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Modeling, Design and Characterization of a Multi-Pixel Digital SiPM for PET Applications
Get PDFPositron Emission Tomography (PET) scanners provide functional three-dimensional images of the body that are extremely useful in cancer and brain research. The goal of this work is the modeling, design and characterization of a CMOS-based photodetector for PET. To this aim, first a model for the energy resolution and coincidence resolution time (CRT) for digital, SPAD-based detectors is developed.
Then, a top-to-bottom detector architecture is proposed, containing an innovative in-pixel com-pression technique that allows for high fill-factor (FF) and efficient readout. At the top-level of the architecture, an integrated discriminator monitors the photon flux for incoming gamma events, enabling an event-based readout scheme. The first complete implementation of this archi-tecture is described, the SPADnet-I sensor, which is composed by an 8×16 pixel array, each of around 0.6 × 0.6 mm2 with 720 SPADs, resulting in a pixel FF of 42.6%. The sensor can obtain the discrete photon flux estimation at up to 100 Msamples/s, which are used by the discriminator and also output at real-time.
The complete characterization of the sensor is presented, and the best sensor configuration was found to be at 84% of the SPADs enabled (disabled starting with the highest DCR one), with 2 V SPAD excess bias and 150 ns integration time. This configuration results in an energy resolution of 10.8% and a CRT of 288 ps, the latter which was obtained with a new, hardware-friendly time of arrival (ToA) estimation algorithm, also described in this thesis.
Finally, the sensor model, validated by the experimental results, is used to predict the perfor-mance of possible modifications in the sensor, and some design improvements are suggested for a future implementation of the architecture
Scultura a Padova: 1540-1620 circa. Monumenti e ritratti
No full textThe dissertation is about the sculpture in Padua from 1540 to 1620 ca., mainly focusing on the production of funerary monuments and portrait busts. Crucial is the connection between the sculptors, the Veneranda Arca di Sant'Antonio and the intellectuals related to the Studio or to the academies of the city. The stylistic development and peculiar forms are compared with those of the contemporary Venice
A new Lagrangian method for transport in porous media (to model chemotaxis in porous media)
No full textAs recently shown in laboratory bench scale experiments, chemotaxis, i.e.the movement of microorganisms toward or away from the concentration gradient of a chemical species, could have a fundamental role in the transport of bacteria through saturated porous media. Chemotactic bacteria could enhance bioremediation by directing their own motions to residual contaminants in less conductive zones in aquifers. The aim of the present work is to develop a proper numerical scheme to define and to quantify the
magnitude and the role of chemotaxis in the complex groundwater system framework. We present a new class of meshless Lagrangian particle methods based on the Smooth
Particle Hydrodinamics (SPH) formulation of Vila & Ben Moussa, combined with a new Weighted Essentially Non-Oscillatory (WENO) reconstruction technique on moving point
clouds in multiple space dimensions. The purpose of this new scheme is to fully exploit the advantages of SPH among traditional meshbased and meshfree schemes and to overcome
its inapplicability for modeling chemotaxis in porous media.
The key idea is to produce for each particle first a set of high order accurate Moving Least Squares (MLS) reconstructions on a set of different reconstruction stencils. Then, these reconstructions are combined with each other using a nonlinear WENO technique in
order to capture at the same time discontinuities and to maintain accuracy and low numerical dissipation in smooth regions. The numerical fluxes between interacting particles are subsequently evaluated using this MLS-WENO reconstruction at the midpoint between two particles, in combination with a Riemann solver that provides the necessary stabilization of the scheme based on the underlying physics of the governing equations. We propose
the use of two different Riemann solvers: the Rusanov flux and an Osher-type flux. The use of monotone fluxes together with a WENO reconstruction ensures accuracy, stability,
robustness and an essentially non oscillatory solution without the artificial viscosity term usually employed in conventional SPH schemes. To our knowledge, this is the first time that the WENO method, which has originally been developed for mesh-based schemes in the Eulerian framework on fixed grids, is extended to meshfree Lagrangian particle
methods like SPH in multiple space dimensions.
In the first part, we test the new algorithm on two dimensional blast wave problems and on the classical one-dimensional Sod shock tube problem for the Euler equations of compressible gas dynamics. We obtain a good agreement with the exact or numerical reference solution in all cases and an improved accuracy and robustness compared to
existing standard SPH schemes. In the second part, the new SPH scheme is applied to advection-diffusion equation in
heterogeneous porous media with anisotropic diffusion tensor. Several numerical test case shows that the new scheme is accurate. Unlike standard SPH, it reduces the occurrence of negative concentration.
In the third part, we show the applicability of the new scheme for modeling chemotaxis in porous media. We test the new scheme against analytical reference solutions. Under the
assumption of complete mixing at the Darcy scale, we perform different two-dimensional conservative solute transport simulations under steady-state conditions with instant injection showing that chemotaxis significantly affect the quantification of field-scale mixing
processes
Optimization-based Strategy for Polyomino Subarraying in Wideband Phased Array Design
No full textPhased antenna arrays provide ultimate performance in areas where high directivity and electronic scanning are needed. That performance is achieved by involving large number of radiators as well as corresponding control units. As a result, such systems become bulky and heavy. In order to reduce the number of control units, elements are grouped in subarrays with one of the control units, such as time delay, put at the subarray level. The drawback of this approach is that if elements are grouped into subarrays producing repetitive patterns in the array structure, radiation pattern of such array will be affected by undesired grating lobes. To eliminate that effect, subarrays of irregular shapes, such as polyominoes, are used. Still, those structures are an object for optimization. This work aims at applying optimization techniques like genetic algorithm to the problem of finding optimal structures of phased antenna arrays composed of polyomino-shaped subarrays. For this purpose a new mathematical model, new algorithm and optimization methods are developed. Application of those techniques showed significant advances in radiation characteristics, in particular sidelobe level. Also new features were enabled, for example, multi-beam radiation pattern forming
Cross-Domain and Cross-Language Porting of Shallow Parsing
No full textEnglish was the main focus of attention of the Natural Language Processing (NLP) community for years. As a result, there are significantly more annotated linguistic resources in English than in any other language. Consequently, data-driven tools for automatic text or speech processing are developed mainly for English. Developing similar corpora and tools for other languages is an important issue. However, this requires significant amount of effort.
Recently, Statistical Machine Translation (SMT) techniques and parallel corpora were used to transfer annotations from a linguistic resource rich languages to a resource-poor languages for a variety of Natural Language Processing (NLP) tasks, including Part-of-Speech tagging, Noun Phrase chunking, dependency parsing, textual entailment, etc.
This cross-language NLP paradigm relies on the solution of the following sub-problems:
- Data-driven NLP techniques are very sensitive to the differences in training and testing conditions. Different domains, such as financial news-wire and biomedical publications, have different distributions of NLP task-specific properties; thus, the domain adaptation of the source language tools -- either the development of models with good cross-domain performance or tuned to the target domain -- is critical.
- Another difference in training and testing conditions arises with cross-genre applications such as written text (monologues) and spontaneous dialog data. Properties of written text such as punctuation and the notion of sentence are not present in spoken conversation transcriptions. Thus, style-adaptation techniques to cover a wider range of genres is critical as well.
- The basis of cross-language porting is parallel corpora. Unfortunately, parallel corpora are scarce. Thus, generation or retrieval of parallel corpora between the languages of interest is important. Additionally, these parallel corpora most often are not in the domains of interest; consequently, the cross-language porting should be augmented with SMT domain adaptation techniques.
- The language distance play an important role within the paradigm, since for close family language pairs (e.g. Romance languages Italian and Spanish) the range of linguistic phenomena to consider is significantly less compared to the distant family language pairs (e.g. Italian and Turkish). The developed cross-language techniques should be applicable to both conditions.
In this thesis we address these sub-problems on complex Natural Language Processing tasks of Discourse Parsing and Spoken Language Understanding. Both tasks are cast as token-level shallow parsing.
Penn Discourse Treebank (PDTB) style discourse parsing is applied cross-domain and we contribute feature-level domain adaptation techniques for the task. Additionally, we explore PDTB-style discourse parsing on dialog data in Italian are report on challenges. The problems of parallel corpora creation, language style adaptation, SMT domain-adaptation and language distance are addressed on the task of cross-language porting of Spoken Language Understanding.
This thesis contributes to the task with the language-style and domain adaptation techniques for machine translation of spoken conversations using off-the-shelf systems like Google Translate, SMT systems trained on both out-of-domain and in-domain parallel data. We demonstrate that the techniques are beneficial for both close and distant language pairs. We propose the methodologies for the creation of parallel spoken conversation corpora via professional translation services that considers speech phenomena such as disfluencies. Additionally, we explore the semantic annotation transfer using automatic SMT methods and crowdsourcing. For the later, we propose the computational methodology to obtain acceptable quality corpus without the target language references and the low worker agreement
An analysis of the role of financialisation, China and stockholding in agricultural commodity price movements
No full textA wide range of commodity prices experienced a large peak in 2007/08, including many agricultural commodities. Since then agricultural commodity prices have remained at relatively high levels compared to the recent past and several agricultural commodities (e.g. maize and sugar) have experienced further, although smaller peaks. Studies of the recent commodity price movements have focused on financialisation, China, low stocks and biofuels. This thesis explores the role of three of these factors in agricultural commodity price formation, namely, financialisation, China and stockholding. Biofuels were analysed in work for my previous employer.
The second chapter uses Granger-causality methods to assess whether index investor positions influence agricultural futures prices. Four extensions are explored that might overcome the limitations of Granger-causality tests in this context. Firstly, the analysis is extended to less liquid markets. Liquid markets tend to be relatively efficient and for those markets the semi-strong form of the Efficient Market Hypothesis suggests that prices should not be forecastable as new information is swiftly impounded in prices. Even if index investment had price impact finding Granger-causality based on weekly data would be surprising. In less liquid markets prices are likely to adjust more slowly to new information.
Three further extensions are examined: long-horizon tests, using relative returns instead of returns and an index of positions instead of positions in individual markets. The analysis supports the conclusion that no impacts are discernible for liquid markets. However, Granger-causality is established in the less liquid soybean oil and livestock markets. Thus, there is clear evidence that index investment has been a factor influencing the level of grains and livestock prices in illiquid markets over the five years 2006-11.These results lead to the conjecture that index investment does also have price impact in liquid markets but that market efficiency prevents the detection of this impact using Granger-causality tests.
Chapter 3 investigates the potential impacts of changes in Chinese stockholding and self-sufficiency policies on world wheat prices. Whether or not the decrease in world grain stocks around the middle of the 2000s has had a significant impact on world prices in the last decade is controversial. Since most of the decrease in stocks was due to decreases in China, the main issue is the role that China has played on world markets. So far most studies that address this question have relied on informal arguments rather than formal modelling of the stocks-trade-price relationships. More recently the impact of possible changes to China’s self-sufficiency policy on world markets has become the focus of attention.
Trade and other grains policies are closely linked. Any impact of changes in grain policy in China will be transmitted to the world market through changing trade patterns. The model shows that a move away from autarky reduces stock levels in China and in the rest of the world resulting in lower global stock levels. In the two scenarios where China imports but does not export global stocks decline by 23 and 32 per cent, respectively, compared to autarky. In the two free trade scenarios, global stocks reduce by 38 and 44 per cent, respectively, compared to autarky. These reductions in stock levels when a country moves from autarky to trading with the rest of the world do not lead to an increase in price variation and, more importantly for policy-makers, do not lead to increases in extreme price movements. In the free trade scenarios, extreme price movements are even reduced despite much lower stock levels.
Chapter 4 introduces a new approach to testing the competitive storage model which has been the main workhorse of the analysis of the role of storage in commodity price formation over the last decades. The relationship between storage and prices is complex. Stock levels depend on current prices and expected future prices. At the same time, the current prices and the futures expected price depend on the level of stocks carried forward from the current period to future periods.
The main approach to testing the competitive storage model has been the comparison of the characteristics of the predicted price series with actual commodity price series. In this study a different approach is taken. A relatively simple model is taken to the experimental laboratory. Participants in the experiment are asked to make storage decisions within a competitive storage model framework. Participants’ behaviour in the experiment deviated from the behaviour predicted by the competitive storage model in a number of ways. The predicted relationship between the amount of wheat available and storage is non-linear in the model but is linear in the experiment. In addition, storage is more sensitive to “wheat” in storage than “wheat” harvested when the model suggests that the effect of wheat in storage and wheat from harvest should the same. Furthermore, average storage tends below the optimal level and storage does not vary as much as predicted by the competitive storage model. The resulting price series tend to be more variable than would be the case if stockholders behaved according to the competitive storage model
Structure and properties of nanostructured materials from atomistic modeling and advanced diffraction methods
No full textMatter at the nanoscale exhibits peculiar properties, often not shown by the bulk counterpart, and strongly coupled to the specific size, shape and structure of the atomic aggregate. Particularly, the enormous surface-to-volume ratio implies boosted reactivity with respect to the environment, while the electronic confinement might cause quantum effects to dominate physical properties.
Characterization techniques are of course essential to investigate properties at the atomic scale.
Scattering techniques have tremendously evolved in the recent past benefiting from third and fourth generation light sources, producing beams with unprecedented spatial and temporal resolution. In a different realm, atomistic simulations have also greatly evolved deriving advantages from both recent theories and modern computing units.
In this framework, a detailed description of the system in a spatial and temporal range compatible with lengths probed by scattering techniques is provided. In a single sentence, the subject of this Thesis is the effort of tying atomistic methods and scattering techniques so to increase the comprehension around size, shape and structure of nanostructured particles
A three dimension hyporheic model of the River Bure: Understanding the nutrient dynamics and the role of streambed heterogeneity
No full textThe hyporheic zone is often defined as the zone where mixing of surface water and groundwater occurs in shallow sediments beneath and adjacent to rivers. This mixing contributes to create unique biogeochemical conditions that may attenuate contaminants from either upstream surface water or groundwater under gaining and losing conditions. Hyporheic exchange results from differences in the channel near-bed head as it varies in space in response to interactions between surface flow and bed topography, with the interaction with the water table playing an important role too. Reactions of contaminants in groundwater also dependent on mixing between surface and subsurface water, which occurs in this zone. Therefore, representation of the profile of upwelling and downwelling exchange between surface water and groundwater have important consequences for contaminant transport. The present work studies nitrogen fate within a restored reach of the River Bure, Norfolk, United Kingdom. To this end, we confront numerical simulations of the hyporheic flow and tracer transport with field measurements of surface flow properties, nearby groundwater table and nitrogen compound concentration. We numerically model mixing between hyporheic flow paths induced by sediment, bedform, meanders on riverbed, and flow paths of adjacent upwelling of deeper groundwater. Results of the analysis indicate that despite the coarse topographical data and with limited surface water hydraulic data it is possible to define the spatial extent of hyporheic exchange and potential mixing zones for contaminants as a function of residence time. The proposed work has the potential to depict high residence time zones and biogeochemical reactivity in homogeneous and heterogeneous sediments. Furthermore, fieldwork analysis shows that in this site the hyporheic zone have a little effect on nutrient concentration. In addition, hydraulic modeling results indicate that streambed discharge significantly influences hyporheic exchange. Especially, the residence times under average stream discharge conditions are higher than the ones under the high discharge conditions mostly on the part that has riffle - pool morphology. From hydraulic point of view, heterogeneous domain has higher connectivity than the homogeneous fine sand subsurface set. Such that, subsurface flow has tendency to flow through high hydraulic conductivity zones, which is defined as tunneling effect, therefore, low conductivity zones have minor effect on hyporheic flow. The predictions based on the DaO2
index proves that heterogeneous sediment formations have more aerobic potential, however, prevailing anaerobic conditions occur mostly vicinity of low hydraulic conductivity zones
Positronium in the AEgIS experiment: study on its emission from nanochanneled samples and design of a new apparatus for Rydberg excitations
No full textThis experimental thesis has been done in the framework of AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy), an experiment installed at CERN, whose primary goal is the measurement of the Earth's gravitational acceleration on anti-hydrogen. The antiatoms will be produced by the charge exchange reaction, where a cloud of Ps in Rydberg states interacts with cooled trapped antiprotons. Since the charge exchange cross section depends on Ps velocity and quantum number, the velocity distribution of Ps emitted by a positron-positronium converter as well as its excitation in Rydberg states have to be studied and optimized.
In this thesis Ps cooling and emission into vacuum from nanochannelled silicon targets was studied by performing Time of Flight measurements with a dedicated apparatus conceived to receive the slow positron beam as produced at the Trento laboratory or at the NEPOMUC facility at Munich.
Measurements were done by varying the positron implantation energy, the sample temperature and the nanochannel dimensions, with the aim of finding the best parameters to increase Ps fraction having velocity lower than 5 10^4 m/s.
Preliminary data were analyzed in order to extract the Ps velocity distribution and its average temperature.
More, an original method for evaluating the permanence time of Ps inside the nanochannels before being emitted into vacuum, was described. A first rough evaluation based on the performed measurements is reported and this result will be useful to investigate the Ps cooling process and to synchronize the laser pulse for Ps excitation in the AEgIS experiment.
In order to perform measurements of Ps excitation in Rydberg states, a new apparatus for bunching positron pulses, coming from the AEgIS positron line, was designed and built.
COMSOL and SIMION softwares were used for designing a magnetic transport line and an electron optical line, which extracts positrons from the magnetic field and focus them on the nanochanneled Si sample.
More, a buncher device, which spatio-temporally compresses the positron bunches, was built and a fast circuit for supplying the 25 buncher electrodes with a parabolic shaped potential was designed and tested.
According to the simulations, at the target position the device will deliver positrons with an energy ranging from 6 to 9 keV, in bunches of 5 ns duration and a spot of 2.5 mm in diameter.
By using this apparatus, first measurements for the optimization of Ps excitation in Rydberg states and studies on the Ps levels with and without magnetic field will be performed. At a later stage investigations of Ps spectroscopy or of Ps laser cooling with the same apparatus could be achievable for the first time
Stabilized optomechanical systems for Quantum Optics
No full textThe optomechanics field of research has been gathering a lot of momentum during the last couple of years. Recent experimental results show that the field is finally entering the quantum era. In this context, we have worked to develop new and competitive optomechanical devices.
We have also worked towards the generation and observation of ponderomotive squeezing and we have identified, and experimentally demonstrated, an optomechanical effect that can ease the achievement of this goal. Finally, we have developed a stabilization technique that have been instrumental for the success of two experiments: the implementation of the Wiener-Kolmogorov data analysis and the squeezing of a mechanical thermal oscillator