Dissertations / Theses on the topic 'Plastic Scintillators'

A plastic scintillation detector exhibiting minimum interference using two different optical fibre light guides have been designed, constructed and evaluated for dosimetry (potentially in-vivo) of the high energy beams used in radiotherapy practice. One detector system contains the radiation resistant SiO2 optical fibre, while the other contains PMMA fibre, which has less resistance to radiation. Each fibre is connected to an independent photo diode. Also, each fibre is connected to a cylindrical water equivalent scintillator measuring 3mm in diameter and 10mm in length. The scintillator is coated with a thin, optically reflective coating. The light arising from the scintillator is transmitted by the fibre optic light guide and is detected at the photodiode. Each fibre has its own focusing and optical filtering techniques based on the fibre acceptance angle, so the photodiode for SiO2 and PMMA are the same. The photodiodes are connected to an electronics box with a digital integrator and counting system. The interference radiation could be minimised using inherent optical filtration and digital integration consistent with pulses from the linear accelerator, thereby avoiding the use of a second optical fibre to compensate for background signals. The SiO2 and PMMA fibre light guides have different properties especially with respect to flexibility. The dose distribution in water of each fibre light guide coupled scintillator is measured and shows good agreement with ionisation chamber results. Spatial resolution and water equivalence are the most important properties of miniature scintillator detectors. It is shown that these systems are not energy dependent, they do not disturb the main radiation beam, are independent of beam angle and have very good linearity with dose. The current study may stimulate the use of SiO2 and PMMA fibre coupled plastic scintillation detectors in medical dosimetry applications. The results of the PMMA scintillation detector are good and it is a promising detector for in vivo measurements due to its flexibility and low cost.

The T2K Neutrino Oscillation experiment consists of a νμ beam produced at JPARC and two detectors: one placed280m downstream, the near detector, and Super-K, which sits 295km downstream. The goal of the experiment is measure the θ₁₃ mixing angle and the phase δ found in the MNSP neutrino mixing matrix by measuring the disappearance of νμ from the original neutrino beam and appearance of νe in the beam as it reaches Super-K. The near detector contains a FGD (Fine Grained Detector) which is designed to provide target mass for the ν μ and then track the particles which come out once the νμ has reacted. The FGD consist mainly of nominally sized 1 cm x 1 cm x 200 cm plastic scintillator bars, which are read out by wavelength-shifting optical fibers (which are in turn read out by MPPC photosensors) threaded through each bar. The bars are glued into a series of XY layers. In November 2006 we successfully extruded 11900 Scintillator Bars at CELCO Plastics in Surrey, BC. On site QA found only 2.5 % of the bars to fall outside of bar specifications. During the November bar production run we scanned 4-10 of each 100 bars within a day of them coming off the production line. We found a variation in the light yields of the bars to be 5%. We set up an aging test for the bars. Three separate measurements were consistent with a 2% per year aging rate. In December 2006 we put our new bars in the M11 beam line at TRIUMF. The light yield of the bars was measured using MIPS. With the beam hitting the far end of the bar and no reflector on the end of the bar the light yield and was found to be adequate for the FGD. I predicted, using a simulation, the difference in light collected by 2 sizes of MPPC, one 1 mm sq and one 1.3 mm sq with a gap of 0.4 mm between fiber and photosensor to be 25%. This was in good agreement with Kyoto groups direct measurement.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

An investigation into the luminescent response of thin film plastic scintillators as a function of their method of preparation is made. Investigations are carried out on NE102A and NE118 using four different methods of preparation. It is found that the Birks model for luminescence as a function of film thickness successfully explains the response in three of the four methods of preparation, but fails to explain the response of thin films prepared on a glass surface. These films show an unexpected non-linearity in their behaviour. It is proposed that the behaviour in these films can be explained in terms of the existence of surface regions in these films. A model based on the existence of these surface regions is prepared. It is further proposed that, in general, the luminescent response of thin films of plastic scintillator is dependent on their method of preparation.

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

A plastic scintillator detector was developed and tested in a 6MV photon beam. The detector comprised a BCF60 plastic scintillator, Polymetyl-Methacrylate Resin optical fibre and photodiode SFH250. The detector was used to measure an inplane profile for the photon beam at a depth of 1.5 cm for a field size of 10x10cm2 at 100 cm SSD. The photon beam was delivered by a Siemens linear accelerator. A comparison was made with the results obtained by cylindrical chambers CC04 and CC13, commercial diode PFD and a stereotactic diode SFD, all from the manufacturer IBA-Wellhöfer. An analysis was performed using the Gamma Evaluation method and the agreement was acceptable for a criterion of Distance To Agreement = 2 mm and Dose Difference = 2%.

This thesis investigates the ability of transparent surface coatings to reduce xenon diffusion into plastic scintillators. The motivation for the work is improved radioxenon monitoring equipment, used with in the framework of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty. A large part of the equipment used in this context incorporates plastic scintillators which are in direct contact with the radioactive gas to be detected. One problem with such setup is that radioxenon diffuses into the plastic scintillator material during the measurement, resulting in an unwanted memory effect consisting of residual activity left in the detector. In this work coatings of Al2O3 and SiO2, with thicknesses between 20 and 400 nm have been deposited onto flat plastic scintillator samples, and tested with respect to their Xe diffusion barrier capabilities. All tested coatings were found to reduce the memory effect, and 425 nm of Al2O3 showed the most promise. This coating was deposited onto a complete detector. Compared to uncoated detectors, the coated one presented a memory effect reduction of a factor of 1000. Simulations and measurements of the expected light collection efficiency of a coated detector were also performed, since it is important that this property is not degraded by the coating. It was shown that a smooth coating, with a similar refractive index as the one of the plastic, should not significantly affect the light collection and resolution. The resolution of the complete coated detector was also measured, showing a resolution comparable to uncoated detectors. The work conducted in this thesis proved that this coating approach is a viable solution to the memory effect problem, given that the results are reproducible, and that the quality of the coating is maintained over time.

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

Cette thèse concerne la mesure neutronique passive des coïncidences de fissions spontanées du plutonium dans des fûts de déchets radioactifs, avec des scintillateurs plastique pour pallier aux compteurs gazeux à 3He classiques, devenus très coûteux. Les scintillateurs sont environ 1000 fois plus rapides avec un temps de détection de l’ordre de la nanoseconde, mais très sensibles aux rayonnements gamma et aux coïncidences parasites de diffusions neutroniques et gamma entre détecteurs voisins (diaphonie). La faisabilité est étudiée par simulation avec le code MCNPX PoliMi en modélisant un dispositif ayant un angle solide de détection de 4π sr autour d’un fût de déchets de 118 L, rempli de matrices de déchets métalliques ou organiques. Un blindage de quelques centimètres de plomb devant les détecteurs a été optimisé pour limiter le taux de comptage des rayonnements gamma issus du fût. La faisabilité théorique est démontrée sur le nombre de coïncidences triples détectées en 1500 s de mesure et la proportion due aux fissions spontanées face aux réactions parasites (α,n). Les premières constituent près de 80 % du signal total et plus de 90 % après rejet de la diaphonie, en paralysant pendant 10 ns les deux détecteurs voisins d’un premier détecteur ayant initié une fenêtre de coïncidence. Pour des masses croissantes de plutonium, une perte de linéarité du nombre de triplets apparaît au-delà de 10 g, surtout due aux coïncidences accidentelles d’impulsions gamma et à la multiplication neutronique par fissions induites. A 100 g, la surestimation de la masse de plutonium n’excède cependant pas 30 % en répartition homogène. Par contre, si cette masse est concentrée en amas au centre de la matrice organique, une compétition entre multiplication neutronique et auto-absorption gamma conduit à la sous-estimer d’environ 20 %. Par ailleurs, un point chaud de 1 g de plutonium (effets de multiplication et coïncidences accidentelles négligeables) en différentes localisations dans la matrice conduit à des variations de signal inférieures à 30 % par rapport à la répartition homogène. Ces écarts augmentent pour certains cas pénalisants comme du plutonium au milieu d’une pièce métallique épaisse. Enfin, tous ces effets dépendent de la matrice et de l’épaisseur de l’écran de plomb, lesquelles affectent la composition des triplets en impulsions neutroniques et gamma. L’étude expérimentale est menée sur une maquette comprenant huit scintillateurs plastique (angle solide de détection inférieur à 2 π sr) protégés par 5 cm de plomb du fût rempli de sources ou échantillons de matière nucléaire, dans des matrices étalon en acier ou bois. Des tests avec sources de 252Cf et d’AmBe reproduisant un ratio alpha de 1,2 entre émissions neutroniques par fission spontanée et réaction (α,n), comme pour les simulations, confirment les tendances obtenues par calcul sur le nombre et la proportion de coïncidences utiles, les effets de matrice et de localisation. Les comparaisons expérience-calcul avec des plaquettes de plutonium et des sources de 137Cs ou 60Co montrent que le modèle numérique impacte fortement la diffusion gamma sur les extrémités des briques de plomb, la multiplication neutronique et l’auto-absorption gamma dans la matière nucléaire. La simulation reproduit toutefois correctement l’ordre de grandeur du signal, ce qui valide les estimations de performances avec le dispositif idéal (angle solide de 4 pi sr). Par ailleurs, les raies corrélées du 60Co, s’il est présent dans le colis, génèrent de nombreux triplets parasites et un écran de 10 cm de plomb serait nécessaire, sans qu’il soit toutefois possible de dépasser 10 MBq de 60Co. Enfin, une étude du bruit de fond environnant (émissions gamma du génie civil, coffres de sources à proximité, rayonnements cosmiques) conduit à préconiser aussi un blindage de plomb externe de 3 à 5 cm d’épaisseur
This PhD thesis presents a faisibility study of passive neutron coincidence counting for detecting coincidences of spontaneous fission of plutonium on radioactive waste drums with plastic scintillators as an alternative to traditional 3He gas counters, which became very costly. Plastic scintillators have a short time of detection of the order of nanosecond, but display a very high sensitivity to gamma rays and to parasitic coincidences due to neutronic and gamma-ray diffusions between neighbors detectors (cross talk). Feasibility is firstly studied by simulation with the MCNPX PoliMi code by modeling a system having a solid angle of detection of 4 pi sr around a 118 L-waste drum filled with metallic-and-organic waste matrices. A-few-cm-thick-lead screen in front of the detectors has been optimized for limiting count rate of gamma rays from the drum. Theoretical feasibility is demonstrated on the number of triple coincidences detected in 1500 s of measurement and on the proportion due to spontaneous fissions compared to parasitic (α,n) reactions. The former represent almost 80 % of total signal and more than 90 % after cross-talk rejection, by paralyzing the two adjacent detectors of a first detector having generated a coincidence window. For increasing masses of plutonium, deviations from linearity on the number of triples appear beyond about 10 g, essentially due to accidental coincidences of gamma-rays pulses and to neutron multiplication by induced fissions. However with 100 g of plutonium, overestimation of plutonium mass does not exceed 30 % in homogeneous distribution. On the other hand if this mass is concentrated in a cluster at the center of the organic matrix, a competition between neutron multiplication and gamma-rays-self-absorption leads to underestimate it of about 20 %. In addition, a hot spot of 1 g of plutonium (for which multiplication and accidental coincidences are negligible) in several localizations in the matrix leads to variations of signal lower than 30 % compared with homogeneous distribution. These deviations increase for some penalizing cases, as plutonium at the middle of a thick, metallic part. Finally, all these effects depend on the matrix and the thickness of the lead screen, which affect the composition of triples in neutron and gamma-ray pulses. Experimental study has been lead on a facility involving eight plastic scintillators (solid angle of detection lower than 2 pi sr) shielded by 5 cm of lead from the drum filled with sources and nuclear material samples in standard matrices of steel or wood. 252Cf and AmBe sources with an alpha ratio of 1.2 between neutron emissions by spontaneous fission and (a,n) reactions, as for simulations, confirm the trends obtained by computations on the number and the proportion of useful coincidences, matrix and localization effects. Comparisons of experiment with calculations with plutonium plates and sources of 137Cs or 60Co show that numerical model impacts strongly gamma-ray diffusions on the edge of lead blocks and neutron multiplication and gamma-ray-self-absorption in the nuclear material. Simulation reproduces correctly the order of magnitude of the signal, which validates the performance estimations with the ideal facility (solid angle of 4 pi sr). Furthermore, correlated gamma rays of 60Co, if it is present in the drum, generate numerous parasitic, triple coincidences And a 10 cm–thick lead screen would be necessary although it will not be possible to go beyond 10 MBq of 60Co. Lastly, a study of surrounded background (gamma-ray emissions from structural constructions and gamma-ray sources stored close to the detectors, cosmic rays) lead to recommend in addition a 3-to-5-cm-thick, external lead screen

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

Deux nouvelles familles de complexes de cuivre(I) cationiques, de formules [Cu(NHC)(N^N)][X] et [Cu(P^P)(N^N)][PF6], ont été synthétisées avec des ligands 2,2’ bis pyridyl pontés, chélates à six chaînons, facilement modulables. Ces complexes présentent des émissions à l’état solide centrées entre 455 et 520 nm (bleu à vert), avec de larges décalages de Stokes et des rendements quantiques pouvant atteindre 0,86. De plus, l’émission via un phénomène de fluorescence retardée activée thermiquement (TADF) a été prouvée pour les complexes [Cu(NHC)(N^N)][X]. Les premiers scintillateurs plastiques dopés avec des complexes de cuivre(I) détectant les radiations nucléaires de type gammas ont été obtenus avec des complexes de formule générale [Cu(P^P)(N^N)][PF6]. Les complexes de formule [Cu(NHC)(N^N)][X] ont permis l’obtention des premières Cellules Electrochimiques Luminescentes (LECs) émettant dans le bleu et incorporant des complexes de cuivre(I). Enfin, les complexes de formule [Cu(NHC)(N^N)][X] à ligand 2,2’ dipyridylamine présentent une activité cytotoxique envers différentes lignées de cellules cancéreuses et apportent la possibilité d’une action ciblée sur les cellules tumorales via l’ajout d’un vecteur. La polyvalence de ces complexes de cuivre(I) repose sur les ligands 2,2’-bis-pyridyl pontés, chélates à six chaînons, dont la synthèse est facile d’accès et les propriétés électroniques et structurales sont modulables
New cationic copper(I) complexes of general formula [Cu(NHC)(N^N)][X] and [Cu(P^P)(N^N)][PF6] were developed with 6-membered-ring 2,2’-bis-pyridyl derivatives as ligand. These complexes exhibited blue (420 nm) to green (520 nm) emissions in solid state, with large Stokes shifts and photoluminescence quantum yields up to 0.86. Furthermore, the emission of the [Cu(NHC)(N^N)][X] complexes via a thermally activated delayed fluorescence (TADF) was demonstrated. The first plastic scintillators incorporating copper(I) complexes and detecting gamma radiations were obtained with [Cu(P^P)(N^N)][PF6] complexes. Application of the [Cu(NHC)(N^N)][X] complexes to the LEC technology led to the first copper(I)-based blue emitting device. In the last chapter, we also demonstrated that copper(I) complexes [Cu(NHC)(N^N)][X] bearing a 2,2’-dipyridylamine as N^N ligand exhibited high cytotoxycity against different cancer cells lines. These complexes paved the way for the design of a new type of copper(I) anti-cancer agents with the opportunity to increase the selectivity against cancer cells via a vectorization of the N^N ligand. The versatility of these copper(I) complexes demonstrated in this work relied on the easy to handle and highly modular 2,2’-bis-pyridyl ligands

Thesis (M.S. in Applied Physics)--Naval Postgraduate School, June 2010.
Thesis Advisor(s): Smith, Craig F. ; Second Reader: Bowden, Nathaniel S. "June 2010." Description based on title screen as viewed on July 16, 2010. Author(s) subject terms: Antineutrino detection, Inverse Betad Decay, neutron capture, lithium gadolinium borate. Includes bibliographical references (p. 71-73). Also available in print.

Quality assurance is a vital part of modern radiation therapy. This thesis deals with the development of a detector system for the quality assurance (QA) of modern external beam radiation therapy. The system consists of a plastic scintillator, a commercial camera and a computer. Different available organic scintillators were initially evaluated to select the most suitable scintillator for our design. Subsequently, many optical artefacts in our prototype design were evaluated and possible correction methods were presented to reduce the impact of the optical artefacts. The basic characteristics of the system (e.g. the reproducibility and response to changes of dose) were assessed in a series of low energy x-rays and high energy proton irradiations. Photographs of the scintillation light distributions were acquired using the detector system for low and high energy photons, electrons and protons and compared with the depth-dose curves measured with an ionisation chamber. During proton irradiation, there was a reduction in the light intensity in the Bragg peak region because the protons‘ high linear energy transfer (LET) leads to quenching where less light is produced than expected. We developed an approach which used Birks equation to correct for the quenching using the Monte Carlo code, Geant4. LET was modelled in Geant4 and was combined with the measured scintillation light to calculate Birks constant. We then used the derived value of Birks constant to correct the measured scintillation light distribution for quenching using Geant4. The results show that the light output increased linearly with the x-rays and proton dose with a correlation coefficient greater than 0.99. The system is stable and provides reproducible results to within 1% in all type of radiation. Good agreements were obtained between the scintillation and the ionisation chamber depth dose curves for both photon and electron beams if depth-scaling factor was considered for the depth dose for electrons. However, energy dependence was seen with low energy x-rays due to the mechanism of interaction at these energies depending on the material's mean atomic number. For protons, no energy and dose rate dependencies were observed for the dose rates and energies used in this work. The results show that Geant4 simulation offered an effective way to correct for quenching for any desired energy. The quenched simulated scintillation results are in good agreement with the measured scintillation results and with the variation in the position of the Bragg peak is less than 0.7%. The results show that the system has the advantage of providing 2D visualisation of individual radiation fields and responded linearly to dose for low energy x-ray beam (50-100 kV) but suffers from energy dependency. The detector system provides acceptable depth dose curves for high energy photons and electron beams but could be enhanced if the optical artefact is corrected for. In addition, we developed an effective way to correct for quenching during proton irradiation. The technique provides a convenient method for rapid, convenient, routine quality assurance for clinical proton beams.

In this report the neutron-gamma discrimination capabilities of the new type of solid organic scintillator, EJ-299-33, was investigated using several pulse-shape discrimination (PSD) techniques. Among others, the analog zero-crossing method andthe digital charge-comparison and integrated-rise-time method were tested. The parameters of the digital PSD methods were optimised individually and the figure-of-merit was measured for each method and compared in different energy windows. The photoelectron yield of the setup was measured using two different photomultiplier tubes (PMT), a 3 inch diameter ET 9821 and a 5 inch diameter ET 9390KB. The highest photoelectron yield was measured with the ET 9390KB, which was the PMT used for the neutron-gamma discrimination capability measurements. In this work, four decay constants were found for the scintillator decay times. These were found by fitting average neutron and gamma-ray waveforms with the convolution of severeal exponential functions, that describes the light emission intensity of the scintillator, with an approximation of the PMT response function. Thebest agreement was found for the assumption that the scintillator light emission intensity is governed by four decay constants. The intensity of the two slowest components contain information about the incident particle.

Thesis (Ph.D.); Submitted to Univ. of California, Berkeley, CA (US); 1 Jun 2003.
Published through the Information Bridge: DOE Scientific and Technical Information. "LBNL--54350" Scielzo, Nicholas David. USDOE Director. Office of Science. Nuclear Physics (US) 06/01/2003. Report is also available in paper and microfiche from NTIS.

In questo lavoro è stata studiata la risposta a particelle al minimo di ionizzazione di un rivelatore, che sarà utilizzato per l'identificazione di muoni in SHIP (Search for HIdden Particles), esperimento proposto al CERN e ancora in fase di approvazione. Il rivelatore è basato su una striscia di scintillatore lunga tre metri accoppiata ad una fibra ottica Wave Lenght Shifter (WLS) e letta da due fotomoltiplicatori al Silicio (SiPM), posti ad entrambi i capi. Lo scopo principale del lavoro è la misura della raccolta di luce del rivelatore e la valutazione della lunghezza di attenuazione della striscia. I risultati mostrano che, nonostante l'attenuazione, la configurazione scelta per il rivelatore è tale da garantire una buona raccolta di luce lungo tutta la striscia.

Cette thèse étudie la détection de matière nucléaire avec la technique de la particule associée pour l’inspection de bagages abandonnés ou de conteneurs maritimes dans le domaine de la sécurité. Le principe consiste à mesurer, avec des scintillateurs plastique, les coïncidences entre particules de fissions induites par des neutrons de 14 MeV produits par un générateur basé sur la réaction 2H(3H,n)4He et équipé d’un détecteur alpha à localisation pour déterminer le temps d’émission et la direction du neutron opposé. La détection d’au moins trois particules de fission en coïncidence avec la particule qui permet de discriminer les matières nucléaires des matériaux bénins. Le système d’acquisition et les outils de simulation ont été qualifiés en passif avec des sources radioactives puis en actif avec le générateur et diverses cibles, validant les estimations de performances de systèmesd’inspection de bagages abandonnés ou de conteneurs maritimes réalisées par simulation numérique avec le code MCNP-PoliMi. Il est ainsi possible de détecter en quelques minutes, quelques kg d’uranium au centre d’un container rempli d’une matrice fer mêmesi l’échantillon est masqué par du plomb, à l’aide du signal des neutrons prompts de fission. La détection est plus difficile dans les matrices organiques en raison de la diffusion des neutrons interrogateurs et de fission sur les noyaux d’hydrogène. Par ailleurs, l’utilisation de scintillateurs plastiques à la place des compteurs gazeux à 3He a été évaluée pour caractériser le plutonium dans les colis de déchets radioactifs par mesure passive des coïncidences. La détection des neutrons de fission est beaucoup plus rapide,ce qui permet de minimiser le bruit accidentel dû aux réactions (,n). Les scintillateurs sont cependant plus sensibles aux rayonnements gamma et à la diaphonie entre détecteurs voisins, ce qui nécessite d’exploiter les coïncidences de multiplicité 3 avec un traitement des données spécifique pour limiter la diaphonie
This thesis investigates the detection of Special Nuclear Materials (SNM) by neutroninterrogation with the Associated Particle Technique (APT). 14 MeV neutrons areproduced from the 3H(2H,n)α fusion reaction in a sealed tube neutron generatorembedding a position-sensitive alpha detector. The alpha detector determines thedirection of the nearly opposite neutron and its time of flight. The detection of at leastthree prompt fission particles in coincidence with the tagged neutron signs the presenceof SNM. The acquisition system and simulation tools have been qualified in passive modewith radioactive sources and active mode with the generator and various targets,validating the simulation of inspection systems with MCNP-PoliMi. Calculations showthat the detection of a few kilograms of shielded SNM with the ATP is possible in ironcargo container, with the prompt fission neutrons signal. Detection is more difficult inorganic matrices due to tagged- and prompt fission neutrons scattering on hydrogennuclei. Furthermore, the use of plastic scintillators instead of 3He counters was studied tocharacterize the plutonium in the radioactive waste by passive coincidences measurement.Measurements at fast time scales of fast-neutrons instead of the long time scales ofthermal-neutrons reduce random coincidences that can occur with high (,n) reactionrate. The scintillators are however sensitive to gamma rays and cross-talk betweenadjacent detectors. Therefore, we used data-analysis algorithms to minimize cross-talkcontribution to measured three-fold coincidences

Il presente elaborato di tesi si pone come obiettivo lo studio, attraverso l'utilizzo di un telescopio per raggi cosmici, della risoluzione temporale di rivelatori di particelle basati su SiPM (Silicon PhotoMultiplier) accoppiati a scintillatori plastici. In particolare, dopo aver effettuato uno scan in tensione dei SiPM, sono state analizzate diverse modalità di accoppiamento degli stessi rispetto agli scintillatori e due differenti dimensioni degli scintillatori stessi. Una sempre maggiore precisione nella misura di informazione temporale mediante l'uso di questo tipo di rivelatori è di fondamentale importanza in vari e numerosi ambiti: dagli esperimenti sulle misure del tempo di volo (TOF, Time Of Flight) in fisica nucleare e subnucleare, alla loro applicazione in tecniche di diagnostica medica avanzata (ad esempio per la PET, Positron Emission Tomography) in campo medico. I risultati raggiunti dall'analisi della risposta temporale dei SiPM in accoppiamento agli scintillatori plastici si sono rivelati più vantaggiosi per la configurazione che vede i SiPM posizionati sui lati opposti dello scintillatore rispetto a quella in cui sono adiacenti allo stesso lato. In generale, è risultato più conveniente operare con SiPM mantenuti a basse tensioni di bias. Infine, anche se sono stati ottenuti buoni risultati per gli scintillatori di dimensioni 0.5x0.5x1.0 cm^3, la migliore risoluzione temporale, di 65±10 ps, si è osservata con SiPM accoppiati agli estremi opposti di scintillatori di dimensioni 2.0x3.0x2.0 cm^3.

Over the past 20 years, the detection of neutrino oscillation has reported a lot of important results. The oscillation phenomenon itself has been well proved by various experiments. Some oscillation parameters has been measured and now in the area of precise determination. On the other hand, some new questions like the possibility of the existence of light sterile neutrinos and unexpected 5 MeV bump were raised during the measurement. The Neutrino Lattice Experiment (NuLat) is a detector based on the Raghavan Optical Lattice (ROL). It should be able to offer a compact design of an effective detector with good mobility. It can be extremely useful in the short baseline reactor neutrino oscillation detection community to resolve several confusing issues. In this thesis, we present the calibration results we got from the first active NuLat detector and show what kind of improvements we need for the next version of the NuLat detector based on these results.
Ph. D.

The neutrino is, by its nature, an elusive particle that requires massive detectors with small backgrounds to capture a handful of events. Nevertheless, neutrino experiments stand at the heart of the current mysteries of particle physics and astrophysics. These include the origin and size of neutrino mass, the existence of additional types of neutrinos, CP violation and the matter--antimatter asymmetry, the amount of metals in the Sun's core, and the existence of non-nuclear energy sources in the Sun. This dissertation concerns the the use of a novel detector technology, the Raghavan Optical Lattice (ROL), in the Low-Energy Neutrino Spectrometer (LENS) and Neutrino Lattice (NuLat) experiments. LENS will measure the solar neutrino luminosity and the Sun's core metallicity using a ROL with indium-loaded liquid scintillator. NuLat will probe the existence of light sterile neutrinos with masses of $ \sim 1\,\mathrm{eV} $ using a ROL made from $ ^{6}\mathrm{Li} $-loaded plastic scintillator. For LENS we present an overview of the experiment and the present the ROL construction results from the LENS R\andD program. In particular we will present results from the micro- and mini-LENS prototypes. For both LENS and NuLat we present the development of an event reconstruction algorithm for ROLs and we apply these to the expected signals for these experiments. For NuLat we present an overview of the experiment including its theory of operation and its sensitivity to sterile neutrino oscillations. Finally, we present work toward the full-sized NuLat detector through bench-top tests and construction of the NuLat demonstrator.
Ph. D.

This thesis describes the measurement technique of fast-neutron tomography for assessing spatial distributions of steam and water in two-phase flows. This so-called void distribution is of importance both for safe operation and for efficient use of the fuel in light water reactors, which compose the majority of the world’s commercial nuclear reactors. The technique is aimed for usage at thermal-hydraulic test loops, where heated two-phase flows are being investigated under reactor-relevant conditions. By deploying portable neutron generators in transmission tomography, the technique becomes applicable to stationary objects, such as thermal-hydraulic test loops. Fast neutrons have the advantage of high transmission through metallic structures while simultaneously being relatively sensitive to the water/void content. However, there are also challenges, such as the relatively low yield of commercially available fast-neutron generators, the tendency of fast neutrons to scatter in the interactions with materials and the relatively low efficiency encountered in fast-neutron detection. The thesis describes the design of a prototype instrument, FANTOM, which has been assembled and demonstrated. The main design parameters have been optimized to achieve maximal signal count rate in the detector elements, while simultaneously reaching an image unsharpness of ≤0.5 mm. Radiographic projections recorded with the assembled instrument are presented, and the performance parameters of FANTOM are deduced. Furthermore, tomographic reconstruction methods for axially symmetric objects, which is relevant for some test loops, have been developed and demonstrated on measured data from three test objects. The attenuation distribution was reconstructed with a radial resolution of 0.5 mm and an RMS error of 0.02 cm-1, based on data recorded using an effective measurement time of 3.5 hours per object. For a thermal-hydraulic test loop, this can give a useful indication of the flow mode, but further development is desired to improve the precision of the measurements. Instrument upgrades are foreseen by introducing a more powerful neutron generator and by adding detector elements, speeding up the data collection by several orders of magnitude and allowing for higher precision data. The requirements and performance of an instrument for assessment of arbitrary non-symmetric test loops is discussed, based on simulations.

Two measurements have been made, addressing gaps in knowledge for 235U(n,f) and 238U(n,f). The energy distribution for prompt fission neutrons is not well-understood below 1 MeV in 235U(n,f). To measure the 235U(n,f) prompt fission neutron distribution, a pulsed neutron beam at the WNR facility in Los Alamos National Laboratory was directed onto a 235U target with neutron detectors placed 1 m from the target. These neutron detectors were designed specifically for this experiment and employed a unique geometry of scintillating plastic material that was designed to reject backgrounds. Fission fragments were detected using an avalanche counter. Coincidences between fission fragment production and neutron detector events were analyzed, using a double time-of-flight technique to determine the energy of the prompt fission neutrons. A separate measurement was made, investigating the neutron-induced fission cross section for 238U(n,f). This measurement also used the pulsed neutron beam at the WNR facility. The neutron flux was normalized to the well-known hydrogen standard and the fission rate was observed for beam neutrons in the energy range of 130-300 MeV. Using an extrapolation technique, the energy dependence of the cross section was determined. These new data filled a sparsely populated energy region that was not well-studied and were measured relative to the hydrogen standard, unlike the majority of available data. These data can be used to constrain the fission cross section, which is considered a nuclear reaction standard.

The observed imbalance of matter and anti-matter in the universe is one of physics' most fundamental unresolved questions. The leading theories to explain this imbalance require CP violation, and the neutron electric dipole moment (nEDM) is a sensitive parameter in its determination. Many new theories of physics beyond the standard model can be constrained or ruled-out by setting limits on the nEDM. Many next generation nEDM experiments require Ultra Cold Neutrons (UCN), produced in superfluid helium. One such experiment is cryoEDM. This thesis explores various types of UCN detection technologies applicable to cryoEDM or any high-density high-efficiency cryogenic nEDM experiment. Cryogenic Phonon Scintillation detectors (CPSD) are modified for this application by operating at 500 mK, and by using a titanium transition edge sensor for phonon signal readout. A CPSD is stabilised in the transition using a novel infra-red light feedback system which reduced the response time to O(100 μs). The detector is characterised and calibrated using an ²⁴¹Am α source. It was found to operate reliably at this elevated temperature and measure an alpha spectrum with 11% resolution at 5.5 MeV. Scintillators are identified as a promising technology for UCN detection at low temperature. Suitable materials that are bright with fast decay times and low γ sensitivity are studied in the temperature range 300 - 6 K. Their light yield to alpha excitation, their decay time characteristics and spectroscopic properties under VUV excitation are investigated. This study includes the first comprehensive investigation of the luminescence properties of plastic scintillators and of ⁶LiF/ZnS(Ag) down to 6 K. It is found that there is no degradation of the luminescence or kinetic properties of these materials across the whole temperature range, revealing them as suitable cryogenic detector materials. Using a plastic scintillator, a prototype UCN detector for operation in liquid helium is designed, manufactured and tested. It is read out using WLS optical fibres to a room temperature photomultiplier. The detector is successfully tested with cold neutrons at the ISIS neutron science facility and found to effectively measure neutrons, with a signal that is clear from background. Recommendations are made for its integration into a cryogenic neutron EDM experiment. This low-cost detector offers a promising method for the passive detection of UCN in a challenging cryogenic environment, with minimal electric interference and low background sensitivity. This technology offers the potential for improved UCN detection efficiency and thus improved sensitivity of the measurement of the neutron EDM.

This thesis investigates surface coatings as xenon diffusion barriers on plastic scintillators. The motivation for the work is improved radioxenon detection systems, used within the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). One type of radioxenon detection systems used in this context is the Swedish SAUNA system. This system uses a cylindrical plastic scintillator cell to measure the beta decay from radioxenon isotopes. The detector cell also acts as a container for the xenon sample during the measurement. One problem with this setup is that part of the xenon sample diffuses into the plastic scintillator material during the measurement, resulting in residual activity left in the detector during subsequent measurements. This residual activity is here referred to as the memory effect. It is here proposed, and demonstrated, that it is possible to coat the plastic scintillator material with a transparent oxide coating, working as a xenon diffusion barrier. It is found that a 425 nm Al2O3 coating, deposited with Atomic Layer Deposition, reduces the memory effect by a factor of 1000, compared an uncoated detector. Furthermore, simulations show that the coating might also improve the light collection in the detector. Finally, the energy resolution of a coated detector is studied, and no degradation is observed. The focus of the thesis is measurements of the diffusion barrier properties of Al2O3 films of different thicknesses deposited on plastic scintillators, as well as an evaluation of the expected effect of a coating on the energy resolution of the detector. The latter is studied through light transport simulations. As a final step, a complete coated plastic scintillator cell is evaluated in terms of memory effect, efficiency and energy resolution. In addition, the xenon diffusion process in the plastic material is studied, and molecular dynamics simulations of the Xe-Al2O3 system are performed in order to investigate the reason for the need for a rather thick coating to significantly reduce the memory effect.

Astrophysical observations give convincing evidence for a vast non-baryonic component, the so-called dark matter, accounting for over 20% of the overall content of our Universe. Direct dark matter search experiments explore the possibility of interactions of these dark matter particles with ordinary baryonic matter via elastic scattering resulting in single nuclear recoils. The ZEPLIN-III detector operated on the basis of a dualphase (liquid/gas) xenon target, recording events in two separate response channels { scintillation and ionisation. These allow discrimination between electron recoils (from background radiation) and the signal expected from Weakly Interacting Massive Particle (WIMP) elastic scatters. Following a productive first exposure, the detector was upgraded with a new array of ultra-low background photomultiplier tubes, reducing the electron recoil background by over an order of magnitude. A second major upgrade to the detector was the incorporation of a tonne-scale active veto detector system, surrounding the WIMP target. Calibration and science data taken in coincidence with ZEPLIN-III showed rejection of up to 30% of the dominant electron recoil background and over 60% of neutron induced nuclear recoils. Data taking for the second science run finished in May 2011 with a total accrued raw fiducial exposure of 1,344 kg days. With this extensive data set, from over 300 days of run time, a limit on the spin-independent WIMP-nucleon cross-section of 4.8 10-8 pb near 50 GeV/c2 WIMP mass with 90% confidence was set. This result combined with the first science run of ZEPLIN-III excludes the scalar cross-section above 3.9 10-8 pb. Studying the background data taken by the veto detector allowed a calculation of the neutron yield induced by high energy cosmic-ray muons in lead of (5.8 0.2) 10-3 neutrons/muon/(g/cm2) for a mean muon energy of 260 GeV. Measurements of this kind are of great importance for large scale direct dark matter search experiments and future rare event searches in general. Finally, this work includes a comprehensive measurement of the energy dependent quenching factor for low energy nuclear recoils in a plastic scintillator, such as from the ZEPLIN-III veto detector, increasing accuracy for future simulation packages featuring large scale plastic scintillator detector systems.

The work covered by this thesis was carried out at the Brasimone ENEA Research Centre, the National Agency for New Technologies, Energy and Sustainable Economic Development, with the aim of optimising a gamma spectrometry system in anticoincidence for the detection of noble gases, in particular the radioactive isotopes of xenon 131mXe, 133Xe, 133mXe and 135Xe. The laboratory of the ENEA Research Center of Brasimone, where the experimental apparatus is found to carry out the measurements of 131mXe, 133Xe, 133mXe and 135Xe, collaborates constantly with the monitoring network and is able to provide, if necessary, data and analysis on noble gases. The signals produced by the interaction of cosmic rays that manage to pass the screen have been recognized as the main cause of the increase of the detector background because they give rise to the Compton continuum and, as a result, they increase the value of detectable MDA. For this reason, a system in anticoincidence has been developed through the use of two plastic scintillators, placed over the shielding of the Hpge detector, which sends pulses recording within a gate located in the germanium multichannel analyzer: at the time the signal arrives from the scintillator, the gate blocks data acquisition to avoid recording pulses generated by cosmic radiation. For both configurations of the system, therefore, both with the anti-coincidence apparatus inactive and in operation, energy, FWHM and efficiency calibrations had to be carried out using a certified multi-peak source. The solution proposed, in conclusion of the thesis, to eliminate any electronic interference with the efficiency of the detector provides for the replacement of the current electronic apparatus with a new fully digitized, made by the company CAEN, Nuclear Electronic Construction Equipment.

Le but de l'expérience EDELWEISS est la détection directe de matière noire sousforme de WIMPs, par l'étude de leur diffusion élastique sur les noyaux de germanium des détecteurs bolomètriques. Le plus problématique des bruits de fond provient des neutrons pouvant mimer l'interaction d'un WIMP dans un détecteur. Ces neutrons sont notamment produits par les rares muons cosmiques de haute énergie qui atteignent le laboratoire souterrain malgré les 4800 m w.e. de roche. Les muons résiduels sont détectés par un système veto de 46 modules de scintillateur plastique entourant l'expérience, qui permet de rejeter la plupart du bruit de fond associé. La détermination précise du bruit de fond neutron résiduel induit par ces muons dans EDELWEISS-III, essentielle pour l'identification des WIMPs, est le but de cette thèse. Le taux de bruit de fond dépend de la géométrie de l'expérience ainsi que des matériaux utilisés, qui ont subi d'importantes modifications depuis EDELWEISS-II. Des simulations GEANT4 du passage des muons dans la nouvelle géométrie ont été réalisées afin d'extraire le taux d'événements induits par les muons dans les bolomètres. Ce taux est en bon accord avec le taux mesuré extrait des données du Run308. En parallèle, une limite inférieure sur l'efficacité du veto muon a été extraite à partir des données bolomètres. Une nouvelle méthode basée sur l'utilisation d'une source d'AmBe a été développée afin d'extraire l'efficacité de chaque module de la simulation. À partir de ces résultats, il a été montré que le bruit de fond attendu est négligeable pour la recherche de WIMPs avec les données du Run308 et ne limitera pas la sensibilité future d'EDELWEISS-III
The aim of the EDELWEISS-III experiment is to detect the elastic scattering of WIMPs from the galactic dark matter halo on germanium bolometers. The most problematic background arises from neutrons, which can mimic a WIMP interaction in a detector. Neutrons are notably induced by high energy cosmic ray muons reaching the underground laboratory despite the 4800 m w.e. of rock overburdened. Remaining muons are tagged using an active muon-veto system of 46 plastic scintillator modules surrounding the experiment, which allows to reject most of the associated background. The goal of this thesis was to give a precise estimation of the irreducible muon-induced neutron background, needed to identify a potential WIMP signal. The expected background depends on the geometry of the experiment as well as on the used materials, both strongly modified since EDELWEISS-II. Geant4-based simulations of muons through the modified geometry were performed to derive the rate of events induced by muons in the bolometer array. This rate has been shown to be in good agreement with the measured one extracted from the Run308 data. In parallel, a lower limit on the muon-veto efficiency was derived using bolometer data only. A new method based on an AmBe source was developed to extract precisely the detection efficiency of individual modules from the simulation. From these results, it was shown that the expected background is negligible for the WIMP search analyses performed with the Run308 data and won't limit the future sensitivity of the EDELWEISS-III experiment

Medidas da radiação gama ambiental foram feitas em algumas regiões da cidade de Ribeirão Preto-SP, de forma a contribuir para o conhecimento acerca dos níveis desse tipo de radiação no estado de São Paulo e no território brasileiro. Escolheu-se determinar as médias anuais de dose efetiva e dose efetiva coletiva, uma vez que estas grandezas levam em conta o risco de possíveis efeitos biológicos associados a exposição à radiação ionizante. Para determinar os valores dessas grandezas, os diversos setores censitários das regiões selecionadas foram mapeados e avaliados individualmente, de acordo com a taxa de dose média absorvida no ar e número de habitantes de cada setor. Utilizou-se um sistema de detecção da radiação gama baseado em cintilador plástico e GPS, que possibilitaram a medida da taxa de dose absorvida no ar a cada segundo, para as coordenadas geográficas selecionadas. Embora este sistema tenha sido desenvolvido para a detecção de fontes artificiais perdidas, ou seja, não tem o objetivo principal de fazer medidas dosimétricas da radiação gama natural, teve seus modos de leitura calibrados e comparados com resultados da literatura (com uma incerteza média de 8,7 %). Dos resultados obtidos, dos mapeamentos realizados nas regiões consideradas, as médias anuais da dose efetiva e dose efetiva coletiva para ambientes ao ar livre foram encontradas com os valores 0,034 ± 0,004 mSv/ano e 25,62 ± 9,25 homem.mSv/ano respectivamente, sendo a primeira cerca da metade da média mundial da dose efetiva recomendada pela UNSCEAR. As médias obtidas são baixas quando comparadas às encontradas em outros estudos sobre dose de radiação gama ao ar livre no estado de São Paulo e em outras regiões do território brasileiro (sendo, de 1,7 até 5,6 vezes menor).
Gamma environmental radiation measurements were done for some urban regions in the city of Ribeirão Preto-SP, in order to contribute with this knowledge on the annual ambiental values of this type of radiation in the São Paulo state and in the Brazilian territory. The quantities chosen were effective dose and effective collective dose, once they take into account the possible biologic damage related to the ionizing radiation energy absorbed. For the assessments of these quantities, selected census sectors were initially mapped out, regarding their average absorbed dose in air and the number of inhabitants living in each one of the sectors. The detector system used is based on an organic plastic scintillator and a GPS, that allows to obtain the absorbed dose rate for each second and their respective geographic coordinates. Even though this system was developed to detect missing artificial gamma sources, in other words, its main function is not for dosimetric measurements, both display modes were calibrated in exposure rate (R/h) and absorbed dose rate (Gy/h) and their readings were compared to results from the literature (with an average uncertainty of 8.7%). From the mapping results of the selected regions, the annual average effective dose and effective collective dose for outdoor environments were obtained, respectively as 0.034 ± 0.004 mSv/year and 25.62 ± 9.25 man.mSv/year. The value for the first average dose is lower (about half) than the worldwide average value published by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), also lower than the values from other regions in the national territory, regarding the gamma dose rate in outdoor environments (from 1.7 up to 5.6 times lower).

Le détecteur MINOS a été développé jusqu'à mi-2013 pour la spectroscopie γ prompte de noyaux très exotiques à partir de réactions d’arrachage de protons. Il est composé d'une cible épaisse d'hydrogène liquide pour augmenter la luminosité et d’une chambre à projection temporelle (TPC) pour reconstruire la position du vertex de réaction et de compenser l'effet de la cible épaisse sur la correction Doppler.La chambre à projection temporelle a été développée avec l'expertise du CEA-Irfu sur les détecteurs gazeux de type Micromegas. Dans un premier temps, différentes solutions pour la TPC ont été testées dans une chambre d'essai avec une source α et des mesures de rayons cosmiques. Des muons cosmiques ont été détectés pour la première fois en utilisant la chambre d'essai en début 2013 et ont validé l'utilisation d'un plan de détection Micromegas. Le premier prototype de TPC a été achevé en mai 2013 et nous avons utilisé un banc de rayons cosmiques pour estimer l’efficacité de la TPC.MINOS a ensuite été expédié au Japon et un test de performance sous faisceau a été réalisée à l'installation médicale HIMAC (Chiba, Japon) avec deux cibles minces au lieu de la cible épaisse d'hydrogène pour valider l'algorithme de reconstruction et la résolution de la position du vertex. Un algorithme de reconstruction de traces basé sur la transformée de Hough a été mis au point pour l'analyse des données, testé avec ces données, et comparé à des simulations.La première campagne de physique avec MINOS a eu lieu en mai 2014, avec SEASTAR. Elle s’est concentrée sur la première spectroscopie des ⁶ ⁶ Cr, ⁷⁰,⁷²Fe et ⁷⁸Ni. L'analyse de la spectroscopie du ⁶ ⁶Cr a révélé deux transitions, assignées aux deux premiers états excités. Une interprétation avec des calculs de modèle en couches montre que le maximum de collectivité quadripolaire se produit à N = 40 le long de la chaîne isotopique de chrome.Le ⁶ ⁶Cr est toujours placé dans la région de l’Îlot d’Inversion à N = 40 et les calculs de modèle en couches ainsi que la comparaison avec des calculs basés sur HFB suggèrent une extension de cet Îlot d’Inversion vers N = 50 en dessous du ⁷⁸Ni. L'analyse des ⁷⁰,⁷²Fe effectuée par C. Louchart (TU Darmstadt, Allemagne) révèle la même tendance que pour les isotopes de chrome. Les données et notre interprétation par le modèle en couches suggère une grande collectivité les Cr et Fe riches en neutrons, éventuellement jusqu'à N = 50, ce qui remettrait en cause la solidité de la fermeture de couche N = 50 en dessous du ⁷⁸Ni
The MINOS device has been developed until mid-2013 for in-beam γ spectroscopy of very exotic nuclei from proton knockout reactions. It is composed of a thick liquid hydrogen target to achieve higher luminosities and a Time Projection Chamber (TPC) to reconstruct the vertex position and compensate for the thick target effect on the Doppler correction.The Time Projection Chamber has been developed with the expertise of CEA-IRFU in gas detectors and Micromegas detectors. At first, different solutions for the TPC were tested in a test chamber with an α source and cosmic-ray measurements. Cosmic rays were detected for the first time using the test chamber in early 2013 and validated the use of a Micromegas detection plane. The first TPC prototype was finished in May 2013, and we used a cosmic-ray bench to estimate the effiiciency of the TPC. The MINOS device was then shipped to Japan and an in-beam performance test was performed at the HIMAC medical facility (Chiba, Japan) with two thin targets instead of the thick hydrogen target to validate the tracking algorithm and the vertex position resolution. A tracking algorithm for the offline analysis based on the Hough transform has been developed, tested with the data, and compared with simulations.The first physics campaign using MINOS took place in May 2014 with SEASTAR. It focused on the first spectroscopy of ⁶ ⁶ Cr, ⁷⁰,⁷²Fe, and ⁷⁸Ni. The analysis of the ⁶⁶Cr spectroscopy revealed two transitions, assigned to the two first excited states. An interpretation with shell model calculations shows that the maximum of quadrupole collectivity occurs at N=40 along the Cr isotopic chain.⁶⁶Cr is still placed in the Island of Inversion region of N=40 and the shell model calculations as well as comparison with HFB-based calculations suggest an extension of this Island of Inversion towards N=50 below ⁷⁸Ni. The analysis of ⁷⁰,⁷²Fe performed by C. Louchart (TU Darmstadt, Germany) reveals the same trend as for Cr isotopes, with a maximum of deformation at N=42. The full data set and our shell-model interpretation suggests a large collectivity for neutron-rich Cr and Fe, possibly up to N=50, questioning the robustness of the N=50 shell closure below ⁷⁸Ni

Tese de mestrado integrado, Engenharia Biomédica e Biofísica (Engenharia Clínica e Instrumentação Médica) Universidade de Lisboa, Faculdade de Ciências, 2019
There is real interest in installing in Portugal a proton therapy facility. Proton therapy has a the potential to spare healthy from unnecessary dose. This technology takes advantage of the existance of the Bragg Peak to be able to obtain a dose distribution closer to what is ideal. Nowadays, ionization chambers are considered the reference in dosimetry. Nevertheless, they offer a poor spatial resolution. Optical fibers, on the other hand, offer a good spatial resolution: with an optical fiber it is possible to achieve a spatial resolution of 2 mm, 1 mm, 0.5 mm or even 0.25 mm. When an optical fiber is irradiated with ionizing radiation it absorbs it and produces scintillation light that is then conducted inside the fiber to a photomultiplier that converts the light in an electrical current. This work’s objective was to develop an experimental setup to study how different optical fibers responds to proton beams, in particular to low energy proton beams. So, this work can be divided in four different stages: development of a software to simulate the interactions of a low energy proton beam with matter; the design and manufacture of a irradiation chamber to block the room light and to make sure that all th light that reachs the photomultiplier is scintillation light; rehabiliation of a X & XY positioning system to be able to place the optical fiber, and irradiation box, with precision; and finally to perform the measurements in the CTN’s Van de Graaff. During the measurements there was made a longitudinal measurement and also a lateral measurement of the beam’s profiles. The lateral measurements were performed to better understand the beam’s geometry and to obtain the points with a higher signal. The measurements of the longitudinal profile didn’t show the beam’s Bragg Peak. This happened for two reasons: first because the beam’s energy is too low and all of the proton’s energy is deposited in the optical fiber and second because the beam’s dispersion means that the number of protons that interact with the optical fiber takes a big fall when the distance is increased. The lateral measurements were able to confirm that the beam used had a gaussian geometry and enabled the calculation of the security distance from the opening of the irradiation box to the Van de Graff’s exit. Although this setup allowed for the measurements to be made. It still needs some improvements: the positioning system needs motors with a bigger torque and the irradiation box needs to be designed in a more user friendly way. The measurements should be done with a more stable proton accelerator.
terapia com raios-X. Durante anos foi vista como aplicável apenas a casos de cancro na base do crânio, para tratamentos feitos em pediatria e para retinoblastomas. Hoje, sabe-se que esta tecnologia pode ser utilizada em todo o tipo de cancros em que a terapia com fotões é utilizada. Em Portugal existe um grande interesse em instalar uma infraestrutura que permita fazer terapia com protões para tratar pacientes oncológicos terapia com protões é uma tecnologia que aproveita a existência do pico de Bragg para poupar, até um certo ponto, os tecidos saudáveis a dose desnecessária. Para conseguir isto é necessário ser capaz de mapear com uma pequena resolução espacial o pico de Bragg para os feixes de protões utilizados. Enquanto que, hoje em dia, as câmaras de ionização são consideradas a referência em dosimetria, estas não oferecem uma pequena resolução espacial. As fibras ópticas, por outro lado, oferecem resoluções espaciais pequenas: 2 mm, 1 mm, 0,5 mm, 0,25 mm. Quando se utiliza uma fibra óptica cintilante para se medir o perfil de dose de um feixe, a fibra é irradiada produzindo luz de cintilação. A luz de cintilação é transmitida dentro da fibra óptica até um fotomultiplicador que converte a luz de cintilação numa corrente eléctrica. O fotomultiplicador é alimentado por uma fonte de alta tensão (o fotomultiplicador utilizado neste trabalho funciona entre os 600 V e os 800 V). A corrente gerada no fotomultiplicador é, finalmente, integrada num electrómetro. A intensidade da luz de cintilação é, em princípio, linear em relação à quantidade de dose absorvida pela fibra. Contudo, devido a fenómenos de quenching a linearidade é perdida quando a fibra interage com partículas com pouco energia, o que corresponde a um grande stopping power. Esta situação é de especial importância pois é na zona do pico de Bragg que os protões atingem os valores mais elevados de stopping power. O objectivo deste trabalho foi estudar as características de fibras ópticas cintilantes, em particular as suas respostas a feixes de protões de baixa energia. Para alcançar este objectivo o trabalho foi dividido em quatro partes distintas: desenvolvimento de um software de simulação da interacção de um feixe de protões com a matéria que permitisse a realização de simulações com energia de feixe até aos 15 MeV; uma vez que os protões utilizados têm pouca energia o que levou a que o ar fosse escolhido como fantoma, foi necessário o design e manufacturação de uma caixa de irradiação que permitisse a interacção do feixe de protões com a fibra óptica e que, ao mesmo tempo, impedisse que a luz ambiente interaja com a fibra óptica; montagem de uma mesa XY para conseguir uma colocação precisa da fibra óptica; realização das medições no acelerador de protões do Campus Tecnológico e Nuclear. Quando se pretende fazer leituras de dose com fibras ópticas não se pode permitir que a luz ambiente interaja com a fibra óptica pois isso altera os valores medidos pela electrónica. No caso das medições de dose com fotões é possível utilizar-se uma manga para impedir esta interacção. No caso de um feixe de protões tal não é possível pois isso altera a energia do feixe de protões antes de estes interagirem com a fibra óptica. A solução encontrada foi desenhar uma caixa de irradiação onde a fibra é introduzida durante as medições. Esta caixa de irradiação tem uma abertura por onde entra o feixe de protões, esta abertura é coberta com uma fina janela de prata ( a janela de prata utilizada nas medições tem 210 nm de espessura). A espessura das janelas de prata foi medida usando a perda de energia de partículas alfa no material da janela. A necessidade de ter uma colocação precisa da fibra óptica foi sentida deste o início, pois sem uma colocação precisa a boa resolução espacial oferecida pela fibra óptica seria inútil pois existiria uma grande incerteza no ponto do espaço em que a fibra é colocada. O sistema de posicionamente escolhido foi uma mesa XY. As simulações realizadas com o pMC foram feitas tendo em conta um volume de fibra semelhante não volume de fibra que é irradiado nas medições experimentais, uma janela de prata de espessura igual à espessura da janela utilizada nas medições experimentais e considerando que o fantoma é composto por ar. Este novo software foi comparado com o software de simulação FLUKA que também foi utilizado nas simulações utilizadas neste trabalho. Durante as medições foi feita uma medição do perfil longitudinal do feixe e foi também feita uma medição do perfil lateral do feixe. A leitura do perfil lateral do feixe foi feita para conseguir entender melhor a geometria do feixe e para conseguir obter os pontos com um maior sinal para se considerar esse ponto para o perfil longitudinal usando a leitura de sinal na fibra óptica colocada em várias posições. As simulações mostraram que existem algumas diferenças entre o pMC e o FLUKA. Estas diferenças podem ser explicadas com o facto de as bases de dados dos stopping power utilizadas no pMC e no FLUKA são diferentes e com o facto de o algoritmo de dispersão ter uma componente lateral menor no pMC. As medições laterais permitiram demonstrar que o feixe possui uma geometria gaussiana e permitiram calcular a distância de seguranc¸a (distância entre a caixa de irradiação e a saída do feixe, necessária para não perfurar a janela de prata) a que a caixa é colocada da saída do acelerador. As medições do perfil longitudinal do feixe não permitiram que o pico de Bragg do feixe fosse observado. Isto acontece por várias razões: primeiro porque a energia do feixe é pequena e dos protões não conseguem atravessar a fibra o que faz com que o que ´e lido seja a energia do protão em vez de a energia deixada pelo protão num determinado volume; segundo porque a dispersão do feixe faz com que o número de protões que interagem com a fibra óptica baixa com a distância e isso baixa bastante a quantidade de energia absorvida pela fibra óptica; a distância entre duas posições consecutivas da fibra é demasiado grande para se poder considerar que temos uma variação pequena de energia. As simulações em que uma geometria gaussiana e uma desalinhamento da fibra óptica em relação à saída do feixe é considerada são as que mais se aproximam das medições experimentais. Como conclusão podemos dizer que apesar de ter sido possível realizar uma série de medições com este dispositivo experimental ainda é necessário realizar alterações ao mesmo. A caixa de irradiação deve ser mais fácil de utilizar, principalmente no que toca à introdução das fibras. O sistema de posicionamento precisa de ser melhorado pois os motores não oferecem torque suficiente para fazer com que percorra toda a distância da calha. É necessário utilizar outro feixe de protões pois o Van de Graaff utilizado no CTN apresenta muitas oscilações na intensidade do feixe.

The purpose of this research was to develop and test a radiation detector to perform beta dosimetry and spectroscopy. The detector utilizes plastic scintillator volumes to produce scintillation light in proportion to the amount of energy deposited in them, and a large-area avalanche photodiode to convert the light to electrical signals. Pulse processing electronics transform the electrical signals into a format useful for analysis, and various software programs are used to analyze the resulting data. The detector proved capable of measuring dose, as compared to Monte Carlo n-Particle simulations, to within about 50% or better, depending on geometry and source type. Spectroscopy results, in conjunction with MCNP-based spectral enhancement methods, proved the detector capable of recording beta spectra with endpoint energies greater than about 250 keV. The detector shows promise for further development as a portable beta detector for field use in beta-contaminated areas.
Graduation date: 2005

A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2016.
Plastic scintillators play a key role in reconstructing the energy and tracks of hadronic particles that impinge the Tile Calorimeter of the ATLAS detector as a result of high energy particle collisions generated by the Large Hadron Collider of CERN. In the detector, plastic scintillators are exposed to harsh radiation environments and are therefore susceptible to radiation damage. The radiation damage effects to the optical properties and structural damage were studied for PVT based commercial scintillators EJ200, EJ208, EJ260 and BC408, as well as PS based UPS923A and scintillators manufactured for the Tile Calorimeter. Samples of dimensions 5x5x0.3 mm were subjected to irradiation using 6 MeV protons to doses of approximately 0.8 MGy, 8 MGy, 25 MGy and 25 MGy using the 6 MV tandem accelerator of iThemba LABS. Results show that damage leads to a reduced light output and loss in transmission character. Structural damage to the polymer base and the formation of free radicals occur for doses ≥ 8 MGy leading to reduced scintillation in the base and re-absorption of scintillation light respectively. Scintillators containing a larger Stokes shift, i.e. EJ260 and EJ208 exhibit the most radiation hardness. EJ208 is recommended as a candidate to be considered for the replacement of Gap scintillators in the TileCal for the 2018 upgrade.
LG2017

A dissertation submitted to the Faulty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. April 2017.
Plastic scintillators, situated in the Tile Calorimeter (TileCal) of the ATLAS detector at the Large Hadron Collider (LHC), play an integral part in the detection of diffractive, energetic hadronic particles that result from high energy proton-proton collisions. As these particles traversetheplastic, theresultantdecayofthepolymerbaseemitsluminescentlightwhichacts as a signature of this interaction. However, the deleterious radiation environment in which the plastics are situated ultimately degrade the plastic through the formation of free radicals which initiate chemical reactions and alter the structure of the plastic. Radical formation was studied using electron paramagnetic resonance (EPR) spectroscopy in six plastic scintillator types of either polystyrene (PS) or polyvinyl toluene (PVT) base, and computational EPR studies were conducted on two small monomer structures and two large polymer, plastic-like computer models. Damage was simulated in the computational models by removing an increasing number of hydrogen atoms. Plastic samples, of volume 500 ⇥500 ⇥ 250 µm, were to subjected protons accelerated to 6 MeV using the tandem accelerator at iThemba LABS, Gauteng, to increasing target doses of 0.8 MGy, 2.5 MGy, 8.0 MGy, 25 MGy, 50 MGy, and 80 MGy. The experimental EPR data taken after two weeks of the sample exposure to air indicate the presence of peroxy-type radicals that initiate chemical reactions, discolour the plastic, and decrease the efficiency of the plastic. Furthermore, the data suggests that damaged PS and PVT samples are susceptible to different mechanisms of radiation damage. However, results pertaining to the decrease in the g-factor and the increase in normalised EPR intensity suggest that all plastics behave similarly using an EPR analysis as a function of dose. Thus, the EPR analysis could not identify a specific plastic that would perform better than the existing plastics used in the TileCal. The computational chemical potential results indicate that electron transfer between damaged pristine and damaged models is possible. In the two small damaged models, the computational EPR data indicate the presence of a various stable akyl-like radicals depending on the site from which the hydrogen atoms are removed. In the two large damaged models, these results indicate a number of alkyl-, benzyl-, and cyclohexadienyl-like radicals.
LG2017

A scintillation counting system has been constructed with the use of BC-400 and EJ-212 series plastic scintillators along with a subminiature photomultiplier tube to investigate the effect of increasing plastic scintillator thickness on system-integrated counts. Measurements have been carried out using four different gamma sources with different energies ranging from 6keV to 1.332MeV and a Ni-63 beta source of maximum energy of 66keV. A simulation was also carried out in MCNP4a to verify the number of H-3 beta particles with max energy 18.6keV that would reach the plastic scintillator in a vacuum setting as well as in an air medium. Scintillator thicknesses ranged from 10μm to 2500μm. The response of the system was determined by measuring the integrated counts as a function of scintillator thickness. The results of these measurements showed the expected positive linear correlation between scintillator thicknesses and integrated counts for all the gamma sources while the slopes of the correlations of each gamma source was a function of the source energy. The beta particle response showed an initial increase of counts with scintillator thickness followed by a slight decrease. The MCNP simulation confirmed an analytical calculation of the fraction of H-3 beta particles for a given air concentration that would reach the scintillator. These results in conjunction with the experimental findings were used to assess the potential of a plastic scintillator system forming the basis of a tritium monitor for the detection of tritium in high-energy gamma backgrounds for Canadian nuclear power workers.
UOIT

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science, 2020
The CERN’s Large Hadron Collider (LHC) is the largest particle accelerator in the World. This comprises a variety of experiments to investigate the basic structure of matter in the universe. A Toroidal LHC Apparatus (ATLAS) is one of the experiments at CERN that is used to detect particles that result from the proton-proton collisions. The ATLAS detector is a general-purpose detector. During the maintenance periods, the ATLAS Tile Calorimeter (TileCal) detector modules are repaired/fixed. The Front-End (FE) electronics of these TileCal modules are tested and maintained using the MobiDICK system and if they are faulty they are repaired or replaced. The FE readout system, which is housed in the super-drawers, is testedand verified using the MobiDICK system. The upgrade of the LHC to the High-Luminosity LHC (HL-LHC) will provide great opportunities to explore new physics beyond the Standard Model. This poses significant challenges to the detector and the Trigger and data acquisition system (TDAQ). The TileCal readout electronics will be replaced during the Phase-II upgrade to cope with the HL-LHC’s increased luminosity. In this study we focus on the certification of the Demonstrator at the particle test beams. The Demonstrator’s latest electronics are being commissioned to be integrated into ATLAS during the LS2. This study presents the early readout certification of the Demonstrator. Due to their properties, such as fast time response and high optical transmission, plastic scintillators are used in particle detectors. The fast pulse generation allows for fast readout, and the intensity of the light is proportional to the energy deposit. Scintillators are used to measure energies and to reconstruct the particles’ path through the luminescence process due to ionizing radiation. However, long exposure to ionizing radiation can cause permanent damage to the scintillators. In this study we focus on the effects of neutron irradiation of plastic scintillators similar to that of the lifetime of the HL-LHC. The plastic scintillator being investigated are the UPS-923A blue scintillators and green scintillators. The optical properties of the irradiated samples are studied using the techniques of light yield measurements
CK2021

For efficient beta detection in a mixed beta gamma field, Monte Carlo simulation models have been built to optimize the thickness of a plastic scintillator, used in whole body monitor. The simulation has been performed using MCNP/X code and different thicknesses of plastic scintillators ranging from 150 to 600 um have been used. The relationship between the thickness of the scintillator and the efficiency of the detector has been analyzed. For 150 m thickness, an experimental investigation has been conducted with different beta sources at different positions on the scintillator and the counting efficiency of the unit has been measured. Evaluated data along with experimental ones have been discussed. A thickness of 300 um to 500 um has been found to be an optimum thickness for better beta detection efficiency in the presence of low energy gamma ray.
UOIT

This thesis describes the development and implementation of a novel method for the dosimetric verification of intensity modulated radiation therapy (IMRT) fields with several advantages over current techniques. Through the use of a tissue equivalent plastic scintillator sheet viewed by a charge-coupled device (CCD) camera, this method provides a truly tissue equivalent dosimetry system capable of efficiently and accurately performing field-by-field verification of IMRT plans. This work was motivated by an initial study comparing two IMRT treatment planning systems. The clinical functionality of BrainLAB’s BrainSCAN and Varian’s Helios IMRT treatment planning systems were compared in terms of implementation and commissioning, dose optimization, and plan assessment. Implementation and commissioning revealed differences in the beam data required to characterize the beam prior to use with the BrainSCAN system requiring higher resolution data compared to Helios. This difference was found to impact on the ability of the systems to accurately calculate dose for highly modulated fields, with BrainSCAN being more successful than Helios. The dose optimization and plan assessment comparisons revealed that while both systems use considerably different optimization algorithms and user-control interfaces, they are both capable of producing substantially equivalent dose plans. The extensive use of dosimetric verification techniques in the IMRT treatment planning comparison study motivated the development and implementation of a novel IMRT dosimetric verification system. The system consists of a water-filled phantom with a tissue equivalent plastic scintillator sheet built into the top surface. Scintillation light is reflected by a plastic mirror within the phantom towards a viewing window where it is captured using a CCD camera. Optical photon spread is removed using a micro-louvre optical collimator and by deconvolving a glare kernel from the raw images. Characterization of this new dosimetric verification system indicates excellent dose response and spatial linearity, high spatial resolution, and good signal uniformity and reproducibility. Dosimetric results from square fields, dynamic wedged fields, and a 7-field head and neck IMRT treatment plan indicate good agreement with film dosimetry distributions. Efficiency analysis of the system reveals a 50% reduction in time requirements for field-by-field verification of a 7-field IMRT treatment plan compared to film dosimetry.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

The significance of the radioxenon memory effect in the context of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty is introduced as motivation for the project. Existing work regarding xenon memory effect reduction and thin film diffusion barriers is surveyed. Experimental techniques for radioxenon production and exposure, as well as for thin film deposition on plastic by plasma enhanced chemical vapor deposition (PECVD), are detailed. A deposition rate of 76.5 nm min⁻¹ of SiO₂ is measured for specific PECVD parameters. Relative activity calculations show agreement within 5% between identically exposed samples counted on parallel detectors. Memory effect reductions of up to 59±1.8% for 900 nm SiO₂ films produced by plasma enhanced chemical vapor deposition and of up to 77±3.7% for 50 nm Al₂O₃ films produced by atomic layer deposition are shown. Future work is suggested for production of more effective diffusion barriers and expansion to testing in operational monitoring stations.
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