Συμβολή στην ανάπτυξη φασματοσκοπικού ανιχνευτή γ-ακτινοβολίας με δυνατότητα προσδιορισμού της χωρικής κατανομής των πηγών ραδιενέργειας
Creators: name: Καΐσας, Ιωάννης Μ.
Publisher: name: Aristotle University of Thessaloniki
Year: 2017
Subjects: Κωδικοποιημένα Διαφράγματα
Τροποποιημένη ομοιογενώς πλεονάζουσα διάταξη
Ψηφιδωτοί ανιχνευτές (CdTe)
Διεγχειρητική πυρηνική ιατρική
Coded Apertures
Modified Uniformly Redundant Arrays (MURA)
Pixel Detectors (CdTe)
Intra-Operative Nuclear Medicine

description: Η παρούσα Διδακτορική Διατριβή (ΔΔ) μελετά τον εντοπισμό σημειακών και εκτεταμένων ραδιενεργών πηγών γ-ακτινοβολίας στον τρισδιάστατο χώρο με την μέθοδο του τριγωνισμού, αναπτύσσοντας δύο γ-κάμερες με ψηφιδωτούς, φασματοσκοπικούς ανιχνευτές-ημιαγωγούς CdTe και κωδικοποιημένα διαφράγματα (ΚΔ). Ο σχεδιασμός των γ-καμερών και η επιλογή της τεχνικής εντοπισμού των κωδικοποιημένων διαφραγμάτων βασίστηκαν στα χαρακτηριστικά των ψηφιδωτών ανιχνευτών-ημιαγωγών CdTe που διέθετε η ομάδα του ερευνητικού προγράμματος COCAE “Cooperation across Europe for Cd(Zn)Te based Security Instruments” (FP7-SEC 218000), ως ανιχνευτές για την ανάπτυξη και δοκιμή τεχνικών εντοπισμού και ταυτοποίησης ραδιενεργών πηγών στον χώρο. Η ΔΔ μελετά το παραπάνω πρόβλημα: -συνδυάζοντας την τεχνική των κωδικοποιημένων διαφραγμάτων με την μέθοδο τριγωνισμού, για τον εντοπισμό ραδιενεργών πηγών στον τρισδιάστατο χώρο, -αναλύοντας το με κώδικα προσομοίωση Monte-Carlo, που αναπτύχθηκε για την συγκεκριμένη εφαρμογή, με έμφαση στην γρήγορη απόκρισή του και σκοπό την επιλογή των βέλτιστων σχεδιαστικών παραμέτρων των κωδικοποιημένων διαφραγμάτων και του γεωμετρικού συνδυασμού τους με τους ψηφιδωτούς ανιχνευτές, -διεξάγοντας μετρήσεις με σημειακές και εκτεταμένες ραδιενεργές πηγές ακτίνων γ με σκοπό τον χαρακτηρισμό της πειραματικής διάταξης ως προς την απόδοσή της, την ακρίβεια εντοπισμού θέσης και την χωρική διακριτική ικανότητα.
description: This PhD thesis investigates the detection of point and extended gamma radioactive sources in the three-dimensional space, exploiting the triangulation method, applied on two γ-cameras with coded apertures in front of pixelated, CdTe, spectroscopic detectors. Also, the PhD Thesis contributes to the development and the design of the gamma cameras and the selection of the coded aperture technique. These were based on the CdTe pixelated detectors, provided by the COCAE research team, "Cooperation across Europe for Cd (Zn) Te based Security Instruments" (FP7-SEC 218000), as detectors for the development and testing of techniques for identifying radioactive sources in space. The PhD thesis studies the above problem: -combining the technique of coded apertures with the triangulation method to locate radioactive sources in the three-dimensional space, -analyzing it with the Monte-Carlo simulation code, developed for this application, with emphasis on its rapid response and aiming at selecting the optimal design parameters of the coded apertures and their geometrical combination with the pixelated detectors, -conducting measurements with point and extended radioactive sources of gamma rays in order to characterize the experimental device in terms of its performance, positioning accuracy and spatial resolution. The PhD Thesis consists of nine (9) main chapters numbered from one (1) to nine (9), the bibliography and an extensive abstract in English. Chapter 1 lists the theoretical background of semiconductor detectors physics with emphasis on CdTe detectors. Crystal growth techniques are reported for CdTe and CdZnTe. The Schottky interfaces and the ohmic interfaces are discussed as interfaces for the signal transmission of the detectors in the electronics. Also, the different ways of dividing the crystals are highlighted, and emphasis is placed on dividing them into pixels. Chapter 2 lists the theoretical background of coded aperture radiation detection. Chapter 2 begins with a brief reference to alternative Nuclear Medicine guidelines, imaging techniques for radioactive sources of gamma-ray, such as Compton Imaging, and then focuses on the analysis of the technique used in the PhD thesis, i.e. the coded aperture imaging. The principle of coded aperture imaging is being developed and the need for "in-focus" imagery to avoid pseudo-images is highlighted. At the same time, the main tables are presented: mask, shadowgram and correlation matrix. Below is a description of basic coding and decoding concepts, convolution and correlation, and the various modes of decoding the shadowgram. Then, the general image quality factors, as well as the coded aperture special quality factors, such as the Auto Correlation Function (ACF), are presented. The following is a detailed presentation of the different types of coded apertures, the way of designing their motives and their properties, with frequent reference to the relevant literature. Finally, coded aperture applications are presented in Astronomy, Nuclear Medicine and Nuclear Reactor Security and Disassembly and their distinction in near and far field applications. The structure of this chapter and the sequence of sub-modules are intended to fully present the coded aperture imaging technique and to justify the choice of it, in particular the Modified Uniformly Redundant Arrays (MURA) in this PhD Thesis. An attempt is also made for the text of the PhD Thesis to be as autonomous as possible for the reader. Chapter 3 presents the phenomenon of parallax that exists in the detection of radioactive sources due to the different position of the two γ-cameras. The triangulation method used in the algorithm to find the three coordinates of radioactive sources in space is mathematically analyzed. The algorithm uses as input the directions indicated by the peaks of the correlation matrix, based on the theoretical background given in Chapter 2 and the geometric characteristics of the γ-cameras. With the completion of Chapter 3, the theoretical basis for identifying radioactive sources of gamma irradiation in the three-dimensional space has been presented. Chapter 4 presents the parts of which the experimental assembly is composed and their specific features. Specifically, the structure and characteristics of the two identical CdTe pixelated semiconductor detectors that were used experimentally are presented. Also, the geometry and structure of the coded apertures (masks), that were constructed and used, are presented and their innovassion, regarding the inversion of the No-Two-Holes-Touching (NTHT). Finally, an experimental setup used for the testing and characterization of CdTe semiconductor detectors is presented. Chapter 5 describes the simulation code of the experimental device, developed in MATLAB software. The validation of its localization results was tested by intercomparison of them with the experimental ones and with results derived by another simulation code, more detailed, but also slower, developed in SWORD. The structure of the simulation is then presented and the sub-routines of the code and the function of each one are analyzed. In particular, there is a basic routine that calls the following sub-routines: producing shadowgrams, correlating them with Table G, i.e. with the mask structure, source localizing by the triangulation method. Also, this code has been adapted to exploit shadowgrams shadowgrams of experimental measurements or shadowgrams produced by other simulation codes. The above modification works not as a simulation, but as a code for the processing of experimental shadowgrams and image reconstruction of the distribution of radioactive sources in space. Chapter 6 analyzes the experimental results of energy spectroscopy and study of signal pulses, CdTe semiconductor samples. It shows the degradation of the energy spectrum as a function of the application time of the polarization voltage on the samples and assumes the reasons of that. Also in this chapter is presented the energy resolution of CdTe semiconductor detectors with interfaces of molybdenum and molybdenum oxide. The energy resolution correlates with the applied polarization voltage and the leakage current, as indicated by the characteristic curve I-V of each sample. Chapter 7 presents measurements for point source detection with the two γ-cameras described in Chapter 4. Specifically, an Am-241 point source located at various positions within FCFOVS (Full coded field of view of the assembly) and the localization accuracy studied which came up better than 1%. Also, the detection efficiency of the experimental assembly is studied, and the results of localization accuracy and signal to noise ratio (SNR) are related to the shadowgram capture time. Chapter 8 presents measurements of the same experimental assembly for the detection of spatially extended sources. Moreover, the change of the image quality factors studied by simulation, as a function of the geometric characteristics of the sources. There are two different masks designed for "in-focus" imaging at different heights from the γ-cameras. Regarding the change in SNR versus the extent of the source, a maximum is reached for a certain size of the source and an analitical, deterministic explanation of the phenomenon is successfully attempted. Also, an image improvement based on the previous observation is suggested by applying a kernel filter with dimensions of ACF in the correlation table. Applying this filter eliminates the noise with an "egg cup" structure seen in the particular NTHT coded apertures with circular projections of their elements. Chapter 9 provides a brief overview of the PhD Thesis presents the findings and analyzes its contribution to the detection of point and extended radioactive sources, specifically in the field of Nuclear Medicine, Radiation Protection and Nuclear Facilities dismantling. In particular, it is proposed the construction of a mobile system consisting of two γ-cameras for localization of radio-labeled lymph nodes or thyroid gland remains, as well as localization of radioactive sources in remote field applications. Also, the conditions and the technical specifications of the γ-cameras for future research are listed. Finally, a list of English terminology found in the text with representative terms of the Greek language, a table of abbreviations-acronyms and the Bibliography are presented.