Needs, trends, and advances in scintillators for radiographic imaging and tomography

Scintillators are important materials for radiographic imaging and tomography (RadIT), when ionizing radiations are used to reveal internal structures of materials. Since its invention by R\"ontgen, RadIT now come in many modalities such as absorption-based X-ray radiography, phase contrast X-ray imaging, coherent X-ray diffractive imaging, high-energy X- and $\gamma-$ray radiography at above 1 MeV, X-ray computed tomography (CT), proton imaging and tomography (IT), neutron IT, positron emission tomography (PET), high-energy electron radiography, muon tomography, etc. Spatial, temporal resolution, sensitivity, and radiation hardness, among others, are common metrics for RadIT performance, which are enabled by, in addition to scintillators, advances in high-luminosity accelerators and high-power lasers, photodetectors especially CMOS pixelated sensor arrays, and lately data science. Medical imaging, nondestructive testing, nuclear safety and safeguards are traditional RadIT applications. Examples of growing or emerging applications include space, additive manufacturing, machine vision, and virtual reality or `metaverse'. Scintillator metrics such as light yield and decay time are correlated to RadIT metrics. More than 160 kinds of scintillators and applications are presented during the SCINT22 conference. New trends include inorganic and organic scintillator heterostructures, liquid phase synthesis of perovskites and $\mu$m-thick films, use of multiphysics models and data science to guide scintillator development, structural innovations such as photonic crystals, nanoscintillators enhanced by the Purcell effect, novel scintillator fibers, and multilayer configurations. Opportunities exist through optimization of RadIT with reduced radiation dose, data-driven measurements, photon/particle counting and tracking methods supplementing time-integrated measurements, and multimodal RadIT.Comment: 45 pages, 43 Figures, SCINT22 conference overvie

Studies for a proton tomography scanner

iMPACT, innovative Medical Proton Achromatic Calorimeter and Tracker, is a University of Padova and INFN project, funded by the European Research Council. The project aim is to design, develop and prototype an extremely fast and accurate proton Computed Tomography Scanner, with the ultimate goal of enabling the realization of a clinically viable proton Computed Tomography (pCT) system. Proton Computed Tomography is an extremely promising technique able to reconstruct density maps (images) of the human body with minimal dose release and high tissue density accuracy, a particularly critical feature in cancer hadron-therapy treatment planning. Hadron-therapy is a leading edge technique where protons or heavy-ions, instead of X-rays, are used to target and destroy the tumor within the human body. By exploiting the peculiar energy deposition distribution such highly ionizing, heavy particles exhibit, it is in fact possible to confine within a volume of few mm3 most of the energy released, hence sparing the healthy tissues surrounding the tumor. However, despite all its beneficial aspects, hadron-therapy is not yet widespread as other more established procedures, such as X-ray therapy. One of the reasons is that the current X-ray Computed Tomography (X-ray CT), currently used to produce body density maps, cannot deliver maps accurate enough to exploit the intrinsic accuracy of the hadron treatment. To precisely aim the hadron energy release with millimeter precision, it is in fact necessary to possess very accurate knowledge of the density it traverse before reaching the tumor. The idea standing behind the development of a pCT scanner is that using the same energy loss behaviour for both the imaging process and the treatment would improve the performance of the latter, the physical interaction process being the same. Currently, several pCT scanner prototypes are being developed around the world; pCT scanner technology however is still far from being applicable in a clinical environment, mainly due to the slow acquisition rates. The iMPACT project therefore plans to develop a pCT scanner able to overcome such limitations, leading the way toward medically sound apparatuses. This thesis work begins by displaying both limitations and advantages of the hadron-therapy technique; the pCT state-of-the-art is then reviewed, highlighting positive features as well as constraints that limit its applicability. The current state of the iMPACT scanner, which embeds a tracker system and a calorimeter, is illustrated and discussed. The thesis then focuses on the development of the calorimeter part of the scanner. The development of a Monte Carlo simulation is presented together with a calibration procedure based on data collected at proton beam tests. Additional studies with proton data are presented with an outlook on future developments.ope

Application of Silicon Photomultipliers to Positron Emission Tomography

Historically, positron emission tomography (PET) systems have been based on scintillation crystals coupled to photomultipliers tubes (PMTs). However, the limited quantum efficiency, bulkiness, and relatively high cost per unit surface area of PMTs, along with the growth of new applications for PET, offers opportunities for other photodetectors. Among these, small-animal scanners, hybrid PET/MRI systems, and incorporation of time-of-flight information are of particular interest and require low-cost, compact, fast, and magnetic field compatible photodetectors. With high quantum efficiency and compact structure, avalanche photodiodes (APDs) overcome several of the drawbacks of PMTs, but this is offset by degraded signal-to-noise and timing properties. Silicon photomultipliers (SiPMs) offer an alternative solution, combining many of the advantages of PMTs and APDs. They have high gain, excellent timing properties and are insensitive to magnetic fields. At the present time, SiPM technology is rapidly developing and therefore an investigation into optimal design and operating conditions is underway together with detailed characterization of SiPM-based PET detectors. Published data are extremely promising and show good energy and timing resolution, as well as the ability to decode small scintillator arrays. SiPMs clearly have the potential to be the photodetector of choice for some, or even perhaps most, PET systems

Monolithic Perimeter Gated Single Photon Avalanche Diode Based Optical Detector in Standard CMOS

Since the 1930\u27s photomultiplier tubes (PMTs) have been used in single photon detection. Single photon avalanche diodes (SPADs) are p-n junctions operated in the Geiger mode. Unlike PMTs, CMOS based SPADs are smaller in size, insensitive to magnetic fields, less expensive, less temperature dependent, and have lower bias voltages. Using appropriate readout circuitry, they measure properties of single photons, such as energy, arrival time, and spatial path making them excellent candidates for single photon detection. CMOS SPADs suffer from premature breakdown due to the non-uniform distribution of the electric field. This prevents full volumetric breakdown of the device and reduces the detection effciency by increasing the noise. A novel device known as the perimeter gated SPAD (PGSPAD) is adopted in this dissertation for mitigating the premature perimeter breakdown without compromising the fill-factor of the device. The novel contributions of this work are as follows. A novel simulation model, including SPICE characteristics and the stochastic behavior, has been developed for the perimeter gated SPAD. This model has the ability to simulate the static current-voltage and dynamic response characteristics. It also simulates the noise and spectral response. A perimeter gated silicon photomultiplier, with improved signal to noise ratio, is reported for the first time. The gate voltage reduces the dark current of the silicon photomultiplier by preventing the premature breakdown. A digital SPAD with the tunable dynamic range and sensitivity is demonstrated for the first time. This pixel can be used for weak optical signal application when relatively higher sensitivity and lower input dynamic range is required. By making the sensitivity-dynamic range trade-off the same detector can be used for applications with relatively higher optical power. Finally, an array has been developed using the digital silicon photomultiplier in which the dead time of the pixels have been reduced. This digital photomultiplier features noise variation compensation between the pixels

Nanoscale mosaicity revealed in peptide microcrystals by scanning electron nanodiffraction.

Changes in lattice structure across sub-regions of protein crystals are challenging to assess when relying on whole crystal measurements. Because of this difficulty, macromolecular structure determination from protein micro and nanocrystals requires assumptions of bulk crystallinity and domain block substructure. Here we map lattice structure across micron size areas of cryogenically preserved three-dimensional peptide crystals using a nano-focused electron beam. This approach produces diffraction from as few as 1500 molecules in a crystal, is sensitive to crystal thickness and three-dimensional lattice orientation. Real-space maps reconstructed from unsupervised classification of diffraction patterns across a crystal reveal regions of crystal order/disorder and three-dimensional lattice tilts on the sub-100nm scale. The nanoscale lattice reorientation observed in the micron-sized peptide crystal lattices studied here provides a direct view of their plasticity. Knowledge of these features facilitates an improved understanding of peptide assemblies that could aid in the determination of structures from nano- and microcrystals by single or serial crystal electron diffraction

A High-Performance Data Acquisition System for Smart Cameras in Science

This dissertation proposes a novel smart camera platform serving as a flexible data acquisition system for scientific applications. Current technological progress offers increasing performance in the areas we consider, namely high data-throughput, data processing, and detector performance. Prevalent data acquisition solutions typically focus on one of these aspects. However, driven by science, experiments experience increasing demands in terms of data throughput, speed and flexibility. In this dissertation, we introduce a system which, in addition to being able to provide high-speed data transfer, is also capable of interpreting the incoming information at an early stage. In order to demonstrate the full potential of the smart camera platform, we focus on X-ray imaging with synchrotron light sources. X-ray imaging applications can investigate the traits of technological and biological processes over microseconds for radiography, and milliseconds for tomography applications. These applications may require different sensors, and include complex experiment operations. The new smart camera platform is part of a larger project, UFO, which introduces a new concept for X-ray imaging. On-line data assessment is used to provide a data-driven feedback and active management of both the process and data acquisition procedure. This is accomplished using a GPU platform for fast reconstruction, embedded on-camera data processing, and integrating smart camera in a high-throughput data acquisition system. The final design of the smart camera platform consists of a custom high-performance FPGA board, providing continuous data transfer, embedded image processing, and a flexible input stage. In the IMAGE beamline of ANKA, camera is integrated in the new control system, and used in real-life applications. A maximum data-throughput of up to 8 GB/s is achieved. A custom image-based algorithm is implemented in the FPGA, with stringent real-time requirements, able to increase native sensor speed up to five times while reducing the amount of transfered data. Several image sensors are used, with resolutions of up to 20 megapixels and frame rates of up to 5 kfps. The smart camera platform was also used in non-imaging applications, stemming from the flexible input stage. The proposed camera architecture enables the user to modify the current system for any kind of high data-throughput applications, and to modify and implement custom processing algorithms

Feasibility of Geiger-mode avalanche photodiodes in CMOS standard technologies for tracker detectors

The next generation of particle colliders will be characterized by linear lepton colliders, where the collisions between electrons and positrons will allow to study in great detail the new particle discovered at CERN in 2012 (presumably the Higgs boson). At present time, there are two alternative projects underway, namely the ILC (International Linear Collider) and CLIC (Compact LInear Collider). From the detector point of view, the physics aims at these particle colliders impose such extreme requirements, that there is no sensor technology available in the market that can fulfill all of them. As a result, several new detector systems are being developed in parallel with the accelerator. This thesis presents the development of a GAPD (Geiger-mode Avalanche PhotoDiode) pixel detector aimed mostly at particle tracking at future linear colliders. GAPDs offer outstanding qualities to meet the challenging requirements of ILC and CLIC, such as an extraordinary high sensitivity, virtually infinite gain and ultra-fast response time, apart from compatibility with standard CMOS technologies. In particular, GAPD detectors enable the direct conversion of a single particle event onto a CMOS digital pulse in the sub-nanosecond time scale without the utilization of either preamplifiers or pulse shapers. As a result, GAPDs can be read out after each single bunch crossing, a unique quality that none of its competitors can offer at the moment. In spite of all these advantages, GAPD detectors suffer from two main problems. On the one side, there exist noise phenomena inherent to the sensor, which induce noise pulses that cannot be distinguished from real particle events and also worsen the detector occupancy to unacceptable levels. On the other side, the fill-factor is too low and gives rise to a reduced detection efficiency. Solutions to the two problems commented that are compliant with the severe specifications of the next generation of particle colliders have been thoroughly investigated. The design and characterization of several single pixels and small arrays that incorporate some elements to reduce the intrinsic noise generated by the sensor are presented. The sensors and the readout circuits have been monolithically integrated in a conventional HV-CMOS 0.35 μm process. Concerning the readout circuits, both voltage-mode and current-mode options have been considered. Moreover, the time-gated operation has also been explored as an alternative to reduce the detected sensor noise. The design and thorough characterization of a prototype GAPD array, also monolithically integrated in a conventional 0.35 μm HV-CMOS process, is presented in the thesis as well. The detector consists of 10 rows x 43 columns of pixels, with a total sensitive area of 1 mm x 1 mm. The array is operated in a time-gated mode and read out sequentially by rows. The efficiency of the proposed technique to reduce the detected noise is shown with a wide variety of measurements. Further improved results are obtained with the reduction of the working temperature. Finally, the suitability of the proposed detector array for particle detection is shown with the results of a beam-test campaign conducted at CERN-SPS (European Organization for Nuclear Research-Super Proton Synchrotron). Apart from that, a series of additional approaches to improve the performance of the GAPD technology are proposed. The benefits of integrating a GAPD pixel array in a 3D process in terms of overcoming the fill-factor limitation are examined first. The design of a GAPD detector in the Global Foundries 130 nm/Tezzaron 3D process is also presented. Moreover, the possibility to obtain better results in light detection applications by means of the time-gated operation or correction techniques is analyzed too.Aquesta tesi presenta el desenvolupament d’un detector de píxels de GAPDs (Geiger-mode Avalanche PhotoDiodes) dedicat principalment a rastrejar partícules en futurs col•lisionadors lineals. Els GAPDs ofereixen unes qualitats extraordinàries per satisfer els requisits extremadament exigents d’ILC (International Linear Collider) i CLIC (Compact LInear Collider), els dos projectes per la propera generació de col•lisionadors que s’han proposat fins a dia d’avui. Entre aquestes qualitats es troben una sensibilitat extremadament elevada, un guany virtualment infinit i una resposta molt ràpida, a part de ser compatibles amb les tecnologies CMOS estàndard. En concret, els detectors de GAPDs fan possible la conversió directa d’un esdeveniment generat per una sola partícula en un senyal CMOS digital amb un temps inferior al nanosegon. Com a resultat d’aquest fet, els GAPDs poden ser llegits després de cada bunch crossing (la col•lisió de les partícules), una qualitat única que cap dels seus competidors pot oferir en el moment actual. Malgrat tots aquests avantatges, els detectors de GAPDs pateixen dos grans problemes. D’una banda, existeixen fenòmens de soroll inherents al sensor, els quals indueixen polsos de soroll que no poden ser distingits dels esdeveniments reals generats per partícules i que a més empitjoren l’ocupació del detector a nivells inacceptables. D’altra banda, el fill-factor (és a dir, l’àrea sensible respecte l’àrea total) és molt baix i redueix l’eficiència detectora. En aquesta tesi s’han investigat solucions als dos problemes comentats i que a més compleixen amb les especificacions altament severes dels futurs col•lisionadors lineals. El detector de píxels de GAPDs, el qual ha estat monolíticament integrat en un procés HV-CMOS estàndard de 0.35 μm, incorpora circuits de lectura en mode voltatge que permeten operar el sensor en l’anomenat mode time-gated per tal de reduir el soroll detectat. L’eficiència de la tècnica proposada queda demostrada amb la gran varietat d’experiments que s’han dut a terme. Els resultats del beam-test dut a terme al CERN indiquen la capacitat del detector de píxels de GAPDs per detectar partícules altament energètiques. A banda d’això, també s’han estudiat els beneficis d’integrar un detector de píxels de GAPDs en un procés 3D per tal d’incrementar el fill-factor. L’anàlisi realitzat conclou que es poden assolir fill-factors superiors al 90%

A photon counting pixel detector for x-ray imaging

Hybrid semiconductor pixel detector technology is presented in this thesis as an alternative to current imaging systems in medical imaging and synchrotron radiation applications. The technology has been developed from research performed in High Energy Physics, in particular, for the ATLAS experiment at the LHC, planned for 2005. This thesis describes work done by the author on behalf of the MEDIPIX project, a collaboration between 13 international institutions for the development of hybrid pixel detectors for non-HEP applications. Chapter 1 describes the motivation for these detectors, the origin of the technology, and the current state of the art in imaging devices. A description of the requirements of medical imaging on X-ray sensors is described, and the properties of film and CCDs are discussed. The work of the RD19 collaboration is introduced to show the evolution of these devices. Chapter 2 presents the basic semiconductor theory required to understand the operation of these detectors, and a section on image theory introduces the fundamental parameters which are necessary to define the quality of an imaging device. Chapter 3 presents measurements made by the author on a photon counting detector (PCD1) comprising a PCC1 (MEDIPIX1) readout chip bumpbonded to silicon and gallium arsenide pixel detectors. Tests on the seperate readout chip and the bump-bonded assembly are shown with comparisons between the performance of the two materials. Measurements of signal-tonoise ratio, detection efficiency and noise performance are presented, along with an MTF measurement made by the Freiburg group. The X-ray tube energy spectrum was calibrated by REGAM. The performance of the PCD in a powder diffraction experiment using a synchrotron radiation source is described in chapter 4. This chapter reports the first use of a true 2-D hybrid pixel detector in a synchrotron application, and a comparison with the existing scintillator based technology is made. The measurements made by the author have been presented at the 1st International Workshop on Radiation Imaging Detectors at Sundsvall, Sweden, June 1999. The PCD1 operates in single photon counting mode, which attempts to overcome the limitations of charge integrating devices such as CCDs. The pros and cons of the two detection methods are discussed in chapter 5, and a comparison was made of the PCD1 performance with the performance of a commercial dental X-ray sensor. The two detectors are compared in terms of contrast and signal-to-noise ratio for identical X-ray fluences. The results were presented at the 2nd International Workshop on Radiation Imaging Detectors, Freiburg, Germany, 2nd-6th July 2000. The author was involved in the conversion of the LabWindows MRS software to a LabView platform, which was presented in an MSc- thesis in the University of Glasgow by F. Doherty. All image processing, data manipulation and analysis code was written by the author

Characterization of the ALPIDE chip with Helium-4 ions for Proton Computed Tomography

Particle therapy has become an appealing therapeutic option for patients with various tumor types. The physical properties of charged particles allow for an improved dose distribution conformality compared to conventional photon-based radiotherapy. This advantage translates into a reduction of unwanted side effects from radiation treatment and in the long run will improve the patient’s quality of life. A team at the University of Bergen is developing a proton Computed Tomography (pCT) scanner prototype. This technology will primarily work as a supplement to particle therapy as it aims to enhance the accuracy of the pre-calculated dose plans applied during treatment. The pCT system is a Digital Tracking Calorimeter (DTC) consisting of multiple layers of the ALPIDE CMOS Monolithic Active Pixel Sensor with the aim of tracking protons and measure its energy. This thesis studies the ALPIDE chip towards its medical applications in the future DTC. It will describe the clusters created on the chip by helium ions and alpha particles with a focus on the parameters that affect the pixel size of the clusters. Results from an analysis of a helium microbeam indicate that the size of a cluster correlates with the position of the incoming particle on the pixel and hence, the interior location of the energy deposition. These clusters varied in size from 5 to 35 pixels when the beam scanned the chip in µm steps. Moreover, an experiment conducted during this project shows that the size of the clusters is dependent on the temperature of the chip and that the average size of the cluster gets larger with increasing temperature. In the end, the results obtained from an ALPIDE telescope irradiated with high energetic helium beams is described. This experiment shows that the clusters used in tracking increases in size at higher Linear Energy Transfer (LET) of the particles.Masteroppgave i fysikkMAMN-PHYSPHYS39

Development and Analysis of Non-Delay-Line Constant-Fraction Discriminator Timing Circuits, Including a Fully-Monolithic CMOS Implementation

A constant-fraction discriminator (CFD) is a time pick-off circuit providing time derivation that is insensitive to input-signal amplitude and, in some cases, input-signal rise time. CFD time pick-off circuits are useful in Positron Emission Tomography (PET) systems where Bismuth Germanate (BGO)/photomultiplier scintillation detectors detect coincident, 511-keV annihilation gamma rays. Time walk and noise-induced timing jitter in time pick-off circuits are discussed along with optimal and sub-optimal timing filters designed to minimize timing jitter. Additionally, the effects of scintillation-detector statistics on timing performance are discussed, and Monte Carlo analysis is developed to provide estimated timing and energy spectra for selected detector and time pick-off circuit configurations. The traditional delay-line CFD is then described with a discussion of deterministic (non statistical) performance and statistical Monte Carlo timing performance. A new class of non-delay-line CFD circuits utilizing lowpass- and/or allpass-filter delay-line approximations is then presented. The timing performance of these non-delay-line CFD circuits is shown to be comparable to traditional delay-line CFD circuits. Following the development and analysis of non-delay-line CFD circuits, a fully-monolithic, non-delay-line CFD circuit is presented which was fabricated in a standard digital, 2-μ, double-meta], double-poly, n-well CMOS process. The CMOS circuits developed include a low time walk comparator having a time walk of approximately 175 ps for input signals with amplitudes between 10-mV to 2000-mV and a rise time (10 - 90%) of 10 ns. Additionally, a fifth-order, continuous-time filter having a bandwidth of over 100 MHz was developed to provide CFD signal shaping without a delay line. The measured timing resolution (3.26 ns FWITh1, 6.50 ns FWTM) of the fully-monolithic, CMOS CFD is comparable to measured resolution (3.30 ns FWHM, 6.40 ns FWTM) of a commercial, discrete, bipolar CFD containing an external delay line. Each CFD was tested with a PET EGO/photomultiplier scintillation detector and a preamplifier having a 10-ns (10 - 90%) rise-time. The development of a fully-monolithic, CMOS CFD circuit, believed to be the first such reported development, is significant for PET and other systems that employ many front-end CFD time pick-off circuits