Master's Theses

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Radon removal from gaseous xenon for the enriched xenon observatory
(Laurentian University of Sudbury, 2014-03-17) Beauchamp, Eric
Neutrino oscillation experiments have shown de nite evidence for non-zero neutrino masses. However, these experiments only tell us about neutrino mass di erences, and nothing about the absolute masses themselves. The observation of neutrinoless double-beta (0 ) decay, a hypothetical nuclear transition, would provide the rst absolute mass scale measurement of the neutrino outside of cosmology. This decay would imply the neutrino to be a Majorana particle, the rst fermion of its kind. 0 decay would also be the rst observation of lepton number violation. The Enriched Xenon Observatory (EXO) is currently searching for 0 decay in 136Xe with a half-life greater than 1025 years. EXO-200 is the rst experiment of the EXO physics program, which has observed twoneutrino double beta decay (2 ) in 136Xe for the rst time, with a half-life of 2:165 0:075 1021 years [1]. This is the longest measured half-life to date. EXO is now designing a 5-tonne scale detector, nEXO, to be sensitive to the inverted-scale hierarchy. Despite the careful selection of radiopure substances for the detector, the existence of trace levels of 222Rn is inevitable. One of the daughters of 222Rn, 214Bi, can emit photons at the Q-value for 0 decay, making it a critical background. This dissertation investigates the method of Rn removal from gaseous Xe through the use of a Cu wool trap.
Quantum phase transitions and topological orders in spin chains and ladders
(Laurentian University of Sudbury, 2014-03-17) Pandey, Toplal
Dimerized antiferromagnetic spin-1/2 chains and ladders demonstrate quantum critical phase transition, the existence or absence of which is dependent on the dimerization and the dimerization pattern of the chain and the ladder, respectively. The gapped phases can not be distinguished by the conventional Landau long-range order parameters. However, they possess non-local topological string order parameters which can be used to classify different phases. We utilize the self-consistent free fermionic approximation and some standard results for exactly solved models to analytically calculate the string order parameters of dimerized spin chains. As a complement parameter the gapped phases possess the topological number, called the winding number and they are characterized by different integer values of the winding number. In order to calculate the string order parameters and winding numbers in dimerized spin chains and two-leg ladders we use analytical methods such as the Jordan-Wigner transformation, mean-field approximation, duality transformations, and some standard results available for the exactly 1D solve models. It is shown that the winding number provides the complementary framework to the string order parameter to characterize the topological gapped phases.
Low radon permeable gloves and laserball simulations for SNO+
(Laurentian University of Sudbury, 2014-03-19) Carranza-Barnard, Zachariah
The SNO+ experiment is a multipurpose liquid scintillator detector whose rst goal is to measure neutrinoless double beta decay. This thesis describes two important components: simulations to optimize the time window for the prompt peak of an optical calibration source, the \laserball" and the search for gloves to handle calibration sources while maintaining stringent background conditions. Non-direct light found in laserball runs creates challenges for optical calibration. By changing the time pro le from the standard 4ns to an asymmetric pro le of +2 􀀀4 ns this contribution of non-direct light can be reduced up to 45%. Gloves provide an access point to manipulate calibration sources during deployment inside the detector and as barrier to 222Rn, a known background to the experiment. However, typical glove materials are found to permeate large amounts radon. Through a careful selection process the material Silver Shield was chosen for use in SNO+ with a permeation rate of 1:1 10􀀀6 radon atoms/hour.
Background discrimination studies and measurements of droplet and bubble size for the Picasso experiment.
(Laurentian University of Sudbury, 2014-05-13) Dhungana, Navaraj
The Project in Canada to Search for Supersymmetric Objects (PICASSO) searches for cold dark matter through the direct detection of Weakly Interacting Massive Particles (WIMPs) via spin-dependent and spin independent interactions with 19F nuclei. The detection principle is based on the superheated droplet technique; the detectors consist of a gel matrix with millions of liquid droplets of superheated fluorocarbon (C4F10) dispersed in it. In order to reduce the background, it is essential to distinguish the signature of different background particles interacting in the detector. A dedicated setup was developed in order to study the response of the C4F10 droplets in the presence of different backgrounds. The main objectives of this research are to identify the actual size (diameter) of the droplet increases due to phase transition and to check and establish the correlation between the droplet size and the maximum amplitude of the signal. In addition, the alpha-neutron discrimination was studied by observing each event’s image frames and the associated acoustic signal to get the amplitude distribution. The mean ratio of bubble size to droplet size was found to be 5.48, independent of temperature and type of interacting particle. Furthermore, no correlation was found between the droplet size and the maximum amplitude of the signal. As for the discrimination studies, the analysis of the signal events has confirmed that alphas generated outside the active liquid in the gel are much more difficult to discriminate from neutron than when alphas are generated inside the active liquid.
Image quality and dose of an accelerator-integrated kV CBCT systems
(Laurentian University of Sudbury, 2014-08-26) Shaaer, Amani
The ability of an imaging modality to precisely determine patient anatomy and provide reliable information about tumor position is critical in the radiotherapy process. As image-guided radiotherapy (IGRT) becomes more popular in radiation treatment, its overall quality and performance, such as the image quality and amount of dose delivered, need to be assessed. The research described in this dissertation was focused on investigation of the image quality of the planar and cone-beam computed tomography (CBCT) images of two imaging systems commonly used in radiotherapy: Varian On-Board Imager (OBI) and Elekta X-ray Volumetric Imager (XVI). Several imaging quality tests were performed using current clinical imaging protocols provided with both systems and various types of image quality phantoms. CBCT imaging dose of each system was also estimated using standard CT dose index (CTDI) phantoms and several imaging protocols. Overall, the image quality between the OBI and XVI was fairly consistent with each other with the exception of high contrast resolution and Hounsfield Unit (HU) accuracy. CTDI of OBI was higher than that of XVI which was related to the different designs and imaging protocols between the two systems.
Bond mean field theory for electron spin resonance frequency shift analysis
(Laurentian University of Sudbury, 2015-02-24) Rodger, Clifford John
Electron spin resonance (ESR) is an important experimental technique. A comprehensive theory of ESR has been di cult to establish, and as such several di erent approximations are used to predict and explain experimental results. This thesis applies the bond-mean- eld theory to the problem of ESR frequency shift for the one-dimensional antiferromagnetic Heisenberg spin chain with uniaxial exchange anisotropy. We use this theory to calculate the ESR resonance frequency shift as a function of temperature and magnetic eld. We perform numerical calculations using the expression obtained. These results are compared to existing results in the literature; they are in broad agreement with theoretical results such as those of Oshikawa and A eck obtained via bosonisation, but they show discrepancies with experimental results. We agree with the theoretical authors that the discrepancy is due to our use of the simplest-case interaction model.
Evaluating SNO+ backgrounds through 222RN assays and the simulation of 13C(a, n)160 reactions during water phase
(2015-10-14) Rumleskie, Janet
SNO+ is a large multipurpose neutrino detector searching for rare interactions. Some backgrounds come from naturally occurring 222 Rn and its daughters within the 238 U chain. Under development are cryogenic trapping assay systems which will monitor the 222 Rn levels within scintillator, water, N 2 cover gas, and small detector materials. These systems can measure concentrations up to 8×10 −5 Rn atoms/L (1.6 × 10 −17 g 238 U/g LAB) within scintillator, and 4.75×10 −15 g are discussed within. 13 238 U/g H 2 O in water. The status of the assay systems C(α,n) 16 O reactions occur from 222 Rn’s progeny 210 Po, but Monte Carlo simulations predict < 0.4 events within the SNO+ fiducial volume of 5.5 m and 5 - 9 MeV region of interest over a 9 month running period within water.
Neck sense rope system and leaching studies for SNO+.
(2015-11-16) Khaghani, Pouya
SNO+ is a multipurpose scintillation-based neutrino experiment which is located at SNO- LAB, Creighton mine, Sudbury. The primary scientific goal of the experiment is searching for the elusive process of neutrinoless double beta decay of 130Te. In addition to 0vbb decay, SNO+ will be able to detect low energy solar neutrinos, geo- and reactor- anti-neutrinos, as well as supernova neutrinos. During an initial water phase, it will also search for invisible modes of nucleon decay. This thesis briefly introduces neutrino physics and discusses the milestones in chapter 1. Chapter 2 discusses the SNO+ experiment in detail and outlines important developments in SNO+. Furthermore, chapter 3 and chapter 4 describe the two main projects that have been done by the author: i) leaching studies for the SNO+ experiment and ii) the neck sense rope system. Looking for rare events requires very stringent background limits. One of the sources of background originates from 222Rn daughters implanted into the inner surface of the SNO+ acrylic vessel. They can leach into the detector volume and increase the level of internal background. A leaching model has been developed by the author to estimate the activity and contribution to the backgrounds. The model is compatible with the measured value from a lead assay to within 1 sigma. The SNO+ sense rope system is a mechanical system which monitors displacement of the AV neck to within 2mm accuracy. The system has been calibrated and installed underground alongside with the sliding floor. Chapter 4 discusses the system, the performed calibration and the installation procedure in more detail.
X-ray scattering point models for breast cone-beam computed tomography
(2016-03-01) Laamanen, Curtis
The purpose of this work was to determine via simulations the potential use of sim- plified scattering point models in full-field breast Cone-Beam Computed Tomography (CBCT). A many Scattering Point (MSP) per incident beamlet model and a Single Scat- tering Point (SSP) model were tested against Geant4 simulations, as well as against each other. Comparisons were made using both homogeneous as well as heterogeneous phantoms. The homogeneous phantoms were cylinders, 14 cm and 7 cm in diameter and 10.5 cm in length with various fibroglandular(fib):fat compositions. The hetero- geneous phantom was the 14 cm phantom mentioned previously, but modeled as pure fib with a number of smaller embedded cylinders composed of fat. A second configu- ration with the compositions of the main and embedded cylinders swapped was also tested. The simulation used a 60 kV tungsten anode spectrum with a HVL of 3.7 mm Al which irradiated the simulated breast phantom over 300 projections. The detector was modeled as 300 × 300, 1 mm2 energy integrating pixels, with a DQE of unity. Both of the models approximate the cone-beam as a number of individual beamlets (300 × 300 to match with the detector) with scattering points placed along their in- tersections with the phantom. The MSP model incorporated a single scattering point per 1 cm of incident beamlet length within the phantom. The SSP model used an adjustable single scattering point positioned at a fractional depth within the phan- tom. By comparing results from these two scattering point models, values of were determined which would yield SSP model scatter approximations matching those of iii the MSP model. Both models were tested against Geant4 simulations for their ability to adequately estimate the scatter signals upon the detector. The SSP model was also tested for its ability to correct for the cupping artifact in reconstructed images of the heterogeneous phantom, without assuming knowledge of the inner heterogeneous geometry. The Hounsfield Units (HU) obtained with primary photons were 48.5±3.18 and −159 ± 23.0 for fib and fat respectively for one of the heterogeneous phantom con- figurations. Due to the cupping artifact in the reconstructed images which included scatter, these values were −46.6 ± 18.9 and −215 ± 34.0 respectively. Following cor- rections for the single scatter, via the SSP model, the corresponding CT numbers became 52.3 ± 3.67 and −161 ± 23.3. It was encouraging to see that a simple model can minimize the effects of single scattered photons during CBCT of a heterogeneous phantom. The HUs obtained post-scatter correction agreed well with those obtained with primary photons. The preliminary findings encourage further efforts for thoroughly testing these scatter point models’ applicability for obtaining higher quality CBCT images.
Fat subtraction protocol for wide-angle x-ray scatter analysis of breast biopsies
(2016-03-21) McDonald, Nancy
Breast cancer detection often involves the use of mammography to locate sus- picious lesions followed by extraction of some tissue within the lesion via a biopsy procedure. The gold standard method for determining whether the extracted tis- sue is malignant or benign is an histological analysis. However, complimentary x ray methods such as x-ray uorescence (XRF), small-angle x-ray scatter (SAXS) and wide-angle x-ray scatter (WAXS) have been investigated by various groups. The focus of this dissertation was to develop a WAXS fat subtraction protocol for the WAXS analysis of breast tissue biopsies. The WAXS signals of breast tissue could become an additional source of diagnostic information. Healthy breast tissue is composed of fat and broglandular ( brous) tissue. Com- parisons of the WAXS signals of broglandular and cancerous tissue are di cult be- cause biopsies of either type usually contain some fat tissue. The ability to look at WAXS signals independent of the fat contribution could be informative. The goal of this work was to validate a WAXS fat subtraction protocol using an animal tis- sue sample consisting of a mixture of fat and brous tissue. The di erential linear scattering coe cient d s=d of a region of interest (ROI) within the sample was measured via energy dispersive x-ray di raction measurements using a custom built CdTe di ractometer. The mean fractional volume of fat ( fat) within the ROI was estimated using a digital x ray imaging system. The transfer of the sample from the iii WAXS system to the digital system required accurate knowledge of the ROI. The use of fat in a WAXS fat subtraction model then allowed the d s=d of brous tissue to be estimated. The signals obtained via the subtraction protocol agreed well with the signals obtained using pure tissue samples. The scattering coe cient d s=d is a function of the momentum transfer argu- ment x = 1= sin( =2), a variable that combines the dependence of scatter on photon and scatter angle . Accessing a larger x space could provide more information about the nature of breast tissue. Modi cations to the custom built di ractometer were implemented in order to access a larger x space. Speci cally, the capability to measure signals at smaller and the use of higher kV beams were the outcomes. Pre- liminary results obtained with water, polymethyl methacrylate, and polycarbonate samples were promising, yet suggested that better collimation is required between the sample and the detector in order to reduce scatter contamination from objects located downstream from the sample.
The SNO+ supernova calibration source development and testing
(2016-07-19) Darrach, Caitlyn
SNO+ is a kilotonne-scale, liquid scintillator-based neutrino detector housed in the underground facilities of SNOLAB at Creighton Mine, Sudbury. SNO+ is capable of detecting bursts of neutrinos released by nearby core-collapse supernovae among other physics goals. For such an event, stress testing is required to ensure that a burst of supernova neutrino events can be reliably read out and recorded by the electronics and data acquisition system to avoid data pileup and limit event separation. During a supernova, SNO+ needs to be able to record the burst, send a timely alert to the astronomical community, and quickly analyze and interpret the data. The supernova calibration system (SNC+) for SNO+ simulates the light produced by interactions of neutrinos from a supernova within the liquid scintillator target using pulsed, visible light from a laser diode. The SNC+ is a data-driven pulser capable of producing high-powered, ns-scale pulses with repetition rates up to 12.5 MHz. Each photon pulse is expected to deposit energy of up to 70 MeV within the liquid scintillator of the SNO+ detector. The light from the SNC+ laser diode will be delivered isotropically within the SNO+ detector by ber optics and a di using glass laserball. The SNC+ has undergone design, parts procurement, construction, assembly, and initial-stage testing for this thesis research.
Calibration and commissioning of the Helium and Lead Observatory
(2017-01-25) Bruulsema, Colin
The Helium And Lead Observatory (HALO) is a dedicated supernova detector at SNOLAB consisting of 79 tonnes of lead instrumented with 128 3He-filled neutron counters. A burst of neutrinos from a supernova will interact with the lead and result in a burst of neutrons, detectable by the counters. This burst can be identified as a supernova signal. The previously existing HALO Monte Carlo simulation was revised to better represent the detector and evaluate its supernova response. The composition of the paint used on the lead blocks was estimated using new and previous measurements. Other geometry updates were checked with neutron capture simulations to verify their implementation. To verify the detection efficiencies of the Monte Carlo simulation, a 252Cf neutron source was deployed in the 40 copper calibration tubes in the detector. The high neutron multiplicity in Cf fissions allowed for the source strength to be determined along with the neutron capture efficiency by an analysis of the relative population of the detected multiplicities. This verified the Monte Carlo simulation’s results and gave an overall efficiency for the detection of supernova-induced neutrons of (27.61 0.17)%. Backgrounds to the detection of supernovae include neutrons leaking into the detector from the lab, the spontaneous fission of uranium inside the detector, and muon-induced spallation events in and around the detector. These factors along with the false positive rates specified by the Supernova EarlyWarning System (SNEWS) limit the trigger threshold to 4 neutrons detected in two seconds, giving a detection range of about 13.7 kpc for supernovae with an average neutrino energy of 18 MeV and a pinching parameter of 2.
Calibration of the PICO-0.1 bubble chamber and development of coated inner vessels for dark matter search
(2017-06-05) Girard, Frédéric
The detection of dark matter is one of the biggest challenges in modern physics. The PICO experiment aims for the direct detection of Weakly Interacting Massive Particles (WIMPs) with bubble chambers. In this thesis, results from calibrations of the PICO-0.1 test chamber is presented. Calibrations were performed at the Tandem Van de Graaff facility of the Université de Montréal. Monoenergetic neutrons were produced from the 51V(p,n)51Cr reaction with a 1.6 MeV proton beam. Two 3He neutron counters used during calibrations for neutron flux normalization were also calibrated. This result is contributing in improving Monte Carlo simulations of PICO-0.1. Finally, preliminary work was done toward the use of new inner vessel materials with coated surfaces for bubble chambers. A Condensation Bubble Chambers (CBCs) was used as a test-bench. Progress has been made toward the usability of Poly-methyl-methacrylate vessels, but more work is needed to solve spontaneous wall nucleation problems.
Design of a neutron calibration source for the SNO+ experiment
(2017-11-13) Semenec, Ingrida
SNO+ is a multipurpose detector situated at the SNOLAB facility located at Creighton mine 2 km deep. The SNO+ experiment will have three phases: water, pure scintillator and Te-loaded scintillator. With the detector filled with scintillator, solar neutrinos, geo and reactor anti-neutrinos, and supernova neutrinos can be studied. To analyze the data collected by the detector, it is important to have detailed knowledge of the detector response. This is why calibration is a crucial part of the experiment. The detector response to neutrons will allow us to study the anti-neutrino flux coming from reactors in Canada. Anti-neutrinos can be detected via the inverse beta decay reaction which can be tagged using the neutrons it produces. This thesis will discuss the radioactive calibration source Americium Beryllium (AmBe) which produces neutrons and gammas. The existing AmBe source - inherited from the SNO experiment - that will be used in water phase has to be modified for the scintillator and loaded scintillator phases. Simulations were carried out to determine the optimal additional shielding required for the scintillator phase. The optimal shielding was determined to be 2 mm of lead surrounded by 1 mm of stainless steel for the encapsulation. The new design for the AmBe source was finalised. The estimated neutron capture event detection efficiency is 74.22%. The analysis of the source deployment at various positions within the detector and the shadowing effects are discussed as well.
First principle study of the electronic structure of semiconductors for photovoltaic applications: organic-inorganic perovskites
(2018-08-18) Morningstar, Brendan
The ill effects of climate change affect all trends, and the steps taken in the drive to reduce global emissions will reverberate for thousands of years. It is among the most significant and urgent problems we face, and so it is immensely important to call upon existing and near future technologies for generating clean electricity. For now, the most talked-about renewable energy source is solar. It is a massive resource by any standard and it has the potential to play an essential role in decreasing the dependency on crude oil and reducing fossil fuel emissions. Today, the best-performing perovskite cell has reached a power conversion efficiency of 22.1%. This unprecedented rise in efficiency for a photovoltaic technology suggests a sunny outlook, but before a large-scale deployment of the technology, there are still some real questions that must be addressed. The best performing perovskite cells contain lead, which is very toxic and damaging to the environment, and are unstable in humid conditions. Also, the fundamental working of these materials is still largely unknown. The technological base of photovoltaics is becoming progressively dependent on complicated materials, and so it is important to systematically investigate the nature of the electronic structure. In the present work, the electronic structure of five perovskite compounds, MAPbBr3, CsPbX3 (X=Cl, Br, I) and RbPbI3, are systematically studied from first principles using the all-electron, full potential, linearized augmented plane wave ((L)APW) + local orbitals (lo) method as implemented in the WIEN2k code. It is noted that: (i) the band gap of ABX3 increases when A changes from MA to Cs; (ii) as X changes from Br to Cl to I, the band gap increases; and (iii) as A changes from Cs to Rb, the band gap mostly remains the same.
222Rn measurements within the water phase of the SNO+ experiment
(2018-12-06) Woosaree, Pooja
The SNO+ experiment is a large multipurpose scintillator detector. In the first phase of the experiment, close monitoring is done to determine radioactivity background levels, in particular the monitoring of 222Rn as its presence can obscure or mimic physics data. This thesis focuses on a cryogenic technique used to collect and concentrate 222Rn in the water used for the SNO+ experiment. The target level for 222Rn using this system is 3.5 × 10−14 g 238U/gH2O equivalent for the initial water phase. The radon assay technique and resulting measurements are discussed. Further analysis was done to determine the content and locations of areas within the detector emitting higher than expected rate of events. This is informally known as the "hotspot" problem.
Parametrized simulation of charge drift and collection in the nEXO TPC
(2019-03-13) Robinson, Alexander Lars
For the purpose of creating a computationally fast and light-weight simulation of charge drift, diffusion, and collection in the nEXO TPC, a numerical Python package was developed which implements two different models. The models implemented are: diffusion via fragmentation of a charge cloud into point charges, and diffusion via integration of the idealized Gaussian distribution across the anode’s charge-collection pads. Both methods are described in detail, and the latter is compared to a more detailed simulation
SNO+ waterphase burst principal component analysis
(2019-10-30) Rost, Philip Rost
SNO+ is a kilo-tonne scale neutrino detector utilizing much of the same hardware that was used during the SNO experiment. The SNO+ experiment will be conducted using three different target media in three phases: water phase, pure scintillator phase and tellurium loaded scintillator phase. Through all phases SNO+ will be sensitive to a large neutrino burst from a nearby supernova. Data bursts can be caused by a supernova neutrino burst or other physical phenomena such as electronic pickup, static discharge, equipment malfunctions and unintended light injection. This thesis examines data bursts which occurred during the light water phase commissioning of detector operation using a principal component analysis. The principal component analysis showed 3 major groupings within the analysed bursts: bursts during period of time with detector running issues, burst generated due to electronics break downs or light injection and bursts occurring during periods of time where the detector is operated in an abnormal running mode. The analysis in this thesis also shows that many of the data bursts were caused by detector running issues after some initial burst event. Since the state of the detector has been improved, a repetition of this study is recommended with more recent data
Bubble growth dynamics for C3F8 bubble chambers
(2019-12-16) Le Blanc, Alexandre
PICO is a direct dark matter search experiment that utilizes bubble chambers and the acoustics of growing bubbles to discriminate signal from background events [5]. It has been shown that temperature perturbations from equilibrium significantly affect the growth of a bubble in superheated water [17], particularly in the form of an onset delay to perceptible growth. In this thesis the temperature perturbation method is applied to superheated C3F8 to describe the acoustic background discrimination achieved by the PICO experiment. We reproduced the delay in water and confirmed its occurrence in C3F8. We hypothesized that the delay can be used for optical background discrimination. However, it was shown that the model does not apply to the energy deposition of different types of particles as all of them, e.g. incoming alpha particles and neutron induced nuclear recoils, create multiple bubbles along their tracks. Those bubbles, in the end, coalesce to form the observed single visible bubble.
The effects of dietary ingestion of nickel recovery slag as a grit source on avian bone
(2020-06-05) Lapointe, Michel R.
Nickel recovery slag has been ubiquitously deposited in the environment of the Sudbury, Ontario basin giving merit to a study of the impact of this foreign material on wildlife in the area. In this work, the effects of ingestion of this largely metallic grit source on the bone health of Columbia Livia Domestica pigeons was measured. This was accomplished by controlling the diets of two groups of birds, one given exclusively limestone grit, the second given exclusively slag as a grit source. After one year of this controlled diet, the subjects were euthanized and their tibiotarsi were subsequently harvested for testing. Tests performed on the tibiotarsi including breaking strength, Young’s modulus, cortical thickness, density, bone mineral density, and mass spectrometry with a focus on iron and calcium concentrations. Additionally, conventional micrographs and scanning electron micrographs with accompanying energy dispersive spectrometry were collected. Our analyses of the results are consistent with degraded bone physiology in the slag‐fed group compared to the control birds.

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