The RTT80 cone beam x-ray computed tomography system, developed by Rapiscan Systems Ltd, uses switched x-ray sources and fixed offset detector rings to remove the time consuming mechanical rotations of earlier imaging systems. This system produces three-dimensional images in real time. A Geant4 Monte Carlo simulation has been developed to investigate scattered radiation in the uncollimated detector machine, showing high levels of scatter behind highly attenuating objects. A new scatter correction method is proposed which estimates scatter to each detector, in each projection, from 1cm³ voxels of the computerised object. The scatter distributions from different materials are pre-determined using a Geant4 Monte Carlo simulation. The intensity of scatter from each voxel is based on measured data. The method is applied to two simulated test objects, a water box simulated with a monoenergetic input spectrum and a test suitcase simulated with a polyenergetic spectrum. The test suitcase is broken down into separate components to analyse the method further. The results show that the method performs well for low attenuating objects, but the results are sensitive to the intensity values. However, the method provides a good basis for a scatter correction method.

The European XFEL is an X-ray free electron laser research facility that generates ultrashort, high intensity flashes. Three SASE undulator systems are made of undulator segments and intersection components. Permanent magnets that form the segments, may potentially suffer demagnetization due to the stray radiation that comes from the interaction between electrons and the beam pipe. A gamma spectrometer (GR1-A, developed by Kromek) is planned to be placed at the entrance of one of the undulator segments of SASE1. Monte Carlo simulations, using Geant4, have been performed to study gamma radiation flux at possible measurement areas. The results show that in some cells the expected flux is larger than the maximum throughput, while for two segments there are areas which satisfy the limitations of the detector. An improvement of the geometry of the Geant4 code has also been done to make simulations more in line with the real systems. A comparison between dose results from the new and the former code shows that components placed at the intersections have a significant impact on dose distributions, especially quadrupole magnets.

<p>  Present WIMP Dark Matter search strategies are mainly focused on possible direct detection through elastic or inelastic scatterings on atomic nuclei, or with electrons. This approach<br> becomes insensitive to M(DM) < 10 GeV. Indirect DM detection refers to the search for DM-DM or DM-M annihilation, decay debris from DM particles, or other particle production,<br> resulting in detectable species. </p> <p><br>    New physics processes, initiated by cosmic ray or dark matter interactions may be observable in underground indirect search experiments by excess high multiplicity neutron<br> production in nuclear targets. Even for M(DM) < 10 GeV, DM-M interaction is capable of<br> producing large signals, >200 neutrons if the energy is deposited in a Pb target.</p> <p><br></p> <p>  The NMDS-II detector, located at an underground laboratory within the Pyhasälmi<br> complex metal mine in central Finland, collated data for 6504 ± 1 hours at 583 m.w.e.<br> and for 1440 ± 1 hours at 1166 m.w.e.. The detector system consists of a 30 cm cube<br> Pb-target surrounded by 60 He-3 proportional tubes and a two layer Geiger Counter muon<br> detection system. The lead target is used to interact with potential dark matter particles, and<br> neutron numbers are measured with He-3 tubes. The neutron event multiplicity production is<br> compared to Geant4 simulations, starting with the well measured absolute muon momentum<br> and angular distribution flux rate at sea level, then propagating the muon flux through rock<br> while preserving the momentum-angular correlation to a depth 4m above the the detector at<br> the two depth locations. The muon flux modeling is compared to the uncorrelated Miyake<br> model at each depth as verification of the muon propagation simulation. Finally, the Geant4<br> fully simulates the passage of the muon and its induced showers through a model universe<br> 10000 m^2 x 12 m depth, and the simulated response of the detector to the calculated muon<br> flux, is compared with the data. <br>  </p> <p>  The Geant4 prediction and the observed data neutron event multiplicity distributions<br> have matching power law shapes, k × n^(-p), and do not have exponential shapes. For the<br> data collected at 583 m.w.e., p=2.36±0.10 with χ2/DoF = 0.76 and for the simulation<br> p=2.34±0.01 with χ2/DoF = 1.05. At 1166 m.w.e., p=2.29±0.007 for the simulation with  χ2/DoF = 1.16. And for the data the collection with only 6 detected events above multiplicity 5, yields p=2.50 ± 0.35 predicted by the Maximum Likelihood Estimatation method. </p> <p><br></p> <p>  The DM acceptance as a function of mass is found using a proton-Pb spallation model.<br> The dark matter mass is assumed to be equal to the proton kinetic energy and to interact<br> uniformly over the volume of the lead target. The number of excess events is found to be<br> -6.1 ± 5.1, that is no excess events are observed. The upper limit with 90% confidence<br> level is then found assuming 2.3 events. The Poisson estimation then yielding search limits<br> 1.1×10^(-44) cm^(-2) for 10 GeV deposited energy, 1.9×10^(-45) cm^(-2) at 1 GeV and 3.0×10^(-45) cm^(-2) for 500 MeV deposited energy and no acceptance at 100 MeV.<br> </p> <p>    An indirect dark matter search was conducted based on DM-M interactions depositing<br> energy in a Pb-target allowing DM masses to be probed in a region 100 MeV < M(DM) <<br> 10 GeV not accessible to direct dark matter searches. Limits are placed on DM-M energy<br> deposition independent of the DM-M interaction. <br>  <br>  <br>  <br>  </p>

Radiotherapy is one of the most used methods for treating cancer and the most common way to execute such treatments is to irradiate tumors with high energy photons. This can damage healthy tissue along the irradiation line. By using hadron therapy and instead irradiate the tumor with charged particles (protons or Carbon 12 ions), the energy can be concentrated to a more specific place in the body. However, the method is not well studied and the tools available for simulating hadron therapy can be hard to use. When simulating hadron therapy and other nuclear interactions a large amount of calculations need to be executed. Monte Carlo methods is a numerical method to solve equations based on repeated number sampling and is used in the simulation program Geant4. Hadron therapy was simulated with Geant4 and the data was analyzed with the data analysis framework ROOT. New macros and analysis scripts were created with the intention to help new Geant4 users. The aim to make Geant4 easier to use was partially met. The implementation of code for the low energy region of Carbon 12 projectiles was unsuccessful.

The core of a BWR constitutes a highly complex radiational and chemical environment. Nuclear fission is utilized to generate power and as a consequence, large quantities of ionizing radiation are produced. Gamma and beta-particles along with neutrons have the sufficient ranges to escape the fuel rods and deposit energy in the reactor coolant. By doing so, radiolysis is initiated. The radiolysis species present in the coolant can interact chemically with the fuel rod cladding and cause corrosion. Different forms of corrosion are found on BWR fuel, with some effects being very local.  This thesis work outlines a method developed to investigate local corrosion phenomena, with a high spatial resolution. The purpose was to study Zircaloy corrosion and more particularly, to investigate an observed jump in corrosion thickness around the lower enrichment step on BWR fuel rod cladding. The corrosion thickness jump is a very local effect, hence the need for high spatial resolution. Monte-Carlo simulations were performed in a GEANT4-model of an inoperation reactor, to study the energy deposited from ionizing radiation in the coolant, around the low enrichment-step of the fuel rods. The energy dose data was then used as input to lwrChem to compute electrochemical potential and equilibrium concentrations of radiolysis species. These are the quantities needed to compute an equivalent corrosion rate on the cladding surface, although this was not performed within this project. The main focus was to successfully develop the two-program method using GEANT4 and lwrChem and this was achieved.  The project was performed Uppsala University with financial contribution from Vattenfall Nuclear Fuel AB and scientific contribution from Studsvik Nuclear AB.

The COmpton Spectrometer and Imager (COSI) is a space telescope that aims to study the soft gamma regime (0.2-5 MeV), capable of performing spectroscopy, imaging and polarimetry of cosmic gamma sources. COSI will use an active shield made of Bismuth Germanium Oxide (BGO), a known scintillator material, in order to reduce the instrumental background produced by primary and secondary particles. When a cosmic ray particle or gamma interacts with the active shield, they will produce scintillation photons. Thanks to the optical coupling of photomultipliers tubes (PMT) to the BGO walls, it is possible to detect these scintillation photons and remove the background events from the analysis with the Compton telescope. IRAP (Research Institute in Astrophysics and Planetology) is the organization responsible of providing the photodetector (PMT with its electronics and mechanics) design to the COSI mission. The main objective of my internship is to study the performance of the photodetector of the active shield, focusing on the development of simulations with Geant4. The first study investigates the propagation of optical photons inside a PMT, comparing an analytical method versus a Geant4 simulation. Geant4 perfectly simulates the transmission and reflection of photons. However, if some of the photons are absorbed by the materials, Geant4 starts to slightly differ. This happens because the absorption is computed with different physic processes in both models. The second study is focused on obtaining measurements from the PMTs at the laboratory. The pulse amplitude at the preamplifier level is analysed, as well as the transmission of the Si pad. The third and fourth studies are focused on the simulation, with Geant4, of the performance of a PMT optically coupled to two different scintillator materials, CsI(Tl) and BGO, respectively. The correct set-up of a simulation is always critical, especially for a space mission like COSI. Both studies aim to find the correct set-up in order to get the most precise simulation. The results are great, showing a really good correlation between the simulation and the real data. During the fifth study, I updated the photodetector engineering model, called DACS (Detector for the AntiCoincidence System), taking into consideration the new electronic board designs and the previous engineering models. The results obtained in this thesis help to understand the performance of the photodetectors that will be used on the COSI telescope, as well as to show the benefits of using a toolkit like Geant4 in projects with propagation of scintillation photons.

Independent yield distributions of high-energy neutron-induced fission are of importance to achieve a good understanding of fission. Even though the mass and charge yield distributions of thermal neutron-induced fission are well known, there are few experimental data for high-energy neutron-induced fission. In addition to basic research on the fission process, independent yield distributions of high-energy neutron-induced fission play a key role in the development of Generation IV fast nuclear reactors. To facilitate measurements of independent fission yields of high-energy neutron-induced fission, a dedicated ion guide and a proton-neutron converter were developed and put to use in experiments at the isotope separator facility IGISOL in Jyväskylä. In parallel, a simulation model of the system was developed in order to optimize the collecting efficiency of fission products in the ion guide. The model uses the Monte Carlo code MCNPX to simulate the neutron production, the fission model code GEF to simulate the fission process, and GEANT4 for ion transportation. In order to benchmark the simulation model, metal foils were inserted in the ion guide with the purpose of collecting fission products. At the same time, nickel, cobalt and indium foils were located between the pn-converter and the ion guide to record the neutron flux from the pn-converter. After the beam was turned off, and after several days of cooling, g-ray spectroscopy measurements of the foils were conducted using a well shielded HPGe detector. Based on the identified g-ray transitions in the spectroscopy data, the productions of corresponding fission products and neutron activation products were calculated, and then used to benchmark the transportation and collection of fission products, as well as neutron production, in the simulations. The conclusion from the benchmark is that the transportation of fission products in the helium gas, as simulated by GEANT4, agrees very well with the measurement, while the transportation of fission products in the uranium targets agrees with the measurement within 10%. The neutron flux at the high-energy part of the neutron spectrum is overestimated by about 40%.Thanks to the benchmark it has been shown that the predictive power of the model is satisfactory and sufficient for the purpose of modeling the ion guide. Furthermore, the parameters involved in the simulations, such as neutron production, distance between the neutron source and the ion guide, volume of the ion guide and so on, play an important role in the optimization of the setup. However, the lower than expected fission rate suggests that the optimization on these parameters may not be enough to achieve a sufficiently high intensity of fission products, especially for nuclei far from the stability line. To achieve a sufficiently high intensity, an electric field guidance, similar to the RF structure of the CARIBU gas catcher presented in G.Savard et al. Nucl. Inst. Meth. B, 376: 246, 2016, to collect fission products is considered.

<p> In nuclear astrophysics, there are still undiscovered areas involving unstable nuclei, like the nucleosynthesis in exploding stars. The unstable nuclei are extremely difficult to produce in the laboratory for study since they decay away quickly once they are formed. To make the unstable nuclei available in the laboratory, Canada has built one of the best facilities in the world for nuclear astrophysics studies with radioactive beams, called TRIUMF-ISAC. Its recent upgrade to ISAC-2 can produce even heavier radioactive beams with higher energy. To fully utilize the ISAC-2 facility, a high-segmented HPGe γ-ray detector-TIGRESS, has been developed to study exotic nuclei using the well-established technique of Coulomb Excitation.</p> <p> As an essential part of the TIGRESS facility, auxiliary detectors, such as silicon detectors, are used to improve the sensitivity of the experiments through Doppler correction. Two types of silicon detectors will be used. One is the CD-shaped double-sided silicon strip detector, which is the topic of this thesis. The other one is the silicon barrel detector. This thesis will discuss the computer simulations and experimental tests of the CD detector.</p> <p> Simulations were made using a simulation toolkit GEANT4, which was run under the Linux system. The goal was to test the hypothesis that measurements of the energies of heavy ions in Coulomb excitation may improve the Doppler correction by constraining the scattering location in a thick target. To test the performance of the simulated CD detector, simple simulations were performed for shooting calibration alpha particles from an 241Am source first directly onto the CD and second through a thin target onto the CD. The goal of additional simulations was to attempt to determine the corresponding scattering depth in a thick target by putting an energy cut on the spectrum of the scattered beam detected in the CD. These simulation results verify the possibility of such localization, though the resolution is not very good.</p> <p> Experimental tests for the CD detector are aimed to obtain the energy resolution for each strip element of the CD and to have at least 8 channels instrumented in our test lab before this detector is sent to TIGRESS facility. The test station including electronics and detector systems and all results from initial tests are described in detail in this thesis.</p> / Thesis / Master of Science (MSc)

Un réseau de seize détecteurs ATLAS-MPX a été mis en opération dans le détecteur ATLAS au LHC du CERN. Les détecteurs ATLAS-MPX sont sensibles au champ mixte de radiation de photons et d’électrons dans la caverne d’ATLAS et sont recouverts de convertisseurs de fluorure de lithium et de polyéthylène pour augmenter l’efficacité de détection des neutrons thermiques et des neutrons rapides respectivement. Les collisions à haute énergie sont dominées par des interactions partoniques avec petit moment transverse pT , associés à des événements de “minimum bias”. Dans notre cas la collision proton-proton se produit avec une énergie de 7 TeV dans le centre de masse avec une luminosité de 10³⁴cm⁻²s⁻¹ telle que fixée dans les simulations. On utilise la simulation des événements de "minimum bias" générés par PYTHIA en utilisant le cadre Athena qui fait une simulation GEANT4 complète du détecteur ATLAS pour mesurer le nombre de photons, d’électrons, des muons qui peuvent atteindre les détecteurs ATLASMPX dont les positions de chaque détecteur sont incluses dans les algorithmes d’Athena. Nous mesurons les flux de neutrons thermiques et rapides, générés par GCALOR, dans les régions de fluorure de lithium et de polyéthylène respectivement. Les résultats des événements de “minimum bias” et les flux de neutrons thermiques et rapides obtenus des simulations sont comparés aux mesures réelles des détecteurs ATLAS-MPX. / A network of sixteen ATLAS-MPX detectors has been put in operation in the ATLAS detector at CERN-LHC. ATLAS-MPX detectors are sensitive to a mixed radiation field of photons and electrons in the ATLAS cavern and are covered with lithium fluoride and polyethylene converters in order to increase the detection sensitivity of thermal neutrons and fast neutrons respectively. High energy collisions are dominated by partonic interactions with small transverse moment pT , associated with "minimum bias" events. In our case, the proton-proton collision occurs with an center-of-mass energy of 7 TeV with a fixed luminosity of 10³⁴cm⁻²s⁻¹ in the simulations. We use "minimum bias" events simulation generated by PYTHIA using the framework Athena. Athena’s framework makes a full GEANT4 simulation of the ATLAS detector. We include the positions of each detector in the Athena algorithms for measuring the number of photons, electrons and muons that can reach the detectors ATLAS-MPX. We measure the fluxes of thermal and fast neutrons generated by GCALOR in lithium fluoride and polyethylene regions respectively. The "minimum bias" events results and the flow of thermal and fast neutrons obtained by simulations are compared with real measurements of the ATLAS-MPX detectors.

Les détecteurs à pixels Medipix ont été développés par la collaboration Medipix et permettent de faire de l'imagerie en temps réel. Leur surface active de près de $2\cm^2$ est divisée en 65536~pixels de $55\times 55\um^2$ chacun. Seize de ces détecteurs, les Medipix2, sont installés dans l'expérience ATLAS au CERN afin de mesurer en temps réel les champs de radiation produits par les collisions de hadrons au LHC. Ils seront prochainement remplacés par des Timepix, la plus récente version de ces détecteurs, qui permettent de mesurer directement l'énergie déposée dans chaque pixel en mode \textit{time-over-threshold} (TOT) lors du passage d'une particule dans le semi-conducteur. En vue d'améliorer l'analyse des données recueillies avec ces détecteurs Timepix dans ATLAS, un projet de simulation Geant4 a été amorcé par John Id\'rraga à l'Université de Montréal. Dans le cadre de l'expérience ATLAS, cette simulation pourra être utilisée conjointement avec Athena, le programme d'analyse d'ATLAS, et la simulation complète du détecteur ATLAS. Sous l'effet de leur propre répulsion, les porteurs de charge créés dans le semi-conducteur sont diffusés vers les pixels adjacents causant un dépôt d'énergie dans plusieurs pixels sous l'effet du partage de charges. Un modèle effectif de cette diffusion latérale a été développé pour reproduire ce phénomène sans résoudre d'équation différentielle de transport de charge. Ce modèle, ainsi que le mode TOT du Timepix, qui permet de mesurer l'énergie déposée dans le détecteur, ont été inclus dans la simulation afin de reproduire adéquatement les traces laissées par les particules dans le semi-conducteur. On a d'abord étalonné le détecteur pixel par pixel à l'aide d'une source de $\Am$ et de $\Ba$. Ensuite, on a validé la simulation à l'aide de mesures d'interactions de protons et de particules $\alpha$ produits au générateur Tandem van de Graaff du Laboratoire René-J.-A.-Lévesque de l'Université de Montréal. / The pixelated Medipix detectors have been developed by the Medipix Collaboration to perform real-time imaging. The semiconducting chip is divided into 65536 pixels of $55\times 55\um^2$ for a total active area of nearly $2\cm^2$. Because of their sensitivity to all kinds of particles, sixteen Medipix2 detectors (ATLAS-MPX) have been placed in the ATLAS detector and its cavern to measure for the radiation produced by the head-on proton collisions produced at the LHC. At the next ATLAS upgrade, the ATLAS-MPX network will be extended to include the Timepix detectors, the latest version that allows one to measure the total energy deposited in the semiconductor. To improve data analysis, a Geant4 simulation project of a Timepix detector was initiated by John Id\'arraga at the Université de Montréal. In the framework of the ATLAS experiment, this simulation could be used with Athena, the ATLAS analysis software, and the full ATLAS simulation. Due to their repulsivity, the charge carriers created by an incoming particle in the pixelated detector are spread over the surrounding pixels causing a charge sharing effect. An effective model has been developed to reproduce this effect without resolving the charge drift's differential equation. This model and the \textit{time-over-threshold} mode of the Timepix have been included in the simulation to reproduce the tracks left by the striking particles. First, one had to individually calibrate each pixel of the device with $\Am$ and $\Ba$ sources. The simulation's validation has been performed with low energy protons and $\alpha$ particles delivered by the van de Graaff Tandem at the Laboratoire René-J.-A.-Lévesque of the Université de Montréal.