Dissertations / Theses on the topic 'Proportional counters'

The X-ray energy resolution achieved by conventional charge signal measurements using a gas proportional counter is determined by statistical fluctuations in the primary number of electrons and in the charge multiplication process. Conventional X-ray energy resolution thus increases as (E)-1/2 where E is the X-ray energy. The proportional counter, therefore, gives a very unsatisfactory X-ray energy resolution for X-ray energies below 1 keV. By eliminating the statistical fluctuations due to the charge multiplication, it is possible to improve the X-ray energy resolution by a factor of 2 over that achieved by conventional charge signal measurements. The implementation of this concept requires an 100% detection efficiency for the electrons present in the primary clusters. The present work is based on electron counting method which uses the charge signals due to single electrons avalanching at the anode wire. The main aim of this work was to determine the maximum possible electron counting efficiency. This required a detailed examination of the parameters relevant to the operation of an electron counting system. An experimental chamber consisting of a uniform field drift tube and a coaxial proportional counter was constructed. Experimental work was carried out to determine electron loss mechanisms such as electron loss by capture, electron loss below the discriminator threshold of the electron counting electronics and electron loss due to the finite resolving time of the electron counting electronics. This involved the measurements of electron mobility and electron lifetime at very low drift fields (Ed/p 0.02 V/cm Torr) for a number of different counter gas mixtures. Single electron response was also examined for these counter gas mixtures at a wide range of charge gains. It was found possible to achieve 89.0% electron counting efficiency at 1.49 keV using A-CH4(50%). The corresponding X-ray energy resolution was found to be 19.5%FWHM, compared to 28.0%FWHM achieved by the conventional charge signal measurements.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010.
Committee Co-Chair: Hertel, Nolan; Committee Co-Chair: Kahn, Bernd; Committee Member: Kulp, David; Committee Member: Lee, Eva; Committee Member: Wang, Chris. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Due to the prevalence of pulse encoders for system state information, an all-digital proportional-integral-derivative (ADPID) is proposed as an alternative to traditional analog and digital PID controllers. The basic concept of an ADPID stems from the use of pulse-width-modulation (PWM) control signals for continuous-time dynamical systems, in that the controllers proportional, integral and derivative actions are converted into pulses by means of standard up-down digital counters and other digital logic devices. An ADPID eliminates the need for analog-digital and digital-analog conversion, which can be costly and may introduce error and delay into the system. In the proposed ADPID, the unaltered output from a pulse encoder attached to the systems output can be interpreted directly. After defining a pulse train to represent the desired output of the encoder, an error signal is formed then processed by the ADPID. The resulting ADPID output or control signal is in PWM format, and can be fed directly into the target system without digital-to-analog conversion. In addition to proposing an architecture for the ADPID, rules are presented to enable control engineers to design ADPIDs for a variety of applications.

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP
FAPESP:01/06837-2

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

This project was designed to determine the feasibility of constructing a multichamber proportional counter. A multi-chamber detector is designed to increase the total surface area which will increase the number of radiation interactions that occur per unit dose. Surface area can be changed without changing the detector volume by subdividing the active volume into several smaller volumes that can then be used as mini detectors whose data can be summed and used to determine the absorbed dose. This will allow the total surface area to remain the same as that of the more common 12.5 cm (5 in.) spherical detector and a decreased total volume resulting in a more compact detector design. However, subdividing those volumes causes problems with electric field fringing at the ends of the mini detectors. In order to correct this, guard ring and field tube designs which operate at a lower voltage than the detector cathode were tested. Results from this study showed that the field tube design provided the best overall resolution but it only outperformed the other designs by a maximum of 5%. However the field tube design doubles the length of the detector which would result in a larger overall detector package. The performance of the single and double ring configurations was suitable for radiation monitoring applications. These findings show that it is feasible to use an array of subdivided detector volumes instead of a spherical detector.

Knowledge of energy deposition in biological cells at nanometer dimensions is essential to understand the biological effects of radiation. This work has resulted in the development of a practical tool to study such energy deposition experimentally, at nanometer dimensions. The main contribution of this research is the design of a cylindrical wall-less proportional counter of 1mm height by 1mm diameter. A wall-less detector (also called grid-walled detector) overcomes the so-called “wall effect”, an experimental artifact that introduces distortions in the radiation energy measurements. An important feature of this detector that distinguishes it from other detectors is its modular design. This allows the detector to be repaired or modified, when necessary, without having to completely disassemble it. Novel design techniques were adopted resulting in a functional detector that can simulate cellular sites as small as 10 nanometers, approximately the size of many molecules in the cell. The detector was tested with a 1 microcurie sealed Am-241 source, which primarily emits monoenergetic alpha particles of energy 5.57 MeV. Microdosimetric spectra analysis for alpha particles and its delta rays from Am-241 were performed for simulated site sizes ranging from 500nm to 10nm. Initial studies to validate the detector design have confirmed good detector performance. We believe this work will serve as a vital platform for bridging the experimentally measured energy spectra to the biological effects of alpha and delta radiations.

Doctor of Philosophy
Department of Mechanical and Nuclear Engineering
Douglas S. McGregor
The 3He gas shortage for neutron detection has caused an increase in research efforts to develop viable alternative technologies. 3He neutron detectors cover areas ranging from 10–1000 cm2 in cylindrical form factors and are ideal for many nuclear applications due to their high intrinsic thermal neutron detection efficiency (> 80%) and gamma-ray discrimination (GRR ≤ 1 x 10-6) capabilities. Neutron monitoring systems for nuclear security applications include Radiation Portal Monitors (RPM’s), backpack, briefcase, and hand-held sensors. A viable replacement technology is presented here and compares three neutron detectors, each with different neutron absorber materials, to current 3He standards. These materials include Li and/or B silica aerogels, LiF impregnated foams, and metallic Li foils. Additionally, other neutron absorbing materials were investigated in this work and include LiF coated Mylar, B foils, BN coated carbon foam, and BN coated plastic honeycomb. From theoretical calculations, the Li foil material showed the greatest promise as a viable 3He alternative, thus a majority of the research efforts were focused on this material. The new neutron detector was a multi-wire proportional counter (MWPC) constructed using alternating banks of anode wires and 95% enriched 6Li foils sheets spaced 1.63 cm apart. In total, six anode banks and five layers of foil were used, thus an anode wire bank was positioned on each side of a suspended foils. Reaction products from the 6Li(n,α)3H reaction were able to escape both side of a foil sheet simultaneously and be measured in the surrounding gas volume concurrently. This new concept of measuring both reaction products from a single neutron absorption in a solid-form absorber material increased the intrinsic thermal neutron detection efficiency and gamma-ray discrimination compared to coated gas-filled detectors. Three different sizes of Li foil MWPC neutron detectors were constructed ranging from 25–1250 cm2 and included detectors for RPM’s, backpacks, and hand-held systems. The measured intrinsic thermal neutron detection efficiency of these devices was approximately 54%, but it is possible to exceed 80% efficiency with additional foils. The gamma-ray discrimination abilities of the detector exceeded 3He tubes by almost three orders of magnitude (GRR = 7.6 x 10-9).

Les compteurs proportionnels sphériques sont utilisés pour la recherche de matière noire par la collaboration NEWS-G depuis 2013 avec le détecteur SEDINE installé au Laboratoire Souterrain de Modane. Ce premier détecteur a permis d'établir une nouvelle limite de la section efficace d'interaction WIMP-nucléon pour des WIMPs de masse inférieur à 0.6 GeV/c². Depuis, la collaboration travail à la mise au point d'un nouveau détecteur. Cette thèse se concentre sur deux aspects améliorés pour le second détecteur. L'étude du bruit de fond observé par SEDINE a permis de conclure que celui-ci est dominé par le présence de la chaine de désintégration du Pb²¹⁰ sur les surfaces et dans le matériaux composant le détecteur et son blindage. Ceci a permis de sélectionner des matériaux et des procédés de fabrication permettant de réduire le bruit de fond du prochain détecteur. Le second développement concerne l'anode du détecteur. Placée au centre de la sphère, l'anode assure la formation du signal. Les derniers développements montrent la capacité de ce composant à assurer la détection d'électrons uniques en étant stable et avec une bonne résolution
The spherical proportional counters have been used by the NEWS-G collaboration since 2013 for dark matter search with the SEDINE detector installed at the Laboratoire Souterrain de Modane. This first detector allowed for establishing a new limit on the WIMP-nucleon cross section for WIMPs of mass less than 0.6 GeV/c². Since then, the collaboration has been working on the development of a new detector. This thesis focuses on two improved aspects of the second detector. The study of the background observed by SEDINE led to the conclusion that it is dominated by the presence of Pb²¹⁰ decay chain on the surfaces and in the materials making up the detector and its shielding. This allowed for the selection of materials and manufacturing methods to reduce the background noise of the next detector. The second development concerns the detector anode. Placed in the center of the sphere, the anode ensures the formation of the signal. The latest developments show the ability of this component to ensure the detection of single electrons while being stable and with good resolution. This study made it possible to develop a new generation of detector which will be installed at SNOLAB in 2020

The Sudbury Neutrino Observatory (SNO), a heavy water Cherenkov experiment, was designed to detect solar Boron-8 neutrinos via their elastic scattering interactions on electrons, or charged current and neutral current (NC) interactions on deuterium. In the third phase of SNO, an array of Helium-3 proportional counters was deployed to detect neutrons produced in NC interactions. A simulation of the current pulses and energy spectra of the main kinds of ionization events inside these Neutral Current Detectors (NCDs) was developed. To achieve this, electron drift times in NCDs were evaluated with a Monte Carlo method, and constrained by using wire alpha activity inside the counters. The pulse calculation algorithm applies to any ionization event, and takes into account processes such as straggling, electron diffusion, and propagation through the NCD hardware. A space charge model was developed to fully explain the energy spectra of neutron and alpha events. Comparisons with data allowed the various classes of alpha backgrounds to be identified, and gave evidence for the spatial non-uniformity of Uranium-238 and Thorium-232 chain nuclei in the counter walls. The simulation was applied to determine the fractional contents of the main types of alpha backgrounds in each NCD string. The number of neutron capture events in the array was extracted via a statistical separation, using Monte Carlo generated alpha background pulse shape parameter distributions and minimal energy information. The inferred total Boron-8 solar neutrino flux is: φ_NC< = 5.74 ± 0.77 (stat) ± 0.39 (sys) x 10⁶ cm^-2s^-1 in agreement with Standard Solar predictions and previous SNO results.

Detecteur gazeux courbe a lame anode. Fonctionnement en mode proportionnel resultant de phenomenes de compensation electrostatique dus a l'extension des lames a condition d'utiliser des melanges gazeux adequats

Cílem této práce je seznámení se s problematikou detekce ionizujícího záření a jeho detekcí zejména pomocí proporcionálních detektorů a navrhnutí algoritmu pro jejich klasifikaci a jeho následná implementace do FPGA. V první části práce je obecně popsáno, jak jednotlivé typy plynem plněných detektorů fungují. V druhé části je věnován prostor algoritmům pro klasifikaci pulzů, které se objevují v literatuře. Následuje návrh vlastního algoritmu, jeho rozbor a rozebrání výsledků. Ve třetí části následuje popis samotné realizace navrhnutého řešení na platformě RedPitaya. Je zde rozebrána celková architektura navrhnutého systému, detailně popsány jak jednotlivé bloky v logice FPGA, tak i ostatní skripty zajišťující zpracování naměřených výsledků a jejich vizualizaci.

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Le détecteur proportionnel sphérique gazeux, SPC (Spherical Proportional Counter), est un nouveau concept de détecteur de particules. Ses principales caractéristiques sont : un seuil très bas en énergie indépendant du volume (faible capacité électronique), une bonne résolution en énergie, une grande robustesse et une seule voie de lecture. SEDINE, un détecteur bas bruit de fond, destiné à la recherche de matière noire légère, a été fabriqué et installé au Laboratoire Souterrain de Modane. Il est actuellement opérationnel et vise à mesurer les évènements rares à bas seuil en énergie. La sensibilité dans la détection d’événements rares étant à basse énergie directement corrélée au niveau du bruit de fond du détecteur, la diminution du seuil en énergie ainsi que celle du bruit de fond ont été la problématique principale de cette thèse. Un effort important a été consacré à la mise en opération du dispositif expérimental. Plusieurs paramètres de détection ont été optimisés : homogénéité du champ électrique dans le volume de l’enceinte, tenue aux étincelles, niveau du bruit de fond électronique et l’étanchéité du détecteur. Le détecteur a été optimisé pour assurer un fonctionnement avec un gain stable à haute pression. La modification du blindage, les nettoyages de l’enceinte du détecteur et l’ajout d’une tente anti-Radon ont permis de réduire significativement le bruit de fond de SEDINE. Les progrès accomplis ont permis d’augmenter la sensibilité du détecteur à basse énergie à une valeur comparable, pour des WIMPs à basse masse, aux autres expériences de recherche souterraines. Nous présentons donc des résultats avec un bruit de fond mesuré, dans la région du keV, qui nous permet de donner une figure d’exclusion compétitive pour la production de la matière noire légère
The Spherical gaseous detector (or Spherical Proportional Counter, SPC) is a novel type of a particle detector, with a broad range of applications. Its main features in- clude a very low energy threshold which is independent of the volume (due to its very low capacitance), a good energy resolution, robustness and a single detection readout channel. SEDINE, a low background detector installed at the underground site of Laboratoire Souterrain de Modane is currently being operated and aims at measuring events at a very low energy threshold, around 40 eV. The sensitivity for the rare events detection at low energy is correlated to the detector background and to the decreasing the level of energy threshold, which was the main point of this thesis. A major effort has been devoted to the operating of the experimental detector. Several detection parameters were optimized: the electric field homogeneity in the sphere, keeping clear of sparks, the electronic noise level and the leak rate of the detector. The detector is optimized for operation with a high pressure stable gain. The modification of the shield, cleanings of the detector and the addition of an anti-Radon tent have significantly reduced the background of SEDINE. Progress has increased the sensitivity of the detector at low energy up to a value comparable to the results other underground research experiences for the low mass WIMPs. We will present the results with a measured background in the region of keV, which has allowed us to show a competitive figure of exclusion for the production of light dark matter

The effective dose equivalent, H (or the effective dose, E ) to an individual is the primary limiting quantity in radiation protection. However, techniques for measuring H for neutrons have not been fully developed. In this regard a new tissue equivalent proportional counter (TEPC) based on a gas electron multiplier (GEM) for measuring H*(10), which is a conservative estimate of H, for neutrons was designed and constructed. The deposited energy distribution for two different neutron sources (a Cf-252 source and a AmBe source) was measured using the new TEPC. The measurements were performed using two different proportional gases: P-10 gas and a propane-based tissue equivalent gas at various pressures. A computer simulation of the new TEPC, based on the Monte Carlo method, was performed in order to obtain the pulse height distributions for the two neutron sources. The simulated results and the measured results were compared. Results show that the experimental results agree with the computational results within 20% of accuracy for both Cf-252 and AmBe neutron sources. A new model GEM-based TEPC was developed for use in obtaining H*(10). The value of H*(10) for the Cf-252 source and for the AmBe source using experimental measurements was obtained. These results are presented in this study. The study shows that the GEM-based TEPC can successfully estimate H*(10). With these results and some refinements, this GEM-based TEPC can directly be used as a neutron rem meter.

Lab measurements and Monte Carlo simulations have been carried out for the evaluation of the Straw-type detectors used in the NA62 experiment at CERN. In addition, analyses of experiment data was used in corrections to improve the reconstruction of particle tracks, ultimately leading to improved resolution of the detector system as a whole. 97.7 percent of the Straws were aligned to within 30 microns, quantified as the deviation from zero of the mean of the inherent residual distribution of each Straw. A drift time dependence on where along the Straw the particle ionized have been corrected for; before the correction the dependence was as big as 6 ns. A radius-drift time relation based on the leading edge timing distribution has been deduced and implemented. Upon implementation artifacts from the piecewise fits used became evident. An alternative approach using residuals has been put forward.

Cette etude a ete consacree a la realisation d'un compteur proportionnel au tissu biologique. Ce detecteur sensible aux neutrons et aux gamma, est destine a mesurer l'equivalent de dose dans le cadre de la radioprotection. L'analyse microdosimetrique des impulsions permet de calculer la dose absorbee et le facteur de qualite, en faisant eventuellement la discrimination entre les deux types de particules

In der vorliegenden Arbeit wird erstmals das QQDS-Magnetspektrometer für die höchstauflösende Ionenstrahlanalytik leichter Elemente am Helmholtz-Zentrum Dresden-Rossendorf umfassend vorgestellt. Zusätzlich werden sowohl alle auf die Analytik Einfluss nehmenden Parameter untersucht als auch Methoden und Modelle vorgestellt, wie deren Einfluss vermieden oder rechnerisch kompensiert werden kann. Die Schwerpunkte dieser Arbeit gliedern sich in fünf Bereiche. Der Erste ist der Aufbau und die Inbetriebnahme des QQDS-Magnetspektrometers, der zugehörige Streukammer mit allen Peripheriegeräten und des eigens für die höchstauflösende elastische Rückstoßanalyse entwickelten Detektors. Sowohl das umgebaute Spektrometer als auch der im Rahmen dieser Arbeit gebaute Detektor wurden speziell an experimentelle Bedingungen für die höchstauflösende Ionenstrahlanalytik leichter Elemente angepasst und erstmalig auf einen routinemäßigen Einsatz hin getestet. Der Detektor besteht aus zwei Komponenten. Zum einen befindet sich am hinteren Ende des Detektors eine Bragg-Ionisationskammer, die zur Teilchenidentifikation genutzt wird. Zum anderen dient ein Proportionalzähler, der eine Hochwiderstandsanode besitzt und direkt hinter dem Eintrittsfenster montiert ist, zur Teilchenpositionsbestimmung im Detektor. Die folgenden zwei Schwerpunkte beinhalten grundlegende Untersuchungen zur Ionen-Festkörper-Wechselwirkung. Durch die Verwendung eines Magnetspektrometers ist die Messung der Ladungszustandsverteilung der herausgestreuten Teilchen direkt nach einem binären Stoß sowohl möglich als auch für die Analyse notwendig. Aus diesem Grund werden zum einen die Ladungszustände gemessen und zum anderen mit existierenden Modellen verglichen. Außerdem wird ein eigens entwickeltes Modell vorgestellt und erstmals im Rahmen dieser Arbeit angewendet, welches den ladungszustandsabhängigen Energieverlust bei der Tiefenprofilierung berücksichtigt. Es wird gezeigt, dass ohne die Anwendung dieses Modells die Tiefenprofile nicht mit den quantitativen Messungen mittels konventioneller Ionenstrahlanalytikmethoden und mit der Dickenmessung mittels Transmissionselektronenmikroskopie übereinstimmen, und damit falsche Werte liefern würden. Der zweite für die Thematik wesentliche Aspekt der Ionen-Festkörper-Wechselwirkung, sind die Probenschäden und -modifikationen, die während einer Schwerionen-bestrahlung auftreten. Dabei wird gezeigt, dass bei den hier verwendeten Energien sowohl elektronisches Sputtern als auch elektronisch verursachtes Grenzflächendurchmischen eintreten. Das elektronische Sputtern kann durch geeignete Strahlparameter für die meisten Proben ausreichend minimiert werden. Dagegen ist der Einfluss der Grenzflächendurchmischung meist signifikant, so dass dieser analysiert und in der Auswertung berücksichtigt werden muss. Schlussfolgernd aus diesen Untersuchungen ergibt sich für die höchstauflösende Ionenstrahlanalytik leichter Elemente am Rossendorfer 5-MV Tandembeschleuniger, dass die geeignetsten Primärionen Chlor mit einer Energie von 20 MeV sind. In Einzelfällen, wie zum Beispiel der Analyse von Bor, muss die Energie jedoch auf 6,5 MeV reduziert werden, um das elektronische Sputtern bei der notwendigen Fluenz unterhalb der Nachweisgrenze zu halten. Der vierte Schwerpunkt ist die Untersuchung von sowohl qualitativen als auch quantitativen Einflüssen bestimmter Probeneigenschaften, wie beispielsweise Oberflächenrauheit, auf die Form des gemessenen Energiespektrums beziehungsweise auf das analysierte Tiefenprofil. Die Kenntnis der Rauheit einer Probe an der Oberfläche und an den Grenzflächen ist für die Analytik unabdingbar. Als Resultat der genannten Betrachtungen werden die Einflüsse von Probeneigenschaften und Ionen-Festkörper-Wechselwirkungen auf die Energie- beziehungsweise Tiefenauflösung des Gesamtsystems beschrieben, berechnet und mit der konventionellen Ionenstrahlanalytik verglichen. Die Möglichkeiten der höchstauflösenden Ionenstrahlanalytik werden zudem mit den von anderen Gruppen veröffentlichten Komplementärmethoden gegenübergestellt. Der fünfte und letzte Schwerpunkt ist die Analytik leichter Elemente in ultradünnen Schichten unter Berücksichtigung aller in dieser Arbeit vorgestellten Modelle, wie die Reduzierung des Einflusses von Strahlschäden oder die Quantifizierung der Elemente im dynamischen Ladungszustandsnichtgleichgewicht. Es wird die Tiefenprofilierung von Mehrschichtsystemen, bestehend aus SiO2-Si3N4Ox-SiO2 auf Silizium, von Ultra-Shallow-Junction Bor-Implantationsprofilen und von ultradünnen Oxidschichten, wie zum Beispiel High-k-Materialien, demonstriert
In this thesis the QQDS magnetic spectrometer that is used for high resolution ion beam analysis (IBA) of light elements at the Helmholtz-Zentrum Dresden-Rossendorf is presented for the first time. In addition all parameters are investigated that influence the analysis. Methods and models are presented with which the effects can be minimised or calculated. There are five focal points of this thesis. The first point is the construction and commissioning of the QQDS magnetic spectrometer, the corresponding scattering chamber with all the peripherals and the detector, which is specially developed for high resolution elastic recoil detection. Both the reconstructed spectrometer and the detector were adapted to the specific experimental conditions needed for high-resolution Ion beam analysis of light elements and tested for routine practice. The detector consists of two compo-nents. At the back end of the detector a Bragg ionization chamber is mounted, which is used for the particle identification. At the front end, directly behind the entrance window a proportional counter is mounted. This proportional counter includes a high-resistance anode. Thus, the position of the particles is determined in the detector. The following two points concern fundamental studies of ion-solid interaction. By using a magnetic spectrometer the charge state distribution of the particles scattered from the sample after a binary collision is both possible and necessary for the analysis. For this reason the charge states are measured and compared with existing models. In addition, a model is developed that takes into account the charge state dependent energy loss. It is shown that without the application of this model the depth profiles do not correspond with the quantitative measurements by conventional IBA methods and with the thickness obtained by transmission electron microscopy. The second fundamental ion-solid interaction is the damage and the modification of the sample that occurs during heavy ion irradiation. It is shown that the used energies occur both electronic sputtering and electronically induced interface mixing. Electronic sputtering is minimised by using optimised beam parameters. For most samples the effect is below the detection limit for a fluence sufficient for the analysis. However, the influence of interface mixing is so strong that it has to be included in the analysis of the layers of the depth profiles. It is concluded from these studies that at the Rossendorf 5 MV tandem accelerator chlorine ions with an energy of 20 MeV deliver the best results. In some cases, such as the analysis of boron, the energy must be reduced to 6.5 MeV in order to retain the electronic sputtering below the detection limit. The fourth focus is the study of the influence of specific sample properties, such as surface roughness, on the shape of a measured energy spectra and respectively on the analysed depth profile. It is shown that knowledge of the roughness of a sample at the surface and at the interfaces for the analysis is needed. In addition, the contribution parameters limiting the depth resolution are calculated and compared with the conventional ion beam analysis. Finally, a comparison is made between the high-resolution ion beam analysis and complementary methods published by other research groups. The fifth and last focus is the analysis of light elements in ultra thin layers. All models presented in this thesis to reduce the influence of beam damage are taken into account. The dynamic non-equilibrium charge state is also included for the quantification of elements. Depth profiling of multilayer systems is demonstrated for systems consisting of SiO2-Si3N4Ox-SiO2 on silicon, boron implantation profiles for ultra shallow junctions and ultra thin oxide layers, such as used as high-k materials

Gas gain measurements were made for nitrogen, xenon and x~non with 2,3 dimethyl-2-butene (DMB) as an additive. Two different methods of measuring gas gain were examined, pulse matching methods and current comparison methods. A preliminary investigation was made into a new approach to the pulse matching method. Previous authors had shaped their test pulses to the same shape as their detector pulses or used a calibration factor to correct for the difference between the shape of the test pulses and the detector pulses. These approaches would be valid only if the detector pulses were always the same shape. The pulses from the detector vary in shape depending upon where the primary ion pairs are formed in the detector sensitive volume. The new approach' I have investigated shapes the detector pulses and the test pulses to the same shape so that the variation in the detector pulse shape becomes irrelevant. This method of measuring gas gain was tested using P-10 and 148Gd as an alpha particle source and gave similar results to gas gains determined by current comparisons. The measurements of gas gain finally used to obtain the data were all performed using the current comparison technique. When the electrons were collected at the central wire and 55Fe was used as the radiation source the ion saturation curves for xenon showed no definite plateaus, only points of inflexion. I found that ion saturation currents could be determined by fitting the observed currents and associated operating voltages to an expression proposed by Johnson. This expression also enabled me to determine the voltage region where gas gain began. The gas gain characteristics of nitrogen and xenon were measured for a range of gas densities. From these measurements it was shown that the gas gain of a cylindrical proportional counter operating in the proportional region is a function only of the reduced field strength at the surface of the anode. Aoyama has proposed an expression for the first Townsend coefficient and he has been able to show that previously proposed expressions are only special cases of his expression. I was able to obtain an excellent fit to Aoyama's expression using my gas gain data for xenon and a value for one of the parameters determined by Kowalski. It is widely held that the mechanism of gas gain in proportional counters is primarily by electron impact. Ionization may also occur in binary mixtures by the non-metastable Penning effect, and the metastable Penning effect. A binary mixture of xenon and DMB was used to investigate these effects. It was found that the gas gain in xenon could be greatly increased by the addition of a small quantity of DMB. A broad peak in gas gain appeared when the concentration of additive was between ~ 0.4% and ~ 0.75% and the molecular number density of the gas filling was 2.7 x 10 [to the power] 25 m-3 (~ 1 atm). When the gas density was reduced to 1.4 x 10 [to the power] 25 m-3 (~ 0.5 atm) the peak became sharper and better defined and reached its maximum when the concentration of additive was ~ 0.43%. Lowering the density to 7 x 10 [to the power] 24 m-3 (~ 0.26 atm) resulted in a very sharp well defined peak centred on an additive concentration of ~ 0.23%. The gas gains used to define these peaks were of the order of 10[ to the power] 5, whilst in pure xenon at identical anode potentials the gas gain was of the order of 10. The mechanisms proposed for these increased gas gains were the non-metastable Penning effect and the metastable Penning effect in the case of high density mixtures, the Pennning effect in the' case of the intermediate density mixtures and for the low density mixtures it was suggested that doubly excited xenon molecules were ionizing the additive. The W-values of the low density xenon plus DMB mixtures were also measured to 5% accuracy. The trend in the mean values of these measurements indicates that the Penning effect is occuring and the minimum value in the W-value occurs at ~ 0.4% which suppports the above proposed mechanisms for the increased gas gains. Detectors filled with mixtures of xenon plus large concentrations of DMB developed double full energy peaks in a relatively short amount of time using the 55Fe source. Passing a large current through the anode removed these double peaks demonstrating that the cause was uneven deposits on the anode. These deposits were probably due to polymerization of free radicals formed in the electron avalanche. Small percentages of DMB did not result in double peaks but drops in gain were observed when the detector was subjected to prolonged usage. The absence of double peaks is attributed to the diffuse nature of the avalanches. If the Penning effect or ionization of the additive by UV photons is taking place the avalanche activity would always completely surround the anode giving rise 'to uniform coatings around the entire circumference of the anode.

Tese de doutoramento em Física, na especialidade de Física Tecnológica, apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Electroluminescence (EL), as the signal amplification method of primary ionisation signal, developed in the 70’s in Gas Proportional Scintillation Counters (GPSC) with noble gas filling, has played an important role in applications to many fields such as X-ray astronomy, plasma physics, medical instrumentation and high-energy physics, up to rising of solid state detectors in the mid 90’s. However, in the last decade EL amplification recovered importance in experiments for rare event detection, such as direct dark matter search and double beta decay. Low count rates and high background imply the need for high signal amplification with statistical fluctuations as low as possible. EL studies are needed for correct detector simulation. While for xenon the literature reported several studies on absolute measurements of EL yield, both experimental and from simulation, yet with disperse results, for argon only one experimental result was reported and its results were ten times lower than the Monte Carlo values from studies performed in our group. The lack of coherent results raised the necessity to study the absolute EL yield in the noble gases xenon and argon thoroughly. In the present work, absolute measurements have been performed for the EL yields of xenon and argon in a uniform electric field GPSC. The obtained results agree with those from Monte Carlo simulation, at room temperature, and with the most recent experimental results measured at cryogenic temperatures reported in the literature, for xenon. We have demonstrated, for the first time that the EL yield, at room temperature, is as high as predicted by the Monte Carlo simulation. Furthermore, studies were performed on the absolute EL yield produced in the strong, variable electric fields of the electron avalanches produced in GEMs and THGEMs. The obtained results have shown that the EL yield values can be more than two orders of magnitude higher than those reached in the commonly used uniform field gaps. This fact can be important if photosensors others than PMTs, with less sensitivity and with area occupancy lower than 100%, are used because of less mass burden and lower radioactivity rates, such as APDs and G-APDs. For the above mentioned studies a simple, straightforward method was used for absolute EL yield measurements, using LAAPDs simultaneously irradiated with EL pulses and X-rays. These X-rays are used as a reference for the absolute determination of the charge carriers produced in the LAAPD by the EL pulses. This technique has been successfully applied, during the last decade, for absolute measurements of primary scintillation yield from organic and inorganic crystals, being suitable for the measurements on EL carried out under the scope of this thesis.
A electroluminescência (EL) em Contadores Gasosos de Cintilação Proporcional (CGCP) com enchimento a gases nobres foi desenvolvida na década de 70 como método de amplificação da ionização primária e desempenhou um papel relevante em aplicações a áreas tão diversas como a astronomia, a física dos plasmas, a instrumentação médica e a física das altas energias. Este protagonismo perdurou até meados da década de 90, época em que surgiram os detectores de estado sólido com qualidade e áreas de detecção melhoradas. A EL reconquistou interesse durante a última década em experiências dedicadas à detecção de eventos raros, como é o caso da procura da matéria negra e do decaimento beta duplo. As baixas taxas de contagem aliadas ao elevado nível de radiação de fundo, característicos destas experiências, definem a necessidade de uma grande amplificação do sinal, associada a baixas flutuações estatísticas, características intrínsecas à amplificação por EL. A correcta simulação de detectores obriga a estudos de EL mais profundos. Na literatura são referidos diversos estudos, tanto experimentais como de simulação, de medições absolutas para o rendimento de EL em xénon, sendo que os resultados apresentavam valores díspares. Para o árgon foi encontrado na literatura um único estudo experimental, cujos valores eram cerca de dez vezes inferiores aos de simulações de Monte Carlo efectuados no nosso grupo. A falta de resultados coerentes suscitou a necessidade de estudar o rendimento absoluto de EL nos gases nobres xénon e árgon. No presente trabalho foram efectuadas medições absolutas para o rendimento de EL em xénon e árgon num CGCP de campo eléctrico uniforme. Os resultados obtidos estão em concordância, tanto com os de simulação de Monte Carlo para a temperatura ambiente como, para o xénon, com os resultados experimentais efectuados a temperaturas criogénicas mencionados na literatura. Mostrou-se, pela primeira vez, que o rendimento de EL à temperatura ambiente é tão elevado como o previsto por simulação de Monte Carlo. Foram igualmente efectuados estudos para o rendimento da EL produzida nos campos eléctricos elevados e variáveis das avalanches de electrões produzidas em GEMs e THGEMs. Os resultados obtidos mostram que estes valores podem ser mais do que duas ordens de grandeza superiores aos obtidos na configuração de campo uniforme. Este facto pode ser importante quando forem utilizados fotosensores que não os PMTs, tais como os APDs ou G-APDs, com menor sensibilidade e/ou uma cobertura em termos de área inferior a 100%, mas que têm níveis de radioactividade menores. Os estudos acima mencionados foram efectuados utilizando um método simples e directo para a medição do rendimento absoluto de EL, com recurso a LAAPDs irradiados em simultâneo com impulsos de EL e raios X. Estes últimos são utilizados como referência para a determinação absoluta dos portadores de carga produzidos no LAAPD pelos impulsos de EL.

Tissue-equivalent proportional counters are frequently used to measure dose and dose equivalent in cosmic radiation fields that include high-Z, high-energy (HZE) particles. The fact that particles with different stopping powers can produce the same energy deposition in the same detector means that the measure of lineal energy cannot provide enough information to evaluate the equivalent dose due to HZE particles. To characterize incident particles by mass and velocity, a multiple-detector system composed of three tissue-equivalent proportional counters simulating different size tissue volumes was proposed to be built. This system took advantage of the well-known fact that lineal energy (y) of a HZE particle depends on the site size, as well as the particle mass and energy. Monte Carlo calculations were used to evaluate lineal energy, using GEANT4, in grid-walled (wall-less) proportional counters with simulated unit density site diameter of 0.1, 0.5 and 2.5 micrometers in a uniform HZE particle field. Uniform beams of 1000 MeV/n and 100 MeV/n 56Fe26+, 28Si14+, 16O8+, 12C6+, 4He2+ ions and proton particles bombarding the detectors were simulated. The results of the calculations were used to determine how much additional information about particle charge and velocity could be obtained from such a detector system. Comparison of simulation results with those of walled detectors was included in the study to illustrate the wall effect. The results shows that the detector system is capable of characterizing HZE particles in a mixed unknown field based on the lineal energy spectra as well as the calculated mean lineal energy. This suggests that it may be practical to use such a system to measure the average particle velocity of HZE particles in space. The parameters used in the simulation are also good references for detector construction. There is only limited experimental data for lineal energy resulting from a large uniform field of HZE particles incident on a wall-less detector. However, the Monte Carlo results are consistent with the experimental data available.

Tissue equivalent proportional counters are frequently used to measure dose and dose equivalent in mixed radiation fields that include neutrons; however, detectors simulating sites 1?m in diameter underestimate the quality factor, Q, for low energy neutrons because the recoil protons do not cross the detectors. Proportional counters simulating different site-sizes can be used to get a better neutron dose equivalent measurement since the range and stopping power of protons generated by neutrons in the tissue-equivalent walls depend on the energy of the primary neutrons. The differences in the spectra measured by different size detectors will provide additional information on the incident neutron energy. Monte Carlo N-particle extended (MCNPX) code was used to simulate neutron transportation in proportional counters of different simulated tissue diameter. These Monte Carlo results were tested using two solid walled tissue equivalent proportional counters, 2mm and 10mm in diameter, simulating tissue volumes 0.1?m and 0.5?m in diameter, housed in a single vacuum chamber. Both detectors are built with 3mm thick tissue equivalent plastic (A-150) walls and propane gas inside for dose measurement. Using these two detectors, the spectra were compared to determine the underestimation of y for large detector, and thereby obtain more information of the incident neutron particles. Based on the MCNPX simulation and experimental results, we can see that the smaller detector produces a larger average lineal energy than the larger detector, which means the larger detector (0.5?m diameter tissue equivalent size) underestimates the Q value for the low energy neutron, therefore underestimates the effective dose. These results confirm the results of the typical analysis of lineal energy as a function of site size.

This thesis presents an independent analysis of the data from 3-He-filled proportional counters from the third phase of the Sudbury Neutrino Observatory (SNO) data. These counters were deployed in SNO's heavy water to independently detect neutrons produced by the neutral current interaction of 8-B solar neutrinos with deuterium. Previously published results from this phase were based on a spectral analysis of the energy deposited in the proportional counters. The work in this thesis introduces a new observable based on the time-profile of the ionization in the counters. The inclusion of this observable in a maximum-likelihood fit increases the potential to distinguish neutrons from backgrounds which are primarily due to alpha-decays. The combination of this new observable with the energy deposited in the counters results in a more accurate determination of the number of neutrons. The analysis presented in this thesis was limited to one third of the data from the proportional counters, uniformly distributed in time. This limitation was imposed to reconcile different time-lines between the submission of this thesis, a thorough review of this work by the SNO Collaboration and results from an independent analysis that is still underway. Analysis of this reduced data set determined that 398 +/- 29 (stat.) +/- 9 (sys.) neutrons were detected in this reduced data-set. The number compares well to the previous analysis of the data, based only on a spectral analysis of the deposited energy, which determined that 410 +/- 44 (stat.) +/- 9 (sys.) were detected in the same time period. The analysis presented here has led to a substantial increase in the statistical accuracy. Assuming that the statistical accuracy will increase when the full data set is analyzed, the results from this thesis would bring the uncertainty in the 8-B solar neutrino flux to down 6.8% from 8.5% in the previously published results. The work from the thesis is intended to be included in a future analysis of the SNO data and will result in a more accurate measurement of the total flux of solar neutrinos from 8-B as well as reduce the uncertainty in the $\theta_{12}$ neutrino oscillation mixing angle.
Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-16 15:56:28.195

Tese de doutoramento em Engenharia Física, ramo de Instrumentacão, apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra
This thesis presents the studies conducted with the objective of developing a new and ruggedized Gas Proportional Scintillation Counter (GPSC) based on high-pressure Xe (5- 20 bar) with a cylindrical geometry for the detection of hard X- and gamma-rays (100 keV to 662 keV). It is to be used in eld applications, where robustness is a requirement, for example in homeland security (detection of illegal transport of radioactive material) or for geological prospection (instrumentation for boreholes). A study of the mobility of ions in gases used in large volume detectors is also presented. In GPSCs, the detection of ionizing radiation is based on the production of scintillation photons as the ampli cation stage, followed by their detection with the help of a photosensor, typically a photomultiplier. GPSCs have an absorption/drift region where the ionizing radiation is absorbed, producing a cloud of primary electrons which is guided by a low electric eld (kept below the excitation threshold of the gas) to the scintillation region, where the electric eld is above the scintillation threshold but below the ionization threshold of the gas. In the scintillation region, they produce a large number of scintillation photons (vacuum ultra-violet photons), emitted during the deexcitation process of the gas atoms. These will eventually reach the photosensor, producing a signal proportional to the energy of the incident radiation. Conventionally, the adopted geometry is planar, since it displays the best energy resolution, but because of the photosensors usually adopted, its use in eld applications is limited. In a recent work, a prototype was developed with a planar geometry with the objective of being more ruggedized for eld applications. The main di erence consisted of the use of a deposited caesium iodide as the photosensor, with the photoelectrons produced by the VUV photons being collected at a grid close to the photocathode. However, this new detector displayed several limitations: low detection e ciency for high energy radiation (above 50 keV); small solid angle subtended by the photosensor; and the high bias voltage needed, which reduced its performance and its application scope. So, to solve these limitations a new detector for higher energies (100-662 keV) was developed using a cylindrical geometry, which is expected to display several advantages. On one hand, the cylindrical con guration allows the number of metallic grids used to be decreased, thus reducing the impact of the internal optical transmission in the detector gain. In addition, the fact that the photocathode is deposited on the inner surface of the detector walls signi cantly increases the solid angle subtended by the photosensor, improving the gain. Also because the radiation is absorbed along the cylinder axis, the detecting e ciency is improved. Moreover, this con guration will, in principle, allow the bias voltage to be minimized for the same gain when compared with the planar geometry. In this work, this new prototype was designed according to the initial performance requirements, constructed and assembled, followed by its characterization with the assessment of the prototype performance using an alpha particle source of 241Am, varying the pressure from 1 up to 3 bar. In the initial stage, the characterization of the 241Am source was performed, followed by the study of the charge collection at the anode and the characterization of the scintillation signal. In this study, it was possible to verify that increasing the E=p above the ionization threshold at the anode surface and slightly above the scintillation one in the collecting region, the energy resolution was improved. In addition, the gain and the signal-to-noise ratio (SNR) of the detector were also determined. Regarding the gain, the experimental values determined were in agreement with the theoretical ones, and at the best possible conditions we were able to reach a gain of 1.9 at 1.05 bar, which gives a good outlook for achieving gains of about 30 at 15 bar. As for the SNR, in the best possible conditions studied, the signal was 10 times greater than the noise, which allowed the minimum detectable energy to be estimated with the detector in the present operating conditions. In parallel with the development of this new detector, the transport properties of ions were also studied to provide information on ion mobility for di erent gas mixtures used or considered for several major experiments (ALICE TPC and TRD, CBM TRD, Next and the future LCTPC), as the information of the mobility of ions in gases is relevant not only for the design and modelling of gaseous radiation detectors, but also in the understanding of the signal formation. This work was developed in the scope of our participation in the NEXT Collaboration and RD51 Collaboration from CERN. The ion drift chamber used in these studies, already available in our laboratory, allows the drift time of this group of ions to be determined with precision and consequently their drift velocity and mobility. Finally, knowing the mobility of these ions and using Blanc's law with the polarization limit of the Langevin's formula, it is possible to identify most of the collected ions. In the scope of this thesis, 5 gas mixtures of interest for the above-mentioned experiments were studied: Xe-N2, Xe-CO2, Xe-CF4, Ar-C2H6 and Ar-CH4. Another interesting result coming from this work is related to the validity of the Langevin polarization formula used to predict the mobility of ions and whose limitations are related to the weak polarizability of some neutrals such as Ne, or by the numerous internal degrees of freedom, responsible for reducing the mobility in gases such as CO2 by about 10%. An alternative method to the use of the Langevin polarization limit, when it fails, is proposed, which will allow a better estimate of the mobility to be obtained.
Esta tese apresenta os estudos realizados com o objectivo de desenvolver e testar um detetor gasoso do tipo contador gasoso de cintilação proporcional (CGCP) baseado em Xe a alta pressão (5-20 bar) com geometria cilíndrica para a detecção de raios-X duros e de radiação gama (100-662 keV), para utilização em ambientes hostis em que a robustez é um requisito, por exemplo em segurança nacional (detecção de transporte ilegal de material radioativo) ou em instrumentação para prospecção geológica. Apresenta também o estudo realizado relativamente às propriedades de transporte de iões em meios gasosos de interesse para detetores de grande volume. Nos CGCPs, a detecção da radiação ionizante assenta na produção de fotões de cintilação como estágio de ampli cação e sua posterior detecção com a ajuda de um fotosensor, geralmente um fotomultiplicador. Os CGCPs são constituídos por uma região de absorção/deriva onde a radiação ionizante é absorvida dando origem a uma nuvem de electrões primários que é guiada por um campo eléctrico baixo (abaixo do limiar de excitação do gás) em direcção à região de cintilação secundária onde, devido ao campo eléctrico mantido acima do limiar de excitação e abaixo do limiar de ionização do gás, são produzidos um elevado número de fotões de cintilação (fotões de VUV). Os fotões emitidos durante o processo de desexcitação dos átomos do meio irão atingir o fotosensor originando um sinal proporcional à energia da radiação incidente. Convencionalmente, a geometria adoptada é a geometria planar por ser a que apresenta a melhor resolução em energia, mas devido à utilização de fotosensores o seu uso em ambiente de campo e limitada. Recentemente foi desenvolvido um protótipo com geometria planar, mais robusto, tendo em vista esse tipo de aplicações, uma vez que utiliza como fotosensor um depósito de iodeto de césio, sendo a carga originada no mesmo pela incidência de fotões VUV, e recolhida numa grelha colocada na sua proximidade. Contudo, este detector revelou algumas limitações: baixa eficiência de detecção para radiação de energia acima de 50 keV, baixo ângulo sólido e elevada tensão de polarização necessária, que condicionam o seu desempenho e a sua gama de aplicações. Para resolver estas limitações foi desenvolvido um novo protótipo para detecção de radiação de energia mais elevada (100-662 keV), optando-se por uma geometria cilíndrica, que apresenta inúmeras vantagens. Por um lado, a con figuração cilíndrica permite diminuir o número de grelhas metálicas utilizadas no anterior protótipo, reduzindo o impacto da transmissão óptica interna no ganho do detector, e melhora signi ficativamente o ângulo sólido. Não menos importante e a melhoria que esta con figuração permite ao nível da e ficiência de detecção, já que nesta geometria a radiação é absorvida ao longo do eixo principal do detector. Também diminui signi ficativamente a tensão de polarização quando comparado com a geometria planar. Depois de um estudo preliminar, o detector foi projetado construído e montado, tendo sido feita a caracterização preliminar do mesmo, à qual se seguiu a veri ficação do seu desempenho com o recurso a uma fonte de partículas alfa de 241Am, para pressões entre 1 a 3 bar. Numa primeira fase, foi feita a caracterização da fonte de 241Am utilizada nos testes, à qual se seguiu o estudo da carga primária recolhida no ânodo, seguindo-se a caracterização do sinal de cintilação. Com este estudo foi possível veri ficar que aumentando o E/p para valores acima do limiar de ionização à superfície do ânodo e ligeiramente acima do limiar de excitação a resolução em energia do detector na região de recolha poderia ser melhorada. Neste trabalho foram ainda estudados o ganho e a relação sinal-ruído (SNR) do detector. Em relação ao ganho, o comportamento observado era o o esperado teoricamente, sendo que nas melhores condições possíveis foi obtido um ganho de 1.9 a 1.05 bar o que nos dá boas perspectivas para alcançar ganhos na ordem de 30 para pressões de 15 bar. Já em relação à SNR, nas melhores condições possíveis, o sinal observado foi 10 vezes superior ao ruído, o que permitiu estimar a energia mínima detetável nestas condições de operação. Em paralelo com o desenvolvimento deste novo detector, foi estudada a mobilidade de iões em diferentes misturas gasosas de interesse ou utilizadas em diversas experiências de grande relevância (ALICE TPC e TRD, CBM TRD, NEXT TPC e a futura LCTPC), uma vez que a informação sobre a mobilidade de iões é relevante não só para o desenho e modelação de detectores, mas também para a compreensão da formação dos impulsos gerados. Este trabalho foi desenvolvido no âmbito da nossa participação na colaboração NEXT e RD51 do CERN. A câmara de deriva de iões positivos utilizada nestes estudos, já existente no laboratório, permite determinar com precisão o tempo de deriva deste grupo de iões e consequentemente a sua velocidade de deriva e mobilidade. Determinada a mobilidade destes iões e utilizando a Lei de Blanc e o limite de polarização de Langevin é possível efectuar a sua identi ficação. No âmbito desta tese foram feitos estudos relativos a cinco misturas com interesse para as experiências mencionadas: Xe-N2, Xe-CO2, Xe-CF4, Ar-C2H6 and Ar-CH4. Neste estudo foi também veri ficada a validade do limite de polarização de Langevin para a estimativa da mobilidade de iões, tendo-se concluído que as suas limitações estão relacionadas essencialmente com a fraca polarizabilidade dos átomos ou moléculas envolvidos como é o caso do Ne, ou pela presença de inúmeros graus de liberdade internos nas mesmas que são responsáveis por reduzir a mobilidade em cerca de 10% em casos como o do dióxido de carbono, CO2. Esta informação torna-se especialmente relevante pois a introdução de correcções permite obter melhores estimativas para a mobilidade em casos onde não existem medidas experimentais.

Tissue equivalent proportional counters (TEPCs) are commonly used for radiation monitoring in areas where a mixture of neutron and photon radiations may be present, such as those commonly encountered in nuclear power plants. In such radiation fields, the dose rate of each component can vary drastically from extremely low to very high. Among these possible combinations of radiation fields with very different dose rates, a mixed field of an intense photon and a weak neutron dose component is the more commonly encountered. This study describes the measurement of lineal energy spectra carried out with a 5.1 cm (2 inch) diameter spherical TEPC simulating a 2 μm diameter tissue site in low energy (33 – 330 keV neutrons) mixed photon-neutron fields with varying dose rates generated by the McMaster University 1.25 MV double stage Tandetron accelerator. The Tandetron accelerator facility was employed to produce neutrons using thick 7Li targets via the 7Li(p, n)7Be reaction. A continuous spectrum of neutrons is generated at any selected proton beam energy which is very narrow at beam energies very close to the threshold of the reaction 1.88 MeV and becomes wider as the proton beam energy moves further away from the threshold energy of the reaction. Dose rates which resulted in dead times as high as 75% for the data acquisition system were employed to study the effect of dose rate on the measured quality factors, microdosimetric averages (y ̅_f and y ̅_D)absorbed dose and dose equivalent. The dose rate at a given beam energy was varied by changing the accelerator beam current. A variety of mixed neutron gamma fields was generated using neutron beams with mean energies extending approximately from 33 keV to 330 keV with the 7Li target using proton beam energies ranging from 1.89 to 2.5 MeV. In direct beams, 478 keV photons which are produced in the 7Li target via inelastic scattering interaction 7Li(p, p'γ)7Li dominate the low LET component of the mixed field of radiation. When a 2 cm thick polyethylene moderator was inserted between the neutron producing target and the counter, the low LET component of the mixed radiation field also contained 2.20 MeV gamma rays originating from 1H(n, γ)2H capture interactions in the moderator. We have observed that high dose rates due to both photons and neutrons in a mixed field of radiation result in pile up of pulses and distort the lineal energy spectrum measured under these conditions. The pile up effect and hence the distortion in the lineal energy spectrum becomes prominent with dose rates which result in dead times larger than 25% for the high LET radiation component. In intense neutron fields, which may amount to 75% dead time, a 50% or even larger increase in values for the measured microsdosimetric averages and the neutron quality factor was observed. This study demonstrates that moderate dose rates which do not result in dead times of more than 20-25% due to either of the component radiations or due to both components of mixed field radiation generate results which are acceptable for operational health physics mixed neutron-gamma radiation monitoring using tissue equivalent proportional counters.
UOIT

Tissue equivalent proportional counters (TEPC) are used for medical and space activities whenever a combination of high and low LET (lineal energy transfer) radiations are present. With the frequency and duration of space activities increasing, exposure to fast heavy ions from galactic cosmic radiation and solar events is a major concern. The optimum detector geometry is spherical; to obtain an isotropic response, but simple spherical detectors have the disadvantage of a non-uniform electric field. In order to achieve a uniform electric field along the detector axis, spherical tissue equivalent proportional counters have been designed with different structures to modify the electric field. Some detectors use a cylindrical coil that is coaxial with the anode, but they are not reliable because of their sensitivity to microphonic noise and insufficient mechanical strength. In this work a new spherical TEPC was developed. The approach used was to divide the cathode in several rings with different thicknesses, and adjust the potential difference between each ring and the anode to produce an electric field that is nearly constant along the length of the anode. A-150 tissue equivalent plastic is used for the detector walls, the insulator material between the cathode rings is low density polyethylene, and the gas inside the detector is propane. The detector, along with the charge sensitive preamplifier, is encased in a stainless steel vacuum chamber. The gas gain was found to be 497.5 at 782 volts and the response to neutrons as a function of angle was constant ±7%. This spherical tissue equivalent proportional counter detector system will improve the accuracy of dosimetry in space, and as a result improve radiation safety for astronauts.

碩士
長庚大學
醫學影像暨放射科學系
99
A proton therapy center is expected to operate in Taiwan about three years later. Microdosimetric study is of particular importance in proton therapy that converts the physical dose into the biological dose. Accordingly, it is imperative to study microdosimetry for proton therapy as a pioneer research in Taiwan. In experimental microdosimetry, the effects of ionizing radiation on biological targets are studied by investigating the statistical distribution of single energy-deposition event at a microscopic level employing a Tissue Equivalent Proportional Counter (TEPC). TEPC is a proportional counter with a cavity and a central anode wire operating with a tissue-equivalent counting gas. But in high dose-rate/fluence radiation fields, such as found in clinical application of linear accelerators or proton therapy facilities, a normal size TEPC cannot be employed because of pile-up of the electronic signals. For the purpose of proton radiotherapy of microdosimetric study, it is desirable to use such a counter which is able to detect and record single energy-deposition event of proton on the chamber. The physical dimensions of a TEPC are an important instrument-design parameter in reducing the effects of dead time and spectrum distortion on account of pulse pile-up. In the past few years, several groups have developed various types of mini-TEPCs. In experience of those literatures, this is usually done by miniaturizing the dimensions of all components of a conventional TEPC. Furthermore, for our design, cylindrical geometry rather than spherical geometry was chosen which results in considerable simplification of the construction of a counter with smaller dimensions. A cylindrical mini-TEPC of 1 mm sensitive volume has been constructed. With such a mini counter, the pulse-height measurements of 30 MeV proton beam have been performed successfully at INER. These results show that promising properties for application of this new type of TEPC, which can be used for the therapeutic proton beam facility of Chang-Gung Memorial Hospital in terms of microdosimetric measurements in the near future.

碩士
長庚大學
醫學影像暨放射科學系
105
In the past decade, many study has shown that the gold nanoparticles (GNPs) have the potential to be a radio-sensitizer in radiation therapy. The radiation enhancement by GNPs is achieved by dose and radical from induced Auger electrons. Nowadays, there is no microdosimetry measurement for the radiation quality enhancement of GNPs in photon radiation. This study used a homemade mini-TEPC in the Chang Gung University (CGU) microdosimetry laboratory to simulate a subcellular target with 1 µm and 0.1 µm diameter irradiated by 192Ir and 250 kVp X-ray photons. This mini-TEPC was used to measure microdosimetry spectra and ready for radiation quality and RBE analysis. A plug with 50 nm thickness of GNPs coating was inserted into the hole on the mini-TEPC. The relative biological effectiveness (RBE) of Human Salivary Gland tumor at the 10% survival level was evaluated by MK model through the relationship between dose mean lineal energy (y ̅_D) and RBE. Microdosimtric analysis results show that the radiation enhancement effects of GNPs caused by y ̅_D increased by 16% and RBE increased by about 4% in the 1 µm subcellular target under 250 kVp X-ray irradiation conditions. For the assessments of the mini-TEPC irradiated by 192Ir, however, the radiation quality enhancement effects of GNPs shows no significant enhancements. This study has demonstrates the feasibility of microdosimetric measurements in photon radiations and evaluated the radiation enhancement of GNPs from the microdosimetric point of view.

The purpose of this study was to determine how well the Monte Carlo transport code FLUKA can simulate a tissue-equivalent proportional counter (TEPC) and produce the expected delta ray events when exposed to high energy heavy ions (HZE) like in the galactic cosmic ray (GCR) environment. Accurate transport codes are desirable because of the high cost of beam time, the inability to measure the mixed field GCR on the ground and the flexibility they offer in the engineering and design process. A spherical TEPC simulating a 1 um site size was constructed in FLUKA and its response was compared to experimental data for an 56Fe beam at 360 MeV/nucleon. The response of several narrow beams at different impact parameters were used to explain the features of the response of the same detector exposed to a uniform field of radiation. Additionally, an investigation was made into the effect of the wall thickness on the response of the TEPC and the range of delta rays in the tissue-equivalent (TE) wall material. A full impact parameter test (from IP = 0 to IP = detector radius) was performed to show that FLUKA produces the expected wall effect. That is, energy deposition in the gas volume can occur even when the primary beam does not pass through the gas volume. A final comparison to experimental data was made for the simulated TEPC exposed to various broad beams in the energy range of 200 - 1000 MeV/nucleon. FLUKA overestimated energy deposition in the gas volume in all cases. The FLUKA results differed from the experimental data by an average of 25.2 % for yF and 12.4 % for yD. It is suggested that this difference can be reduced by adjusting the FLUKA default ionization potential and density correction factors.

碩士
南台科技大學
電子工程系
93
In this paper, the system takes advantage of the X-ray to determine sheetmetal or the degree of internal damage of sample by the position of output pulse. The parts of this digital signal position detection system can be carried out by the field programmable gate array (FPGA). FPGA can be used by the controller in controlling circuit capability and also offers the large volumes of gate arrays to store complex circuit in one chip. Other advantages are high capacity, low power consumption and high secret and programmable. Specifically, (1) X-ray shines on the sheetmetal for producing a reflective pulse signal, (2) this reflective pulse signal is inputted to the pre-amplifier of high sensitive position diction system, (3) the output signal will be transferred and then be downloaded into FPGA. In this digital circuit, signal will be used for determining the message, which is adopted by discriminator circuit. The noise of this signal will be eliminated by infinite impulse response filter (IIR filter), and then the data will be saved in the waveform memory in FPGA Finally, we can determine the degree and the position of internal damage of our sample through the distribution of X-ray energy by reading the data in its unique software.