Area Efficient Multi-Threshold Null Convenction Logic

Multi-Threshold null convention logic (MTNCL) is a commonly used asynchronous paradigm for designing low power NCL circuits. Traditionally, MTNCL circuits implemented using complementary metal oxide semiconductor (CMOS) technique that tends to occupy a large area. To address this limitation, a gate diffusion input (GDI) methodology is introduced for implementing MTNCL circuits. This GDI technique enables complex logic to be implemented using only two transistors that helps to reduce area utilization. In this paper, a novel approach to implement MTNCL designs based GDI methodology is proposed. The proposed approach has been verified by implementing TH23 MTNCL gate. Comparing to the conventional CMOS implementation, the proposed approach shows a 45% reduction in the area overhead

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 Application of Methodologies for Sensitivity Analysis and Uncertainty Evaluation of the Results of the Best Estimate System Codes Applied in Nuclear Technology

Sensitivity and uncertainty analyses can provide quantitatively in a mathematically and physically well-founded way answers to typical scientific and engineering questions such as: how much the model under consideration represents the physical phenomena, how far the calculated results can be extrapolated and etc… Nuclear Power Plant (NPP) technology has been developed based on the traditional defense in depth philosophy supported by deterministic and overly conservative methods for safety analysis. In the 1970s, conservative hypotheses were introduced for safety analyses to address existing uncertainties. Since then, intensive thermal-hydraulic experimental research has resulted in a considerable increase in knowledge and consequently in the development of best-estimate codes able to provide more realistic information about the physical behaviour and to identify the most relevant safety issues allowing the evaluation of the existing actual margins between the results of the calculations and the acceptance criteria. However, the best-estimate calculation results from complex thermal-hydraulic system codes (like Relap5, Cathare, Athlet, Trace, etc..) are affected by unavoidable approximations that are un-predictable without the use of computational tools that account for the various sources of uncertainty. Therefore the use of best-estimate codes within the reactor technology, either for design or safety purposes, implies understanding and accepting the limitations and the deficiencies of those codes. Uncertainties may have different origins ranging from the approximation of the models, to the approximation of the numerical solution, and to the lack of precision of the values adopted for boundary and initial conditions. The amount of uncertainty that affects a calculation may strongly depend upon the codes and the modeling techniques (i.e. the code’s users). A consistent and robust uncertainty methodology must be developed taking into consideration all the above aspects. Three main independent ways to perform the sensitivity and uncertainty analysis of thermal-hydraulic system code calculations have been identified in the present effort with approaches based on: a) propagation of code input errors and statistical treatment of the resulting uncertainty, b) propagation of code output errors and ‘deterministic’ treatment of the resulting uncertainty and c) experimental validation and calibration methodology of complex time-dependent numerical simulation models able to consistently incorporate both computational and experimental uncertainties making extensive use of the concepts of the sensitivity analysis. Taking into consideration the above framework, the main objective of the thesis is to contribute to the further development and qualification of the sensitivity and uncertainty tools for performing deterministic nuclear reactor safety analyses. To this aim, the PhD activity has been subdivided in two main parts mostly related with the different state of advancement and maturity of the involved methodologies. From one side, the goal is to consolidate and strengthen the Code with the capability of Internal Assessment of Uncertainty (CIAU) proposed by University of Pisa and to demonstrate its robustness and achieved maturity level through the application of the methodology in the framework of the BEMUSE (Best Estimate Methods Uncertainty and Sensitivity Evaluation) project promoted by OECD. From the other side, the aim is to develop a fully deterministic method, named CASUALIDAD (Code with the capability of Adjoint Sensitivity and Uncertainty AnaLysis by Internal Data ADjustment and assimilation) based on advanced mathematical tools for performing the sensitivity and the uncertainty analyses internally to the thermal-hydraulic system code. In relation with CIAU, the contributions of the thesis’s work are 1) the extension of the uncertainty database with the addition of twelve new tests and 2) the development of a procedure for the ‘internal’ qualification of the method. Both aspects result in a more accurate CIAU uncertainty evaluation as they contribute respectively to improve the statistical performance and to perform a systematic qualitative and quantitative analysis of the data constituting the CIAU database. In relation with the second goal of the thesis’s work, a comprehensive approach for utilizing quantified uncertainties arising from Integral Test Facilities (ITFs) and Separate Effect Test Facilities (SETFs) in the process of calibrating complex computer models for the application to NPP transient scenarios has been developed. The methodology proposed is capable of accommodating multiple SETFs and ITFs to learn as much as possible about uncertain parameters, allowing for the improvement of the computer model predictions based on the available experimental evidences. The pioneering CASUALIDAD approach can be considered also as an attempt to substitute the empiricism of the CIAU approach in relation with the statistical treatment of the accuracy for deriving the uncertainty values with rigorous mathematical deterministic methods like the advanced sensitivity tools for performing the local (the Adjoint Sensitivity Analysis Procedure, ASAP, or the Forward Sensitivity Analysis Procedure, FSAP) and global (Global Adjoint Sensitivity Analysis Procedure, GASAP) sensitivity analyses and the methodology (the Data Adjustment and Assimilation, DAA) for consistently incorporating observed available information into a predicting model to obtain an improved uncertainty estimation. As a main conclusion from the present effort, it is clear the industrial relevance of the best-estimate plus uncertainty approach compared with the conservative approach. More in detail, in relation with the CIAU uncertainty method, the attained results demonstrate, within the framework of an international initiative, the level of robustness and adequacy of the proposed uncertainty methodology. At the same time, both the achieved advancements and the performed applications, constitute fundamental supports for the use of the CIAU method in the NPP licensing process, like for Angra-2 NPP (Brazil) in the recent past (2000) and for Atucha-2 NPP (Argentina) currently under application. In relation with the CASUALIDAD, for which a demonstrative application (i.e. the blowdown of a gas from a pressurized vessel taking into account the heat transfer through the vessel wall) is presented, the proposed methodology constitutes a major step forward with respect to the generally used expert judgement and statistical methods as it permits a) to establish the uncertainties of any parameter characterizing the system, based on a fully mathematical approach where the experimental evidences play the major role and b) to calculate an improved estimate of the computed response and relative improved (i.e. reduced) uncertainty