Physics / Physique - Master's Theses

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Radon production in the EXO-200 recirculation loop
(2024-01-19) Walent, Max
EXO–200 was a 0νββ experiment which has completed data taking and its next generation nEXO is currently in the design phase. In the enriched liquid Xenon of EXO–200 a steady state of 170 ± 31 222Rn atoms was ob- served which contributed to the background of the detector. This thesis is concerned with finding the source of the radon atoms in the Xenon of EXO–200. It is done through a post-mortem analysis of the plumb- ing emanation. A majority of the plumbing was reconstructed replacing the detection chamber with a Radon detector known as an Electrostatic Counter (ESC) that detects alpha decay products. Several experiments were done with different configurations of the plumbing. Different configurations were compared against each other by blank subtraction to single out components of interest, specifically the purifiers and gas purity monitors (GPMs). New efficiencies for the ESC were calculated for the new detector configuration. The reassembled plumbing generally, showed an average steady-state 222Rn population of 222 ± 62 atoms which is consistent with what EXO- 200 observed during operation. The purifiers averaged a 222Rn activity of 33 ± 23 atoms (6.1 ± 4.2 dpd, 1 dpd= 0.012 mBq, where dpd is decays per day). The GPMs showed a 222Rn activity consistent with zero. An estimate was made of the activity from wetted stainless steel surfaces, it was found, but not proven conclusively, that these surfaces could be the dominant back- ground contributor. Thus the major contributor to the background appears to be external to the purifiers and GPMs, it is expected that this contributor is the wetted stainless steel surfaces.
Analysis of the composition of Halo's 252CF neutron source
(2024-03-20) Nichol, Ben
The Helium and Lead Observatory (HALO) is a supernova neutrino detector located un- derground at SNOLAB. HALO consists of 128 3He neutron counters and 79 tonnes of lead. In the event of a supernova, neutrinos will interact with the lead, releasing neutrons which will be detected by the 3He neutron counters. Each neutron counter in HALO has a different neutron capture efficiency by virtue of its position within the array. These efficiencies were found by previous work using Monte Carlo simulations and verified with a californium neu- tron source. A previous analysis using this source has assumed that it was purely comprised of 252Cf, but recently it has been recognized that several isotopes are present and contribute to the source’s neutron output. A better knowledge of the isotopic composition of the cali- bration source is necessary to more accurately determine HALO’s neutron capture efficiency in different locations. In addition to the 252Cf present in the source, other neutron emitting isotopes of various half-lives and multiplicity distributions are present, such as some 250Cf, which was present at the source’s creation, and 248Cm which is created as an alpha decay product of 252Cf. This demands a more complex model of the source, which is undertaken in this thesis. New data of the 252Cf source was taken with the source in HALO’s central tube. This data, as well as data taken over the past few years, was analyzed with maximum likelihood methods with hopes of learning more about the isotopic composition of the source. It was found that by analyzing the past five years of runs taken with the source in HALO, the decay pattern does not match any expected set of initial conditions. Assuming a central neutron capture efficiency of 48%, the strength of the source appears to be 24.8 ± 0.2 neutrons per second greater than the 7.7 ± 0.1 neutrons per second expected. If the ICP-MS report made at the beginning of the source’s lifetime is correct pertaining to the source’s isotopic composition and the curium was removed from the source on the date provided, then some other phenomena must account for the discrepancy experienced. Various explanations for this discrepancy were tested, including gamma-induced fission, alpha-induced fission, and spontaneous fission of isotopes in the nearby rocks. No method examined appeared to give a strong explanation for this discrepancy.
Extracting the time of core-bounce from core-collapse supernova neutrino signals in current and next-generation neutrino detectors
(2023-09-25) Hill, Remington W.
Core-collapse supernovae (CCSNe) are amongst the most rare and energetic events in the galaxy. In the Milky Way, they are predicted to happen as infrequently as 1.64±0.46 times per century. Over a duration of approximately ten seconds, a CCSN will convert ≈ 99% of its iron core’s gravitational binding energy into neutrinos. The initial wave of neutrinos is powered by the neutronization burst, which is generated by electron capture reactions on the collapsing core, which follows a critical time in the dynamics of a CCSN, core-bounce. It has been 36 years since a CCSN was observed via its neutrinos. With the observation of SN 1987A via its neutrino signal, a global effort has been undertaken to bring together all neutrino detectors under a common goal: providing an early alert to the astronomical community of an impending supernova and, if possible, triangulate to the CCSN using its neutrino signal. This effort is called the SuperNova Early Warning System (SNEWS). Triangulation simulations have recently seen tremendous success in determining where a supernova is positioned from its neutrinos, but these studies have made use of high statistics detectors such as HyperK, JUNO, and DUNE. This work im- plements six analytic techniques into the detectors HALO and HALO-1kT, with the intent of extracting a common reference time across all detectors to use for triangulation efforts. The common reference time chosen is the time of core-bounce (t0), as it is followed by a rapid rise in νe events within νe sensitive neutrino detectors. Our analysis made use of the SNOwGLoBES event rate calculator, which quantifies event rates from supernova neu- trino signals, which was then simulated through each detector’s Monte Carlo simulation code. Various supernova models were taken into consideration to account for systematic uncertainties between different mass progenitors, equations of state, etc. Our analysis determined that for HALO and HALO-1kT, a constant fraction discriminator (CFD) technique was optimal in extracting the time of core-bounce from the neutrino signal at close distances (< 3 kpc), while a negative log likelihood technique was optimal at further distances. At 1 kpc, HALO-1kT had a precision of 543 μs when using the CFD technique to extract t0, which falls within the precision required to triangulate effectively (< 1 ms, which HALO-1kT can obtain out to 3 kpc). With the intent of eventually implementing these techniques into all experiments involved in SNEWS 2.0, SNO+ was incorporated into our analysis in the later stages of this research. A preliminary exploration showed severely degraded performance in contrast to HALO-1kT, where the CFD technique could only obtain millisecond precision, not microsecond. Further analysis is encouraged.
sno+ background study: polonium on acrylic vessel surface and radon assay
(2023-01-18) Yu, Shengzhao
SNO+ is a 780 tonnes organic liquid scintillator neutrino detector located at Vale’s Creighton mine, Sudbury, ON. 2 km overburden of rock above helps to achieve the low cosmic radiation background level of SNO+. Meanwhile, radioactive material in the rock can decay and produce radiation in the region of interest for the search of 0νββ decay. SNO+ is looking for neutrinos at very low energy and thus it is crucial to have a low background environment. My thesis evaluates two kinds of background sources: 222Rn and 210Po. 222Rn is the progeny of 238U in the rock. The water and gas assays are used to monitor the 222Rn concentration in the surrounding cavity water and other parts of the experiment or other gas volumes in SNOLAB. The analysis of the SNO+ data helps to understand the 210Po activity on the internal surface of the detector’s acrylic vessel. The 222Rn level in the cavity water is below the target of 4.5 × 10−13 gU238/gH2O. The Rn levels in the LN2 plant and international dewar are at a 10−4 reduction factor compared to mine air. The 210Po background level in the internal AV is holding a relatively constant level of about 1800 events per second. Spatially, the 210Po backgrounds are more active at the equator and the belly plate regions. The estimated number of 210Pb atoms deposited on the AV inner surface is 1.84 × 1012 .
HALO-1kT prototype He-3 counters: background studies
(2023-02-09) Weima, Esther
The last supernova near our galaxy was in 1987. HALO-1kT will be a low background galactic Supernova detector. HALO- 1kT needs to have low backgrounds to detect a supernova on the far side of the galaxy. A big source of backgrounds is the planned 4.3 km of helium-3 proportional counters. My research tested the prototype proportional counters to ensure their backgrounds are low enough to avoid regular false positives. The first way of testing them was to take the 4 counters underground at SNOLAB to collect 3-4 months of data as well as a 2-day calibration run to see what the base background rate is. Two of the counters were then attached to electrostatic counters and counted to determine the background of the wall material. Those results showed emanation rates of 0.137 Hz to 0.811 Hz in the wall material. While that is clean, it is not clean enough to meet HALO-1kT background goals.
Evaluating 238U external background for SNO+ experiment using radon Aasays and 214Bi analysis
(2022-04-11) Hussain, Syed Muhammad Adil
SNO+ is large multipurpose detector located at SNOLAB filled with liquid scintillator. The scintillator will then be loaded with Te isotope, allowing to look for neutrino-less double beta decay which is extremely rare. This will determine if the neutrino is its own antiparticle. One of the main concerns for these rare event experiments is the presence of backgrounds, which could mask the signals of interest. This thesis will focus on 222Rn, one of the most common backgrounds due to its excessive prevalence in the mine environment. Radon decays into daughter nuclei where the energies lie within the region of interest for neutrino-less double beta decay. The detector is housed in a large cavity that is filled with ultrapure water and has a nitrogen cover-gas in order to avoid contamination. Radon Assay is a technique that was developed for the original SNO experiment to keep track of the radon content within the cavity and the covergas systems. The Assay system itself is well calibrated with low backgrounds. Assays are performed frequently at different positions of the cavity and cover-gas to monitor the radon levels. During a radon assay, radon is cryogenically trapped, concentrated, and shared into a ZnS coated Lucas cell for a period of time and known amount of flow. This Lucas cell is then connected to a PMT, which detects the decayed alphas that are used to calculate the number of radon atoms in the assay. This technique is a crucial part of measuring and monitoring the low backgrounds for the experiment which is then verified from the in-situ Bi214 analysis for accuracy.
Cellular and dosimetric characterization for biological studies of sub-natural background radiation
(2020-08-27) Kennedy, Konnor
Living things have evolved and grown in the presence of natural background radiation. Many biologic studies have been conducted with levels above natural background, but very few have examined sub background levels of radiation. A specialized tissue culture incubator (STCI) was constructed to shield against environmental radiation within SNOLAB. GEANT4 simulations of the STCI results showed a substantial decrease in dose rate compared to above ground and below ground incubator environments at 0.51 nGy/hr (for the STCI) vs 18.51 nGy/hr and 19.1 nGy/hr respectively, which equates to a 37.5 fold and 36.3 fold reduction in dose respectively. Compared to other sub background experimental dose rates (such as those conducted by Gran Sasso), the STCI is lower by a factor of 7.5. Neoplastic transformation assays were run above ground using the CGL1 hybridized cell line, both treated with aluminum and without. This experiment has laid out the protocol for future below ground assays in SNOLAB using the STCI. It has also led to interesting trends worth studying in future experimentation. Compared to the 0 mGy control, the 3000 mGy dose point was statistically significant (p-value <0.05) both with aluminum and without.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.

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