Made available in DSpace on 2014-10-09T12:32:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:10:17Z (GMT). No. of bitstreams: 1 03269.pdf: 4525465 bytes, checksum: 8d08a1f89182180eaf465278796a6732 (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Le contexte de ce doctorat s’inscrit dans la lutte contre les risques de terrorisme nucléaire et radiologique (acronyme NRBC-E). La détection de ces matières dangereuses, car émettrices de neutrons, s’effectue traditionnellement à l’aide de compteurs proportionnels à Hélium-3. Or, l’annonce de la pénurie de ce gaz depuis plus d’une dizaine d’années pousse à concevoir des détecteurs aussi performants. L’émission neutronique étant toujours accompagnée d’un flux gamma, les détecteurs doivent discriminer ces deux contributions. Les scintillateurs plastiques, polymères radioluminescents, peuvent opérer cette séparation. Celle-ci s’effectue alors sur le déclin de l’impulsion lumineuse. Née en 1968, la théorie de Voltz et Laustriat fournit une explication de la discrimination neutron/gamma dans les scintillateurs organiques (« Pulse Shape Discrimination », PSD). Ainsi, le sujet du doctorat est d’appréhender les phénomènes photophysiques ayant lieu dans ces matériaux, plus particulièrement sous forme plastique, après l’interaction neutron/matière ou gamma/matière mais avant l’émission de photons de scintillation. Nous avons d’abord dressé un état de l’art des scintillateurs plastiques discriminants de 1959, année du premier matériau préparé, jusqu’à aujourd’hui. Nombre de compositions chimiques ont été décrites dans la littérature ; ces travaux mettent en évidence les compositions chimiques permettant la discrimination neutron/gamma. Compte-tenu de l’extrême complexité de modéliser l’interaction rayonnement/matière (énergies de l’ordre du MeV) suivie des transferts photophysiques (de l’ordre de l’eV), nous avons caractérisé des scintillateurs plastiques préparés au laboratoire. Ainsi, nous avons mis en place une chaîne d’acquisition numérique permettant la discrimination neutron/gamma. Nous avons ensuite testé l’influence de paramètres intrinsèquement liés à la nature du matériau : la préparation chimique, le volume et le fluorophore secondaire. Nous avons constaté que la reproductibilité des matériaux plastiques est complexe à obtenir. Du reste, le fluorophore secondaire et sa concentration doivent être soigneusement sélectionnés selon le volume du scintillateur afin d’éviter l’auto-absorption. Grâce à des mesures d’absorption transitoire, nous avons identifié le transfert photophysique conférant un rôle important au fluorophore secondaire. Par ailleurs, nous avons évalué l’influence de critères extrinsèques aux scintillateurs plastiques, plus spécifiquement l’influence d’une forte irradiation (10 kGy), sur les propriétés de discrimination neutron/gamma des matériaux. Enfin, grâce à la plateforme ELYSE (CNRS & Université Paris-Sud), nous avons optiquement simulé une trace neutron dans des scintillateurs liquides et plastiques. Grâce au système de détection offrant une spectrométrie 3D en absorption transitoire et en fluorescence, nous avons élaboré une nouvelle théorie photophysique permettant d’expliquer la formation d’états excités triplets significatifs pour la discrimination neutron/gamma. Les travaux présentés ici contribuent à l’appréhension des phénomènes photophysiques responsables de la discrimination neutron/gamma dans les scintillateurs plastiques. / The context of this PhD lies within the framework of fighting against nuclear and radiological threats (CBRN-E acronym). These hazardous materials can emit neutrons. Neutrons can traditionally be detected thanks to a proportional counter based on Helium-3 gas. However, the last decade announced the shortage of this gas, leading therefore scientists to design new detectors, which are as effective as proportional counters. Neutrons are always emitted with a gamma rays flux. So detectors have to discriminate between these two contributions. Plastic scintillators, which are radioluminescent polymers, can effectively operate this separation. This discrimination between neutrons and gamma rays is made thanks the decay of the light pulse. Born in 1968, Voltz and Laustriat’s theory explains neutron/gamma discrimination in plastic scintillators (also named Pulse Shape Discrimination, PSD). Thus, the subject of this PhD is to understand photophysical phenomena in plastic scintillators, which take place after neutron/matter or gamma/matter interaction but before the emission of scintillation photons. We first provided a state of the art of discriminating plastic scintillators as early as 1959 (first prepared material) until nowadays. Many chemical compositions have been described in the literature. All these works highlight the need to finely select the chemical composition allowing neutron/gamma discrimination. It is extremely hard to model the interaction of radiation with matter (energies up to the MeV range) followed by photophysical transfers (up to the eV range). This way, we characterized lab made plastic scintillators. To do this, we set up a digital detection chain for neutron/gamma discrimination measurements. We then tested the influence of intrinsic parameters to the nature of scintillators: chemical preparation, volume and secondary fluorophore have been particularly studied. We noted that scintillators reproducibility is complex to obtain. Furthermore, the secondary fluorophore and its concentration have to be selected according to the volume of the material in order to avoid self-absorption. Thanks to transient absorption measurements, we identified the photophysical transfer which allocates a significant role to the secondary fluorophore. We then evaluated the influence of extrinsic criteria on neutron/gamma properties of plastic scintillators, and specifically high irradiation doses (10 kGy). Finally, thanks to the ELYSE platform (CNRS & Paris-Sud University), we optically simulated a neutron track in liquid and plastic scintillators. Thanks to the detection system offering a 3D spectrometry in transient absorption and fluorescence, we elaborated a new photophysical theory, which can explain the formation of triplet states in plastic scintillators for neutron/gamma discrimination. All these works presented herein contribute to understand the photophysical phenomena, which are responsible of neutron/gamma discrimination in plastic scintillators.

L’expérience Double Chooz, située sur le site de la centrale nucléaire de Chooz dans la région des Ardennes en France, étudie la disparition des antineutrinos. Le but principal de l’expérience est de mesurer avec une grande précision l’amplitude d’oscillation sin2 2θ13 des antineutrinos émis par les deux réacteurs de la centrale de Chooz. La précision de cette mesure dépend fortement d’une connaissance précise des taux et de la forme des spectres des bruits de fond qui contaminent la sélection finale des antineutrinos en particulier dans la gamme d’énergie où l’oscillation des neutrinos est attendue. Nous avons étudié dans la présente thèse le bruit de fond d’origine cosmique dans l’expérience Double Chooz. En effet, les muons cosmiques traversant les détecteurs ou interagissant dans le voisinage immédiat sont la principale source de bruit de fond observés dans l’expérience Double Chooz. Deux types de bruits de fonds sont analysés et présentes dans cette thèse : les neutrons rapides (FN) et la double capture de neutrons (DnC). Des techniques d’identification dédiées ont été développées pour chacun de ces bruits de fond et, par conséquent, les formes spectrales et les taux associés ont été́ déterminés. Les valeurs obtenues dans le cadre de ce travail servent de paramètres d’entrée dans le fit final d’où la valeur de θ13 est extraite. La dernière mesure publiée par la collaboration Double Chooz est sin2 2θ13 = 0.119 ± 0.016. / The Double Chooz experiment is- a reactor antineutrino disappearance experiment located on the site of the Chooz nuclear power plant in the Ardennes region in France. The principal aim of the experiment is a high precision measurement of the oscillation amplitude sin2 2θ13 of the antineutrinos emitted from the two reactor cores of the Chooz power plant. The robustness and accuracy of this measurement depends strongly on a precise knowledge of the rates and spectral shapes of the backgrounds that contaminate the antineutrinos selection over the neutrino oscillation expected region. We have studied in the present thesis the muon induced background in the Double Chooz experiment. Indeed, cosmic muons crossing the detectors or interacting in the neighborhood constitute the main source of background events encountered in Double Chooz. Two distinct backgrounds analysis are presented in this thesis: fast neutrons (FN) and double capture of neutrons (DnC). Dedicated identification techniques have been developed for each of these backgrounds and, consequently, the associated spectral shapes and rates have been determined. The values obtained in this work serve as inputs in the final fit whence the θ13 value is extracted.The latest measurement released by the Double Chooz collaboration is sin2 2θ13 = 0.119 ± 0.016$.

A boron neutron capture enhanced fast neutron therapy assembly has been designed for the Fermilab Neutron Therapy Facility (NTF). This assembly uses a tungsten filter and collimator near the patient¡¯s head, with a graphite reflector surrounding the head to significantly increase the dose due to boron neutron capture reactions. The assembly was designed using Monte Carlo radiation transport code MCNP version 5 for a standard 20x20 cm2 treatment beam. The calculated boron dose enhancement at 5.7-cm depth in a water-filled head phantom in the assembly with a 5x5 cm2 collimation was 21.9% per 100-ppm B-10 for a 5.0-cm tungsten filter and 29.8% for an 8.5-cm tungsten filter. The corresponding dose rate for the 5.0-cm and 8.5-cm thick filters were 0.221 and 0.127 Gy/min, respectively. To validate the design calculations, a simplified BNCEFNT assembly was built using four lead bricks to form a 5x5 cm2 collimator. Five 1.0-cm thick 20x20 cm2 tungsten plates were used to obtain different filter thicknesses and graphite bricks/blocks were used to form a reflector. Measurements of the dose enhancement of the simplified assembly in a water-filled head phantom were performed using a pair of tissue-equivalent ion chambers. One of the ion chambers is loaded with 1000-ppm natural boron (184-ppm 10B) to measure dose due to boron neutron capture. The measured dose enhancement at 5.0-cm depth in the head phantom for the 5.0-cm thick tungsten filter is (16.6 ¡À 1.8)%, which agrees well with the MCNP simulation of the simplified BNCEFNT assembly, (16.4¡À 0.5)%. The error in the calculated dose enhancement only considers the statistical uncertainties. The total dose rate measured at 5.0-cm depth using the non-borated ion chamber is (0.765 ¡À 0.076) Gy/MU, about 61% of the fast neutron standard dose rate (1.255Gy/MU) at 5.0-cm depth for the standard 10x10 cm2 treatment beam. The increased doses to other organs due to the use of the BNCEFNT assembly were calculated using MCNP5 and a MIRD phantom.

An der Neutronen-Flugzeit-Anlage nELBE des Helmholtz-Zentrums Dresden-Rossendorf sollen Reaktionsquerschnitte mit Relevanz für die nukleare Transmutation bestimmt werden. Die Transmutation hochradioaktiver Abfälle aus abgebrannten Brennelementen thermischer Kernreaktoren in schnellen Neutronenspektren hat das Potential die langlebige Radiotoxizität der Abfälle deutlich zu reduzieren. Zum grundlegenden Verständnis der Physik der Transmutation müssen sowohl Spalt- und Neutroneneinfang-Wahrscheinlichkeiten von Brennelementbestandteilen als auch inelastische Streuquerschnitte an Konstruktionsmaterialien im schnellen Neutronenspektrum mit möglichst kleinen Unsicherheiten bekannt sein. Diese Arbeit beschäftigt sich mit der Messung des inelastischen Neutronen-Streuquerschnittes mit Hilfe einer neu entwickelten Doppel-Flugzeit-Methode. Mit einem kombinierten Aufbau aus Plastik- und BaF2-Szintillationsdetektoren werden die beim Streuprozess emittierten Neutronen und Photonen in Koinzidenz erstmalig nachgewiesen und dadurch der bei der Streuung angeregte Zustand des Zielkerns identifiziert. An nELBE wird weltweit einzigartig der Elektronenstrahl eines supraleitenden Linearbeschleunigers, des ELBE-Beschleunigers, zur Erzeugung schneller Neutronen benutzt. Dieser wird auf einen Kreislauf flüssigen Bleis fokussiert, in dem die Elektronen Bremsstrahlung erzeugen, die wiederum Neutronen aus Bleikernen herauslöst. Durch die kurze Zeitdauer der Elektronenstrahlimpulse von ca. 5 ps kann mit einem kompakten Neutronenquellvolumen auch mit einer kurzen Flugstrecke eine gute Zeitauflösung erzielt werden. Das emittierte Neutronenspektrum hat eine einem Maxwell-Boltzmann-Spektrum ähnliche Verteilung und reicht von etwa 10 keV bis etwa 10 MeV. Bei einem verwendbaren Elektronenstrom von 15 μA beträgt die Quell-Stärke etwa 1,6 · 10^11 n/s. Die Neutronen werden kollimiert und auf eine Probe natürlichen Eisens geschossen, die bei einer Flugstrecke von etwa 6 m positioniert war. Die Probenposition ist von einem Array von bis zu 42 BaF2-Szintillationsdetektoren zur Photonendetektion umgeben. In einem Abstand von 1 m sind fünf 1 m lange Plastik-Szintillationsdetektoren zum Neutronennachweis aufgebaut. Zur Bestimmung des einfallenden Neutronenflusses wurde eine 235U-Spaltkammer verwendet, die bei einer Flugstrecke von etwa 4,3 m zwischen Neutronenquelle und Probe aufgestellt war. Die Signale aller Detektoren werden von einer speziell dafür entworfenen VME basierten Datenaufnahmeelektronik verarbeitet und die Zeit- und Ladungs-Werte bestimmt. Aus dem Detektionszeitpunkt des Photons wird die Flugzeit und damit die Energie des einfallenden Neutrons bestimmt. Aus der Zeitdifferenz zwischen der Photonen- und Neutronendetektion ergibt sich die Flugzeit bzw. Energie des gestreuten Neutrons. Mit Hilfe von Kinematik-Rechnungen können die Ereignisse herausgefiltert werden, die der inelastischen Streuung unter Anregung eines bestimmten Kernniveaus eines bestimmten Isotops entsprechen. Aus dem Verhältnis von eingefallenem Neutronenstrom und nachgewiesenen Streuereignissen jeder Kombination aus einem Plastik- und einem BaF2-Szintillationsdetektor wurde entsprechend der Raumwinkelabdeckung der Detektoren der winkel- und energiedifferentielle inelastische Streuquerschnitt bestimmt.

Diss. (sammanfattning) Linköping : Linköpings universitet, 2007. / Härtill 4 uppsatser. I publikationen felaktig serieuppgift: Linköping studies in health sciences ; 989.

Made available in DSpace on 2014-10-09T12:48:41Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:45Z (GMT). No. of bitstreams: 1 09250.pdf: 2620343 bytes, checksum: 7c7a04350dced4d288c23f2472f9b667 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / Linear accelerators of high energy beams have been used to treat tumors in radiotherapy. Some questions about the protection of the patient and occupationally exposed workers have been arisen because high energy bundles generate scattered radiations that are undesirable in treatment. A possible radiation type generated due to the interaction of the beam with accelerator elements is the neutron, mainly fast neutrons. Such radiation can cause serious problems for patients besides cause damage to devices used by them. This work aims investigate the presence of fast neutrons around a mobile accelerator dedicated to intraoperative radiotherapy (IORT) using high energy electron beams. The aim is to detect the possible fast neutrons and further evaluate their distribution within phantom irradiated. For this study a mobile accelerator (LIAC), for electron beams, and a conventional accelerator (Elekta), for the photon beam, were used. Neutron measurements were made using bubble detectors (C-318) which are sensitive only to fast neutrons. In order to simulate the patient two polymethylmethacrylate (PMMA) phantoms, with a volume of 62 x 29 x 24 cm3 and 80 x 30 x 20 cm3, were used. Three detectors were positioned around the LIAC accelerator to measure the fast neutrons radiation generated by the accelerator elements. The measurements inside the phantom were made with three detectors positioned at three different depths, 5, 10 and 15 cm; 1, 5 and 10 cm. Moreover, two distances with respect to the central axis were chosen to the phantons irradiated by LIAC and Elekta: 0 and 4 cm; 0 and 30 cm, respectively. The results from the measurements around the LIAC demonstrate the abscense of fast neutrons. The results for both, LIAC and Elekta, showed that the dose of fast neutrons is mainly concentrated in the central axis and a decrease of 60% and 99%, respectively, in relation to off-axis doses. The highest values found for the different energies of 12, 10 and 8 MeV were 2.44, 1.14 and 0.59 uSv / Gy, respectively, for the LIAC. While the highest values for energies of 15, 10 and 6 MeV for Elekta were 2.98, 1.08 and 0.15 mSv/Gy, respectively. These results also showed the increasing ratio of dose to energy, both for Elekta and for LIAC. The relation between dose and depth also was discussed. It was found that in some cases the neutron attenuation increases with depth as expected while in others the ratio is not clear. The comparison between the doses established in each accelerator showed a difference around 10−3. The data presented in this work, therefore, demonstrate that the LIAC minimally generates fast neutrons within the phantom, when compared with the Elekta, and around were not detected for the LIAC. The neutrons generated by both accelerators presented similar behaviour within the phantom in relation to the energy variation, depth and central axis distance. / Aceleradores lineares de feixes de alta energia são utilizados para tratamento de tumores em radioterapia. Isso tem gerado algumas questões sobre a proteção do paciente e dos trabalhadores ocupasionalmente expostos, pois feixes de alta energia geram radiações espalhadas que são indesejáveis no tratamento. Uma das possíveis radiações geradas devido à interação do feixe com elementos do acelerador é o nêutron, principalmente nêutrons rápidos. Essa radiação pode causar sérios problemas para pacientes caso interajam com o mesmo e ainda podem causar danos a dispositivos utilizados pelo paciente. A proposta desse estudo foi investigar a presença de nêutrons rápidos ao redor de um acelerador móvel dedicado, LIAC, para radioterapia intraoperatória (RT-IO) que utiliza feixes de elétrons de alta energia. Busca-se detectar os possíveis nêutrons rápidos e ainda avaliar a sua distribuição dentro de objeto simulador irradiado. Para este estudo foram utilizados tanto um acelerador móvel (LIAC), para feixes de elétrons, quanto um acelerador convencional (Elekta), para feixe de fótons. As medidas dos nêutrons foram feitas utilizando detectores de bolhas (C-318) que são sensíveis apenas a nêutrons rápidos. Para simular o paciente foram utilizados dois phantons de polimetilmetacrilato (PMMA) com volume de 62 x 29 x 24 cm3 e 80 x 30 x 20 cm3. Em cada posição escolhida, três detectores foram posicionados ao redor do acelerador LIAC para medir a radiação de nêutrons rápidos que eram gerados pelos elementos constituintes do acelerador. As medidas dentro do phantom foram feitas com três detectores posicionados em três profundidades diferentes, 5, 10 e 15 cm; 1, 5 e 10 cm e duas distâncias em relação ao eixo central, 0 e 4 cm; 0 e 30 cm para os phantons irradiados pelo LIAC e Elekta, respectivamente. Os resultados das medidas ao redor do LIAC demonstraram a ausência de nêutrons rápidos. Os resultados tanto para o LIAC quanto para o Elekta mostraram que a dose de nêutrons rápidos é principalmente concentrada no eixo central do feixe e uma queda de 60%, a 4 cm do eixo, e 99%, a 30 cm do eixo, respectivamente. Os maiores valores encontrados para as diferentes energias de 12, 10 e 8 MeV foram 2,44, 1,14 e 0,59 uSvGy para o LIAC, respectivamente. Enquanto que os maiores valores para energias de 15, 10 e 6 MeV para o Elekta foi de 2,98, 1,08 e 0,15 mSVGy, respectivamente. Esses resultados ainda mostraram a relação crescente da dose com a energia, tanto para Elekta quando para LIAC. Outra relação que foi discutida é da dose com a profundidade, porque em alguns casos é nítida a atenuação de nêutrons com o aumento da profundidade e em outros não se demonstra tão regular. A comparação entre as doses estabelecidas em cada acelerador mostrou uma diferença em torno de 10−3. Os dados apresentados nesse trabalho, portanto, demonstram que o LIAC gera minimamente nêutrons rápidos dentro do phantom, se comparado com o Elekta, e ao redor do LIAC não foram detectados. O nêutrons gerados por ambos aceleradores apresentaram comportamento parecido dentro do phantom em relação a variação de energia, profundidade e distância do eixo central. / São Cristóvão, SE

Made available in DSpace on 2014-10-09T12:48:41Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:45Z (GMT). No. of bitstreams: 1 09250.pdf: 2620343 bytes, checksum: 7c7a04350dced4d288c23f2472f9b667 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Thesis (M.S. in Physics)--Naval Postgraduate School, January 1965. / Thesis Advisor(s): Handle, Harry E. "January 1965." Description based on title screen as viewed on June 2, 2010 DTIC Descriptor(s): (Neutron Cross Sections, Thermal Neutrons), (Research Reactors, Reactor Feasibility Studies), Fast Neutrons, Gold, Radioactive Isotopes, Measurement, Perturbation Theory, Neutron Capture, Indium, Standards, Errors, Materials, Neutron Flux, Mathematical Analysis, Cadmium, Reactor Shielding Materials, Computer Programming, Foils (Materials), Reactor Control, Reactor Kinetics, Reactor Start Up Sources. DTIC Identifier(s): AGN-201 Reactors Includes bibliographical references (p. 32). Also available in print.