optimum FPS fixed, the detector energy thresholds were set in small steps from 15 keV to 35 keV and the Currents were set for X-ray tubes in ranges of 0.1 mA to 1.0 mA to find the relationship between voltage and current of the X-ray source and counts per second (CPS). The samples i.e., fat, liver, muscles, paraffin wax, and contrast media were stacked at six different thickness levels in a stair-step chamber made from Plexi-glass. X-ray transmission at six different thicknesses of tissue samples was also examined for five different energy (regions) thresholds (21 keV, 25 keV, 29 keV, 31 keV, and 45 keV) to determine the effect on count per second (CPS). In this study, 12 frames per second is found to be the optimum frame rate per second (FPS) based on the spectral response of an X-ray source and CPS has a linear relationship with X-ray tube current as well. It was also noted that A sample's thickness also affects its X-ray transmission at different energy thresholds. A high sensitivity and linearity of the detectors make them suitable for use in both preclinical and medical applications.Comment: 29 pages and 11 figure

Design, Fabrication and Characterization of a Unipolar Charge Sensing Amorphous Selenium X-ray Detector

Amorphous Selenium is a direct conversion photoconductor that has been widely used in X-ray imaging applications. Due to its high spatial resolution A-Se plays an important role in breast cancer screening and diagnostics, allowing for the detection of small and subtle lesions. However, a-Se has poor collection efficiency due to low carrier mobility and charge trapping resulting from its amorphous structure. The trapped charges can cause memory artifacts, including photocurrent lag, which can persist for several seconds after the X-ray pulse has ended. As a result, a-Se is a challenging material for dynamic imaging applications that require high spatial resolution. The research discussed in this thesis aims to investigate and address the temporal behavior of a-Se photoconductors, specifically the issue of lag, which can lead to image artifacts and degradation of image quality in dynamic imaging applications. The research involves the design of unipolar charge sensing detectors with pixel sizes of 20, 40, 80 and 150 microns to improve energy resolution and the temporal response compared to conventional a-Se detectors. Theoretical analysis and simulations are presented for the unipolar charge sensing detector including weighting potential, charge collection efficiency, pulse height spectroscopy and energy resolution which range from 5% to 2% . The work further discusses the fabrication process of the designed detector in the G2N lab at the university of Waterloo. It discusses the experimental results obtained and the challenges that were faced while fabricating the detector and how they can be overcome in the futur

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples

CsPbBr3 SINGLE CRYSTAL GROWTH FOR GAMMA-RAY DETECTOR

Detection of X-rays, gamma-rays and charged particles is crucial in the situation of homeland security, medical imaging and fundamental research. The thesis introduces the background of radiation and radiation detector. Then the mechanisms of spectral broadening are discussed. Charge collection, which is the main reason that degrade the resolution, is focused in the aspect of unbalanced electron-hole movement, traps and polarization, surface recombination, and noise. This thesis also reports a simple approach of additive-assisted inverse temperature crystallization (ITC) solution method to achieve high-quality CsPbBr3 single crystal growth. The crystals grown from regular precursor solution tend to grow fastest along [002] direction, resulting in long but small crystal bars. The introduction of additive changed the growth behavior, resulting in cuboid shape and large size single crystal. The detector made of solution-grown perovskite CsPbBr3 single crystal is able to acquire energy spectra from cesium-137 (137Cs) with a resolution of 5.5% at 662 keV.Master of Scienc

Transport phenomena in X and γ ray semi-insulator detector: a new charge correction approach

This research is part of the broader project of study and application of II-VI semi-insulating materials and especially of Cadmium and Zinc Telluride (CdZnTe or CZT). The current interest about these ternary compounds, such as semi-insulating materials for high energy photon detectors is mainly due to their high energy-gap that makes these materials ideal for applications at room temperature avoiding noise problems due to leakage current. Within this class of materials CZT is particularly appreciated both for the high-stopping power, due to the high atomic number of its components and its crystal structure, and for the high transport properties if compared to those of similar semiconductors. For these reasons CZT arises as an ideal candidate for high energy detector. The applications are numerous in several areas as security and environmental monitoring, storage of radioactive materials, medical instrumentation, space applications, astrophysics and cosmology. The problems of purity and homogeneity of the material are still far from being resolved. For this reason transport properties are still limited as compared to those of silicon, germanium and gallium arsenide, also in relation to the size of developed sensors (even several cubic centimeters). On increasing the photon energy the mean absorption depth arises with detriment of charge collection and spectroscopic property, with a consequent line broadening. In addition still persist difficulties regarding passivation and realization of contacts that could ensure low noise and an efficient charge collection. The main purpose of this work is to study charge collection processes and signal deterioration causes, improving the growth process and identifying appropriate methodologies for charge deficit correction, in order to create an electronic circuitry for data acquisition and signal correction.\\ The experimental activity was focused on the study of material grown by the Technology Group of IMEM-CNR Institute of Parma. This material have been grown with Boron Oxide Vertical Bridgman technique and used to create high energy detectors (10-700keV). The work can be divided into three main parts: 1) The material characterization, by means of photo-induced current, I-V characteristics and X and γ spectroscopy, to characterize the material and in particular to analyze bulk an superficial defects, impurity levels and the consequent transport properties in devices made by technology group of IMEM Institute. 2) The second one consists of theoretical model assessment to describe the material photo-response and the electronic read-out chain in order to obtain both the shape of the electronic signal and transport property informations. These model could be useful to correct the charge deficit through the information concerning the photon absorption depth in the crystal. 3) The third part concerns the development of data acquisition, filtering system and data elaboration. After a brief introduction about the fundamental issues involved, we make a careful analysis of these three aspects mentioned above. Finally we will discuss the thesis conclusions and the possible developments of this research. Other complementary activities, that play a minor role in the this research, can be found in the appendix.L'attività di ricerca si inserisce nel più ampio filone dello studio e dell'applicazione dei materiali semi-isolanti del tipo II-VI ed in particolare modo del Telloruro di Cadmio e Zinco (Cd1-xZnxTe o CZT). L’interesse attuale nei confronti di questi composti ternari, come materiali semi-isolanti per rivelatori di alte energie, è principalmente dovuta all’ampiezza dell’energy-gap (per altro modificabile variando le proporzioni dei componenti in particolare la frazione di Zinco), che permettendo di ovviare ai problemi di rumore dovuti alle correnti di leakage rende questi materiali ideali per applicazioni a temperatura ambiente. In questa classe di materiali il Cd1-xZnxTe (CZT) è particolarmente apprezzato sia per via dell'elevato stopping-power, dovuto all'elevato numero atomico dei suoi componenti e alla sua struttura cristallina, sia a causa delle elevate proprietà di trasporto, se paragonate a quelle di semiconduttori di questo tipo, e, come tale, si presenta come un valido candidato per il settore sensoristico anche grazie alla possibilità di essere prodotto in cristalli di grandi dimensioni. Le applicazioni sono molteplici nei settori della sicurezza e monitoraggio ambientale, stoccaggio di materiali radioattivi, della strumentazione medicale, delle applicazioni spaziali, dell’astrofisica e della cosmologia. I problemi legati alla purezza e all’omogeneità del materiale sono comunque tutt’altro che risolti a causa soprattutto di proprietà di trasporto limitate, se paragonate a quelle del Silicio, del Germanio e dell'Arseniuro di Gallio, e delle dimensioni macroscopiche dei sensori sviluppati (anche di diversi centimetri cubi). Le limitate proprietà di trasporto, limitando la carica raccolta dagli elettrodi all'aumentare della profondità di assorbimento del fotone, sono infatti la principale causa della riduzione delle proprietà spettroscopiche del materiale all'aumentare dell'energia dei fotoni coinvolti e del conseguente allargamento delle righe loro associate. Inoltre persistono tuttora problematiche relative alla passivazione e alla realizzazione di contatti che non distorcendo il campo assicurino un basso rumore e una efficiente raccolta di carica. L'obiettivo principale del progetto di tesi è quindi lo studio e la modellizzazione del processo di raccolta di carica e l’individuazione delle cause di deterioramento del segnale permettendo il miglioramento della qualità del materiale in fase di crescita e individuando metodologie adeguate per la correzione del deficit di carica raccolta dai rivelatori al fine di creare eventualmente un'elettronica per l’acquisizione e la correzione del segnale. L'attività si è focalizzata principalmente sullo studio del materiale cresciuto all'IMEM dal gruppo di Tecnologia con tecnica B2O3 Vertical Bridgman e utilizzato per realizzare rivelatori per alte energie(10-700KeV). Possiamo suddividere il lavoro essenzialmente in tre parti: 1. Una prima parte di studio sperimentale mediante tecniche di correnti foto-indotte, caratteristiche I-V e spettroscopia X e γ, necessaria alla caratterizzazione del materiale ed in particolare all'analisi dei difetti di bulk e di superficie, dei livelli di impurezza e delle conseguenti proprietà di trasporto sui dispositivi realizzati presso il gruppo di tecnologia dell'IMEM di Parma. 2. Una seconda parte “teorico-simulativa” relativa allo studio e alla simulazione di modelli che descrivano il comportamento del materiale e della catena elettronica di read-out al fine di studiare la forma del segnale elettronico generato, sia per ricavare informazioni sulle proprietà di trasporto sia per correggere il deficit di carica raccolta attraverso l'informazione sulla profondità dell'evento di assorbimento. 3. Una terza parte relativa allo sviluppo del sistema di acquisizione e filtraggio dati e alla creazione dei programmi di fitting necessari alla loro interpretazione. Dopo una breve introduzione sugli argomenti fondamentali coinvolti seguirà una attenta analisi di questi tre aspetti del lavoro di tesi sopra elencati. Infine discuteremo le conclusioni fondamentali a cui siamo giunti e brevemente ne esporremo i possibili sviluppi. Nelle appendici è possibile trovare altre attività di complemento al lavoro di dottorato che però hanno avuto un ruolo minore nello sviluppo del percorso di tesi

Development and Characterization of Lithium Indium Diselenide for Neutron Detection and Imaging Applications

Lithium indium diselenide [LISe] is under development as a single crystal semiconductor detector for neutron detection applications. Enriched in lithium-6, a neutron sensitive isotope, this wide-band gap semiconductor possesses the inherent neutron-gamma discrimination afforded by the thermal neutron capture reaction energy while providing distinct efficiency advantages over lithiated conversion layer detectors. The overarching theme of this work is to characterize the fundamental properties of this material to optimize its performance in neutron detection applications. The work presented here includes the identification of a suitable metallurgical contact for advanced detector fabrication, fundamental electronic property characterization, and proof-of-principle fast neutron imaging performance. Candidate contact materials were deposited through radio frequency magnetron sputtering. The primary metrics used to identify a robust contact were adhesion to the LISe surface and current voltage characteristics. Among the numerous contacts investigated, indium demonstrated the best adhesion properties. Its viability was demonstrated through the fabrication of a pixelated thermal neutron imaging detector (LTNI). Charge generation, transport, and trapping properties were investigated with emphasis on the stability of these properties post-operation in high thermal neutron flux fields. Neutron and alpha spectroscopy, photoinduced current transient spectroscopy, Raman spectroscopy, trap-filled limited voltage, and photoconductivity measurements were used to probe the charge transport and trapping mechanisms. Moderate transport properties were identified with respect to comparable technologies. Defect studies demonstrated that the type and density of defects strongly influenced performance of the detector. Encouraged by the performance of LTNI, an imaging detector was fabricated by coupling a LISe crystal to a 256 x 256 channel Timepix Application Specific Integrated Circuit to maximize spatial resolution. The fast neutron spatial resolution for 9MeV [electron-Volts] neutrons was investigated via a knife edge experiment. The measured efficiency was in agreement with the Evaluated Nuclear Data File cross-section database. The ultimate spatial resolution of the system was determined as 1.55 millimeters via the 10-90% decrease in contrast of the one-dimensional edge spread function. In conclusion, this material has been shown to exhibit suitable properties warranting further development for high efficiency slow neutron applications guided by the results of this work

Lead oxide (PbO) for direct conversion detectors

There is great interest in the utilization of non-crystalline photoconductors for direct conversion medical X-ray imaging detectors. Lead Oxide (PbO) is one of the most promising candidates for application in general radiography and fluoroscopy since it possesses high theoretical X-ray-to-charge conversion gain and high X-ray stopping power due the high Z of Pb (Z = 82). A further advantage of PbO compared with other photoconductors (like polycrystalline layers of HgI2, PbI2, CZT) is the absence of heavy absorption edges up to 88 keV, which inherently offers higher spatial resolution. Therefore, PbO exhibits many of the requirements for an effective detector material. However, although very promising, poly-PbO layers are known to exhibit a relatively high dark current, an incomplete charge collection and a residual signal after the end of X-ray exposure, called signal lag. The reported lag was the main obstacle of this poly-PbO-based detector prototype, since this restricts applications to static imaging only and obscures the full potential of PbO in medical imaging. Another disadvantage is the high porosity of poly-PbO and its structural instability in air, which makes this material challenging for practical utilization in X-ray detectors. To combat the above problems (i.e. lag, porosity and structural degradation in air) we have advanced PbO deposition technology and developed a novel type of amorphous lead oxide (a-PbO). The obtained a-PbO layers have near single-crystal bulk densities and are stable in air. In addition, the layers are capable of withstanding higher electric fields, while at the same time iii exhibiting lower dark currents than previously achieved. Also, the temporal response and charge yield were significantly improved and compare favorably with published results on conventional films of polycrystalline PbO and even a-Se – the only photoconductor currently utilized in direct conversion detectors. Our measured X-ray response show almost complete elimination of signal lag to a level sufficient for the high-speed operations. Our advances in PbO technology allow us to utilize the full potential of PbO for medical X-ray imaging applications