Th.--Sci. phys.--Lille 1, 1977. N°: 387.

Οι φωτοπολλαπλασιαστές Πυριτίου (Silicon Photomultipliers, SiPMs) είναι ελκυστικοί υποψήφιοι για την αντικατάσταση των φωτοπολλαπλασιαστών στην πυρηνική ιατρική απεικόνιση. Οι SiPMs είναι συμπαγείς και μικρού μεγέθους, εξασφαλίζουν υψηλό κέρδος με χαμηλή τάση πόλωσης, ενώ έχουν γρήγορη απόκριση. Επιπλέον, δεν επηρεάζονται από τα ισχυρά μαγνητικά πεδία, γεγονός που επιτρέπει τη χρήση τους στο γοργά αναπτυσσόμενο πεδίο του PET-MRI. Ωστόσο, η ανάπτυξη πρωτότυπων μικρών ανιχνευτικών διατάξεων που βασίζονται στους SiPMs παρουσιάζει μεγάλες σχεδιαστικές απαιτήσεις, εφόσον είναι απαραίτητη η αποδοτική σύζευξη του σπινθηριστή και του SiPM, όπως επίσης και η βελτιστοποίηση της ενίσχυσης και ψηφιοποίησης του σήματος. Σε αυτή την εργασία εξετάστηκε η λειτουργικότητα τριών βασικών μονάδων SiPM της εταιρίας SensL κατά τη σύζευξή τους με διάφορους ανιχνευτές σπινθηρισμού, κατάλληλους για συστήματα SPECT και PET. Ως ραδιενεργές πηγές χρησιμοποιήθηκαν τα ισότοπα 99mTc, 111In και 18F. Το σύστημα ανάγνωσης-επεξεργασίας των λαμβανόμενων σημάτων απαρτίζεται από ένα κύκλωμα προενισχυτή της εταιρίας SensL, ενισχυτή και ADC τύπου ΝΙΜ καθώς και το λογισμικό MPA/WIN, το οποίο διαβάζει δεδομένα από μια κάρτα PCI. Οι παράμετροι που μετρήθηκαν είναι το σχήμα του εξερχόμενου παλμού, ο ρυθμός σκότους (Dark Count Rate, DCR), η ανιχνευτική ικανότητα, η γραμμικότητα και η ενεργειακή διακριτική ικανότητα του συστήματος για διαφορετικές τάσεις πόλωσης (Vbias) και ενέργειες φωτονίων γ. Η βέλτιστη ενεργειακή διακριτική ικανότητα για την ενέργεια των 511 keV ήταν 11%, η οποία αυξάνεται στο 23% για ενέργεια 140 keV. Η τάση πόλωσης επηρεάζει ισχυρά το πλάτος του εξερχόμενου παλμού, ενώ η διάρκεια του παλμού παρουσίασε μικρότερες διακυμάνσεις. Η σχετικά αργή χρονική απόκριση των ενισχυτών NIM περιόρισε τη δυνατότητα μετρήσεων σε όλους τους διαθέσιμους σπινθηριστές, αλλά έδωσε το κίνητρο του σχεδιασμού και της κατασκευής στο εργαστήριο νέων κατάλληλων κυκλωμάτων ενίσχυσης και ψηφιοποίησης γρήγορων παλμών. / Silicon Photomultiplier (SiPMs) detectors are attractive candidates for the replacement of Photomultipliers in nuclear imaging. They are compact, they provide high gain with low bias voltage and have fast response. In addition strong magnetic fields don’t influence them, which allows their use in the emerging field of PET-MRI. The development of small prototypes based on SiPMs has several design challenges, since it is necessary to optimize scintillator and SiPM coupling, as well as signal amplification and digitization. In this work we have investigated the performance of three basic SiPM modules constructed by SensL corporation coupled to various scintillator detectors, suitable for SPECT and PET systems. The radioactive sources that we used were 99mTc, 111In and 18F. The readout system includes the SensL preamplifier circuit, a NIM amplifier and ADC and MPA/WIN software. The parameters that have been measured are output pulse shape, dark count rate (DCR), detection efficiency, linearity and energy resolution for different bias voltages (Vbias) and energies. Best energy resolution for 511keV was 11% and was increased to 23% for 140keV. Vbias strongly affects output pulse amplitude, while time duration has smaller variations. The performance of a custom amplifier is presented and further optimizations in readout electronics are discussed.

No description available.

This thesis reports a rock slope hazard investigation located in the Central Andes of Chile, where two significant rock mass wasting events were recognized. Dating using cosmogenic nuclide for ³⁶Cl showed that the deposits were post-glacial in age, corresponding to the Upper Pleistocene Portillo Rock Avalanche (PRA) and a Holocene rock slump and rockslide. The pre-historic landslide deposits underlie both a key transportation route between Chile and Argentina and an important ski resort. The purpose of this research was to investigate the likely failure mechanism and characterise the runout path and volume of the PRA. The insights gained on the back analysis of the slope were used in later stages to assess the hazard potential of a recurring major rockslide. The distinct element code UDEC was used to evaluate the failure mechanism. Elasto-plastic modelling results showed that sliding and shearing along the bedding planes together with brittle fracturing and shearing through the toe of the slope likely had occurred. Runout simulations were carried out using DAN3D. Combinations of rheologies were tested and ranked based on their ability to represent the current distribution of the debris by means of pre-failure topography reconstruction and volume estimates of the deposits. Results showed that the best basal rheological combination for the PRA was frictional during the rockslide and Voellmy when entrainment became important. In contrast, a constant frictional basal rheology best represented the Holocene rock slump. The performance of the present-day state of the slope was tested under different scenarios. Under static condition the slope proved to be stable indicating a stabilized geometrical profile with time. Also, the slope proved to be stable under increased pore water pressures at its toe. Finally the modelled slope was subjected to a seismic load (M=7.8) and its crest failed due to an outward rotation of blocks, probably aided by topographic amplification. The runout simulations showed that the leading edge of the flow could override part of the International Santiago-Mendoza Corridor with no direct impact to the Portillo Ski Resort. Overall, though, under this highly unlikely dynamic condition for the site, the hazard level is very low. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The aim of this Master's thesis is to design and to implement the security test in considering a resistance of the device under test to the effects of the distributed denial of service attack DDoS SYN Flood. After processing the test results is developed a protocol about security test of the device under test. In this thesis are tested two devices, namely CISCO ASA5510 firewall and a server with the specified name Server. The theoretical part of the thesis discusses the primary types of network attacks such as reconnaissance, gain access and denial of service attacks. Explained the concept of DoS and its principle, further types of DoS attacks and distributed denial of service attacks DDoS.

The aim of this Master thesis is a testing of Cisco ASA\,5510 firewall with affect of Denial of Service attacks. Part of the thesis is a teoretical description some of the attacks and practical tests. Practical part covers basic testing of Cisco ASA with Spirent Avalanche 3100B. Number of TCP connections per second and firewalls throughput on 7th layer of ISO/OSI model were tested. Also the effect of Denial of Service attacks on the throughput. In a next part there is described one possible way how to generate Denial of Service attacks from a Linux server to the firewall. Python scripts were used for generation DoS packets. With those scripts it is possible to generate five types of attacks.

No description available.

Digital silicon photomultipliers (dSiPMs) and single-photon avalanche diode (SPAD) imagers are optical sensing systems formed from the integration of time-to-digital converters (TDCs) with arrays of highly sensitive photodetectors known as SPADs. TDCs are high-performance mixed-signal circuits capable of timestamping events with picosecond level resolution. The digital operation mechanisms of SPADs allow for their outputs to be sent to TDCs, where the timestamps of individual photon detections are recorded. In recent years, time-resolved SPAD-based sensors have been a heavily studied topic due to their exceptional performance potential in biomedical imaging applications, including time-of-flight (ToF) positron emission tomography (PET), fluorescence lifetime imaging microscopy (FLIM), and diffuse optical tomography (DOT). This work targets the optimization of these sensors in low-cost standard complementary metal-oxide-semiconductor (CMOS) processes. Firstly, this thesis provides a detailed review of the work accomplished in CMOS TDCs and their integration in SPAD-based sensors. Next, a feedback time amplification TDC was designed and tested in the TSMC 65 nm process that can achieve < 5 ps timing resolution in a very compact area of 0.016 mm2. The design is then described for a multi-time-gated array of p+/n-well SPADs that aims to mitigate SPAD dark noise while providing high-speed imaging by applying shifted gate windows simultaneously to an array of SPADs. The p+/n-well SPAD is first characterized in a passive quench configuration where it demonstrated a maximum dark count rate of 44.9 kHz, 18.1% peak PDP at 420 nm, and 0.82 ns timing jitter at a 0.7 V excess bias. While the current multi-time-gated prototype is not fully functional, the measurement results for individual pixels of the multi-time-gated array showed a 3.25 ns median gate window with a 2.2x 10-4 dark count probability for a 0.7 V excess bias, with 440 ps timing resolution and ~1 LSBrms timing jitter. Based on the results, limitations of the current design and sources for future improvement are then discussed in detail. / Thesis / Master of Applied Science (MASc) / Medical imaging plays a key role in the diagnosis of diseases like cancer, and as such, the optimized performance of medical imaging systems is a large area of research. Recently, highly sensitive photodetectors known as single-photon avalanche diodes (SPADs) were integrated with high-performance timing circuits known as time-to-digital converters (TDCs) to form digital silicon photomultipliers (dSiPMs) and SPAD imagers. DSiPMs and SPAD imagers are capable of timestamping the detection of individual photons with a very high level of accuracy in order to generate biomedical images. This thesis focuses on the design and measurement of these sensors using standard fabrication processes with the aim of working towards high-performance medical imaging sensors at a low cost. Firstly, we review the results achieved in TDCs and SPAD-based sensors within the recent literature. Following that, we present the design and performance results of a custom-designed TDC that aims to achieve state-of-the-art performance within a small area in order to maintain low-cost and optimal integration with SPADs. Next, the design is described for an array of custom time-gated SPADs with integrated TDCs. Finally, the SPAD is characterized in two different configurations to identify sources of improvement for future design iterations.

No description available.

Single-photon avalanche diodes (SPADs) capable of single photon detection are promising optical sensors for use as receivers in optical wireless communication (OWC) systems. In SPAD-based receivers, the intersymbol interference (ISI) effect caused by dead time is an important drawback that limits performance. In this thesis, we propose two novel SPAD operation receivers to reduce the ISI effect in SPAD-based OWC. To validate the feasibility of these two modes, we design a free-running SPAD front-end circuit with post-layout transient simulation results. This SPAD circuit is improved by a novel mixed passive-active quench and reset front-end circuit that achieves a very short dead time. Based on the traditional free-running mode, we design the clock-driven mode and time-gated mode to reduce the ISI effect through time-controlled operating signals. In this work, we develop a new simulation system to assess the ISI effect in On-Off Keying (OOK) modulated communication and pulse position modulated (PPM) communication. To accurately evaluate these three modes, we build a OWC platform to test our proposed SPAD receiver manufactured by TSMC 65 nm process. The Test results demonstrate that the clock-driven mode and time-gated mode receivers can improve the bit error rate (BER) performance in low data rate communication and high data rate high optical power communication, respectively. Moreover, compared to the free-running mode, the two proposed time-controlled modes achieve higher data rate communication and better noise tolerance ability in SPAD-based OWC. / Thesis / Master of Applied Science (MASc) / Optical communication involves using light as a signal to transmit information, and it is currently a highly popular field of research. However, optical receivers used in this type of communication often require specific conditions, which can limit the overall performance of the communication system. To address this issue, we have developed an optical sensor tailored for optical communication. This sensor boasts exceptional sensitivity, allowing it to detect individual particles of light, thereby substantially reducing the demand for signal intensity in the optical communication system. Moreover, we have devised three operational circuits that enhance the sensor's responsiveness to signals under specific communication conditions. We have created a mathematical model to evaluate the proposed optical sensor and the designed circuits, and subsequently manufactured the optical sensor. Both the simulation results and the actual test outcomes unequivocally demonstrate that our proposed sensor has the potential to enhance the performance of optical communication systems.