Department of Physics

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This digital collection includes theses, dissertations, and faculty publications (specifically open educational resource videos) from the Department of Physics.

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Open Access
A framework for simultaneous photon detector readout system simulations for the deep underground neutrino experiment
(Colorado State University. Libraries, 2020) Christensen, Anne R., author; Buchanan, Norm, advisor; Ross, Kate, committee member; Bangerth, Wolfgang, committee member
This thesis will discuss the changes to the coding framework for the Deep Underground Neutrino Experiment (DUNE). DUNE is simulated in a coding framework, called Liquid Argon Software (LArSoft). The framework simulates the particle event, the photons produced due to interactions and the electronics. The electronic simulation framework for DUNE has been changed to improve functionality and ease of use. The electronics simulation has been modularized so electronic readout models can be directly compared. The changes to the framework will be described and validated in this thesis.
Open Access
A measurement of muon neutrino charged-current interactions with a charged pion in the final state using the NOνA near detector
(Colorado State University. Libraries, 2023) Rojas, Paul Nelson, author; Buchanan, Norm, advisor; Lee, Siu Au, committee member; Harton, John, committee member; Kokoszka, Piotr, committee member
The NOνA experiment is a long-baseline neutrino experiment hosted by Fermilab. The intense NuMI neutrino beam, combined with NOνA Near Detector, provides the opportunity to study neutrino interactions at an unprecedented level. The goal of this analysis is to measure the rate of muon-neutrino charged-current interactions in the NOνA near detector resulting in the production of one muon and at least one charged pion. This thesis will present the result of the double differential cross section measurement of this process in muon kinematics of energy and angle. Excesses in the extracted signal (greater than 25%), relative to the simulation, were found at large scattering angles. These excesses were greater than the estimated uncertainties (∼15%).
Embargo
A measurement of the double-differential electron antineutrino charged-current inclusive cross section in the NOvA near detector
(Colorado State University. Libraries, 2023) Doyle, Derek, author; Buchanan, Norm, advisor; Gelfand, Martin, committee member; Harton, John, committee member; Norman, Andrew, committee member; Pouchet, Louis-Noël, committee member
The neutrino is a fundamental particle of the universe that was first hypothesized in 1930 by Wolfgang Pauli to explain the observed energy distribution of outgoing electrons produced from beta-decay. Since then, it has been discovered that there are at least three types, or flavors, of neutrinos and that they oscillate between these flavors as they travel through space and time. This discovery proved that neutrinos have a non-zero mass and positioned neutrino oscillations to provide a window into understanding the matter/antimatter asymmetry in the universe. Principle to all neutrino measurements is an accurate and robust interaction model over a large range of energies, and measurements to support the model. Of particular importance to the NuMI Off-axis νe Appearance (NOvA) neutrino oscillation experiment is the energy range from 1 to 10 GeV, where Quasi-Elastic (QE), Meson-Exchange Current (MEC), and Deep Inelastic Scattering (DIS) interactions all contribute significantly. Using neutrinos from the Neutrinos at the Main Injector (NuMI) beam and the NOvA near detector, the first double-differential electron antineutrino charged-current inclusive cross section is measured and compared to various interaction models implemented within the genie Generator framework, version 3. Good agreement is observed between measurement and a genie model tuned to NOvA data.
Open Access
A new measurement of the 2S1/2-8D5/2 transition in atomic hydrogen
(Colorado State University. Libraries, 2021) Brandt, Adam D., author; Yost, Dylan C., advisor; Yalin, Azer P., committee member; Roberts, Jacob L., committee member; Field, Stuart B., committee member
High-precision spectroscopy of simple atoms provides input data that can be used to extract fundamental constants and to test Standard Model theory. Hydrogen, the simplest element, has played a historically significant role in the development of fundamental theory and, more recently, provides important data for the proton radius puzzle. In this thesis, we will describe a new measurement of the 2S1/2-8D5/2 transition on a cryogenic hydrogen beam. We will overview the measurement scheme and experimental apparatus, then present analysis and systematic characterization important to the spectroscopy. Finally, we will present our preliminary determination of the proton radius and the Rydberg constant using our value for the 2S1/2-8D5/2 combined with the previously measured 1S-2S transition.
Open Access
A search for Lorentz and CPT violation in the neutrino sector of the standard model extension using the near detectors of the Tokai to Kamioka neutrino oscillation experiment
(Colorado State University. Libraries, 2016) Clifton, Gary Alexander, author; Toki, Walter, advisor; Berger, Bruce, committee member; Eykholt, Richard, committee member; Hulpke, Alexander, committee member
The Tokai to Kamioka (T2K) neutrino experiment is designed to search for electron neutrino appearance oscillations and muon neutrino disappearance oscillations. While the main physics goals of T2K fall into conventional physics, T2K may be used to search for more exotic physics. One exotic physics analysis that can be performed is a search for Lorentz and CPT symmetry violation (LV and CPTV) through short baseline neutrino oscillations. The theoretical framework which describes these phenomena is the Standard Model Extension (SME). Due to its off-axis nature, T2K has two near detectors. A search for LV and CPTV is performed in each detector. The search utilizes charged-current inclusive (CC inclusive) neutrino events to search for sidereal variations in the neutrino event rate at each detector. Two methods are developed; the first being a Fast Fourier Transform method to perform a hypothesis test of the data with a set of 10,000 toy Monte-Carlo simulations that do not have any LV signal in them. The second is a binned likelihood fit. Using three data sets, both analysis methods are consistent with no sidereal variations. One set of data is used to calculate upper limits on combinations of the SME coefficients while the other two are used to constrain the SME coefficients directly. Despite not seeing any indication of LV in the T2K near detectors, the upper limits provided are useful for the theoretical field to continue improving theories which include LV and CPTV.
Open Access
A spectral analysis of the Crab nebula and other sources with HAWC
(Colorado State University. Libraries, 2016) Gussert, Michael, author; Harton, John, advisor; Mostafa, Miguel, advisor; Toki, Walter, committee member; Anderson, Chuck, committee member; Gelfand, Martin P., committee member
The High Altitude Water Cherenkov observatory (HAWC) is an extensive air shower particle detection array designed to study cosmic gamma (γ) rays in the Very High Energy (VHE) regime (100 GeV to 100 TeV). One of the most thoroughly studied sources in this energy range is the Crab nebula, a pulsar wind nebula created by the aftermath of supernova 1054. The core of this analysis revolves around the determination of the differential flux spectrum of the Crab nebula using a process known as forward folding. Forward folding allows energy spectra to be fit without requiring a direct measurement of the primary energy of individual extensive air showers. The energy resolution of HAWC is very poor (on the order of 50% or more), and so this method is ideal for any spectral analysis carried out with HAWC data. The differential spectra are modeled as a power law with a normalization (Φ0), spectral index (γ), and a cutoff energy (Ec): dN/dE = Φ0(E/E0)γe−E/Ec . The normalization of the Crab nebula was found to be 1.03±0.091 0.083 stat ±0.19 sys)×10−12(TeV−1 cm−2 s −1 ) with an index of −2.54 ± 0.095 stat ± 0.27 sys and a cutoff of 91.0 ±174 59 stat with E0 =4.0 TeV. This method was also applied to 11 other sources, and the minimum detection significance required to constrain a spectrum was found to be between 10 and 14 σ.
Open Access
A study of simulated neutrons in the NOνA near detector
(Colorado State University. Libraries, 2020) Jarosz, Jessica, author; Buchanan, Norm, advisor; Wilson, Robert, committee member; Eilertson, Kirsten, committee member
This thesis explores how neutron interactions can be studied in the NOνA near detector, and the potential use of a deuterium-tritium neutron source. Understanding neutron kinematics within the near detector could aid in constraining antineutrino properties in charged-current quasi-elastic interactions. Refining our knowledge of such an interaction decreases systematic uncertainties, which is crucial for precise neutrino oscillation measurements. Monte-Carlo simulations of mono- energetic neutrons were performed to examine energy deposition, scattering, and neutron energy loss mechanisms.
Open Access
Accelerating NOvA's Feldman-Cousins procedure using high performance computing platforms
(Colorado State University. Libraries, 2019) Doyle, Derek, author; Buchanan, Norm, advisor; Harton, John, committee member; Pouchet, Loius-Noël, committee member
In order to assess the compatibility between models containing physically constrained parameters and small-signal data, uncertainties often must be calculated by Monte Carlo simulation to account for non-normally distributed errors. This is the case for neutrino oscillation experiments, where neutrino-matter weak interactions are rare and beam intensity at the far site is low. The NuMI Off-axis νe Appearance (NOvA) collaboration attempts to measure the parameters governing neutrino oscillations within the PMNS oscillation model by comparing model predictions to a small data set of neutrino interactions. To account for non-normality, NOvA uses the computationally intensive Feldman-Cousins (FC) procedure, which involves fitting thousands of independent pseudoexperiments to generate empirical distribution functions that are used to calculate the significance of observations. I, along with collaborators on NOvA and Scientific Discovery through Advanced Computing: High Energy Physics Data Analytics (SciDAC-4) collaborations, have implemented the FC procedure utilizing the High Performance Computing (HPC) facilities at the National Energy Research Scientific Computing Center (NERSC). With this implementation, we have successfully processed NOvA's complete FC corrections for our recent neutrino + antineutrino appearance analysis in 36 hours: a speedup factor of 50 as compared to the methods used in previous analyses.
Open Access
Advancing the capability of high energy Yb:YAG lasers: multilayer coatings, pulse shaping and post compression
(Colorado State University. Libraries, 2022) Wang, Hanchen, author; Rocca, Jorge, advisor; Roberts, Jacob, committee member; Lee, Siu Au, committee member; Marconi, Mario, committee member
Recently, cryogenically cooled Yb-doped amplifiers have been successfully scaled both in output energy and in repetition rate. The performance of such laser systems and their applications can be expanded by advancements in the development of optical coatings, that allow for scaling to higher pulse energies; as well as improvements in pulse shaping that include shorter pulse durations and the generation of programmable sequences of ultrashort pulses. This dissertation focuses on realizing the improvements mentioned above for cryogenic Yb:YAG amplifiers. First it reports the evaluation of ion beam sputtering (IBS) dielectric coatings for Yb:YAG at the environmental conditions in which cryogenic amplifiers are operated. The IBS coatings showed consistent performance in ambient, vacuum and cryogenic conditions, with damage threshold measured 20.4±0.6 J/cm2 for anti-reflection (AR) coating, and 27.4±1.3 J/cm2 for high reflector (HR) coating with 280 ps pulse duration at 77 K under the ISO:21254 standard. Second, a method for synthesizing trains of high energy compressed pulses was demonstrated and used to pump an 18.9 nm Ni-like Mo plasma-based soft x-ray laser more efficiently. The synthesized pulse increased the conversion efficiency of this spatially coherent soft x-ray source by 40%. Finally, femtosecond pulses were generated by post compression using a gas filled hollow core fiber (HCF), in which spectral broadening was achieved by self-phase modulation with an additional contribution from stimulated Raman scattering. Utilizing nitrogen gas as the non-liner medium, 300 mJ, 8 ps pulses were broadened to 3.7 nm and re-compressed to 460 fs by a grating compressor. The propagation and spectral broadening of high energy picosecond pulses in gas-filled HCFs were modeled and the results of simulations were compared with experiments.
Open Access
An analysis of noise in the NOvA near detector
(Colorado State University. Libraries, 2016) Judah, Matthew, author; Buchanan, Norm, advisor; Ross, Kate, committee member; Pallickara, Sangmi, committee member
The NOvA (NuMI Off-axis νe Appearance) long-baseline neutrino experiment utilizes neutrino oscillations to gain insight into the unanswered questions in neutrino physics and further our knowledge of particle physics. The answers can only be arrived at through precise and accurate measurements of neutrino properties. To obtain these high precision results using the NOvA experiment background signals and noise must be understood and characterized. The work described is a study of noise characteristics from the electronics and photosensors used in the near detector of the NOvA experiment. A number of methods for the identification and elimination of noise in the electronics are discussed.
Open Access
Analysis of impact of non-uniformities on thin-film solar cells and modules with 2-D simulations
(Colorado State University. Libraries, 2010) Koishiyev, Galymzhan Temirkhanovich, author; Sites, James R., advisor; Lear, Kevin L., committee member; Leisure, Robert Glenn, 1938-, committee member; Harton, John L., committee member
Clean and environmentally friendly photovoltaic (PV) technologies are now generally recognized as an alternative solution to many global-scale problems such as energy demand, pollution, and environment safety. The cost ($/kWh) is the primary challenge for all PV technologies. In that respect, thin-film polycrystalline PV technology (CdTe, Cu(In,Ga)Se2, etc), due to its fast production line, large area panels and low material usage, is one of the most promising low-cost technologies. Due to their granular structure, thin-film solar cells are inherently non-uniform. Also, inevitable fluctuations during the multistep deposition process of large area thin-film solar panels and specific manufacturing procedures such as scribing result in non-uniformities. Furthermore, non-uniformities can occur, become more severe, or increase in size during the solar-panel's life cycle due to various environmental conditions (i.e. temperature variation, shading, hail impact, etc). Non-uniformities generally reduce the overall efficiency of solar cells and modules, and their effects therefore need to be well understood. This thesis focuses on the analysis of the effect of non-uniformities on small size solar cells and modules with the help of numerical simulations. Even though the 2-D model developed here can analyze the effect of non-uniformities of any nature, only two specific types of microscopic non-uniformities were addressed here: shunts and weak-diodes. One type of macroscopic non-uniformity, partial shading, was also addressed. The circuit model developed here is a network of diodes, current-sources, and transparent-conductive-oxide (TCO) resistors. An analytic relation between the TCO-resistor, which is the primary model parameter, and TCO sheet resistance ρS, which is the corresponding physical parameter, was derived. Based on the model several useful general results regarding a uniform cell were deduced. In particular, a global parameter δ which determines the performance of a uniform solar cell depending on sheet resistance ρS, cell length L, and other basic parameters, was found. The expression for the lumped series resistance in terms of physical parameters was also derived. Primary power loss mechanisms in the uniform case and their dependence on ρS, L, and light generated current JL were determined. Similarly, power losses in a small-area solar cell with either a shunt or a weak-microdiode were identified and their dependence on ρS, JL, and location of the non-uniformity with respect to the current collecting contact was studied. The impact of multiple identical non-uniformities (shunts or weak-diodes) on the performance of a module was analyzed and estimates of efficiency loss were presented. It was found that the efficiency of the module strongly depends not only on the severity and fractional area of non-uniformities but also on their distribution pattern. A numerical parameter characterizing distribution pattern of non-uniformities was introduced. The most and least favorable distribution patterns of shunts and weak-diodes over the module area were determined. Experimentally, non-uniformities may be detected with the help of spatially resolved measurements such as electroluminescence (EL). The 2-D circuit model was also used to develop the general framework to extract useful information from experimental EL data. In particular, a protocol that can help distinguish a shunt from a weak-diode and estimate the severity of the non-uniformity based on the EL data was developed. Parts of these simulation results were verified with experimental EL data obtained by other authors. The thesis also discusses the effect of partial shading (a macroscopic non-uniformity) on the operation and safety of thin-film solar panels. A detailed analysis of the current-voltage characteristics of partially shaded module was performed. Conditions that result in a shaded cell experiencing high reverse voltage were shown. A mathematical formalism was developed to distinguish two extremes: when reverse-bias shunting or breakdown dominates. It was shown that in the shunt-dominated case in extreme situations the voltage across the shaded cell can be quite large (~ 20V). High voltage across the shaded cell results in both high power dissipation and elevated temperature. Depending on the light generated current, the temperature above ambient of the shaded cell can be as high as ~100-300°C, implying potential safety issues. The analysis covered all basic rectangular shade configurations.
Open Access
Barium extraction from liquid xenon on a cryoprobe for the nEXO experiment and a nucleon decay search using EXO-200 data
(Colorado State University. Libraries, 2019) Craycraft, Adam B., author; Fairbank, William M., Jr., advisor; Roberts, Jacob, committee member; Wilson, Robert, committee member; Johnson, Thomas E., committee member
Neutrinoless double beta decay (0νββ) is a theorized decay that is beyond the standard model of particle physics. Observation of this decay would establish the Majorana nature of neutrinos and show violation of lepton number. Nucleon decay is another theorized decay that is beyond the standard model of particle physics that would violate baryon number. Observation of baryon number violation has been pursued for sometime in a wide variety of experiments. EXO-200 is an experiment that utilized a time projection chamber (TPC) filled with liquid xenon (LXe) enriched in the isotope xenon-136 to search for 0νββ. In this thesis, an analysis of EXO-200 data in search of evidence for triple-nucleon decays in ¹³⁶Xe is presented. Decay of ¹³⁶Xe to ¹³³Sb and decay to ¹³³Te were the particular decays searched for in this analysis. No evidence for either decay was found. Limits on the lifetimes of these decays were set that exceed all prior limits. The proposed nEXO experiment will be next generation LXe TPC search for 0νββ. In order to eliminate background events that are not associated with two neutrino double beta decay, a technique to tag the barium-136 decay daughter is under development. In this thesis, continued development is presented of a scheme to freeze the barium daughter in a solid xenon sample on the end of a cryoprobe dipped into LXe and subsequently tag it using its fluorescence in the solid matrix.
Open Access
Barium tagging in solid xenon for the EXO experiment
(Colorado State University. Libraries, 2011) Mong, Brian, author; Fairbank, William, Jr., advisor; Lundeen, Stephen, committee member; Berger, Bruce, committee member; Van Orden, Alan, committee member
Neutrinoless double beta decay experiments are searching for rare decay modes never before observed to uncover the absolute mass of the neutrino, as well as to discover if it is a Majorana fermion. Detection of the daughter nucleus can help provide positive identification of this event over most radioactive backgrounds. The goal of the Enriched Xenon Observatory (EXO) is to measure the rate of 0νββ decay in 136Xe, incorporating 136Ba daughter identification by laser induced fluorescence spectroscopy. Here, we investigate a technique in which the 136Ba daughter is grabbed with a cryogenic probe by freezing it in solid xenon ice, and detected directly in the solid xenon. The absorption and fluorescence spectra of barium in solid xenon were observed for the first time in this work. Identification of the 6s2 1S0 → 6s6p 1P1 transition in both absorption (558 nm) and emission spectra (594 nm) were made. Additional blue absorption and emission lines were observed, but their transitions were not identified. Saturation of the 6s2 1S0 → 6s6p 1P1 transition was not observed with increased excitation rates using resonance excitation at 558 nm. From this a limit on the metastable decay rate was deduced to be greater than 104 s-1. Finally a fluorescence spectrum was obtained from a sample with only 20,000 atoms in the laser beam. With potential improvements of 107 in detection efficiency, single barium atom detection seems possible in solid xenon. A fiber probe detector based on a bare single mode fiber was also constructed and tested with fluorescing dye molecules. Successful detection of a few dye molecules in solution at the probe tip was demonstrated.
Open Access
Brillouin light scattering spectroscopy of phonons, magnons, and magnetoelastic waves
(Colorado State University. Libraries, 2022) Nygren, Katherine Elise, author; Buchanan, Kristen S., advisor; Field, Stuart, committee member; Brewer, Samuel, committee member; Shores, Matthew, committee member
This thesis discusses three projects that involve the propagation of waves through the utilization of an optical measurement technique known as Brillouin light scattering (BLS) spectroscopy. BLS spectroscopy measurements were completed using a six pass tandem Fabry-Pérot interferometer to detect light that has inelastically scattered from vibrational, spin, or magnetoelastic waves in a sample. This measurement method is noncontact, so wires do not need to be connected to the sample, nondamaging (unless the laser power is too high, and only for sensitive samples), and can detect nonlinear signals. The first project uses an antenna called an interdigital transducer to produce a surface acoustic wave. This wave travels across a piezoelectric substrate and couples to a spin wave in a nickel film. The coupled wave known as a magnetoelastic wave is then studied using BLS as a function of the external applied magnetic field. These results are used to help us understand how the magnetic resonance contributes to the coupled wave. Further BLS measurements as a function of distance across the nickel film are used to calculate a decay length of the magnetoelastic wave two orders of magnitude larger than the decay length for a pure spin wave in nickel. Second, we explore a device using a thin film of an organic ferrimagnet called vanadium tetracyanoethylene (VTCNE) that is magnetic at room temperature and has low damping, which rivals damping in high quality YIG films commonly used in microwave applications. Because VTCNE is oxygen sensitive it is encapsulated between two pieces of glass using an epoxy. The encapsulation does not change the damping, however due to magnetostriction, the strain of the epoxy may change the magnetic properties of the film. To understand how the epoxy strain can effect this device and others with similar encapsulation, we study thermal phonons in the encapsulation materials using Brillouin light scattering. The thermal phonon measurements along with phonon simulations allow us to calculate both the wave speeds and the elastic properties of the materials. These calculated properties can then be used to model future VTCNE devices. The final major project uses BLS spectroscopy to study spin waves in a Y-shaped structure of an iron nickel alloy. Using an in-plane externally applied magnetic field and an antenna across the top of the Y, we excite magnons in each arm of the Y, which then propagate into the base of the Y. BLS measurements are taken in each arm and the base of the Y, as a function of the driving frequency, and a 2D spatial map of the spin waves in the Y-structure was obtained to gain additional information on the modes that propagate past the junction of the Y. The BLS data in conjunction with simulations, demonstrate an indirect way to efficiently excite Damon-Eshbach spin waves as well as convert low wavevector spin waves in the arms of the Y into higher wavevector spin waves as they propagate into the base of the Y. The wavevector conversion and more efficient method of generating Damon-Eshbach spin waves are tools that can be exploited in magnonic device designs. Three additional spin wave projects are also discussed briefly. The projects include a yttrium iron garnet (YIG) confined structure, a VO2 film with a metal-insulator-transition near room temperature, and a heavy metal-ferrimagnet-heavy metal sample that should have a strong interfacial Dzyaloshinskii-Moriya interaction.
Open Access
Brillouin light scattering study of linear and nonlinear spin waves in continuous and patterned magnetic thin films
(Colorado State University. Libraries, 2014) Liu, Hau-Jian Jason, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Neilson, James R., committee member
This thesis focuses on the use of the Brillouin light scattering (BLS) technique to measure spin waves or magnons in thin films. BLS is an experimental technique that measures the inelastically scattered light from photon-magnon interactions. Broadly, three different experiments are presented in this thesis: the measurements of spin wave properties in iron cobalt (FeCo), yttrium iron garnet (YIG), and microstructures involving Permalloy (Ni80Fe20) and cobalt nickel (CoNi). First, conventional backward scattering BLS was used to measure the spin waves in a set of Fe65Co35 films that were provided by Seagate Technologies. By fitting the spin wave frequencies that were measured as a function of the external magnetic field and film thickness, the quantum mechanical parameter responsible for short range order, known as the exchange parameter, was determined. Second, nonlinear spin waves were measured in YIG using conventional forward scattering BLS with time resolution. Two nonlinear three wave processes were observed, namely, the three magnon splitting and confluence. The nonlinear power threshold, the saturation magnetization, and the film thickness were determined independently using network analyzer measurements. The spin wave group velocities were determined from the space- and time-resolved BLS data and compared to calculations from the dispersion relations. Back calculations showed the location where the three magnon splitting process took place. Lastly, spin waves in Permalloy and CoNi microstrips were measured using a recently developed micro-BLS. The micro-BLS, with a spatial resolution of 250 nm, allows for measuring the effects on the lateral confinement of spin waves in microstrips. The confinement of spin waves led to modifications to the dispersion relations, which were compared against the spin wave frequencies obtained from the micro-BLS. The Permalloy experiments shows non-reciprocity in surface spin wave modes with opposite wavevectors and provides a quantitative measure of the difference in excitation efficiency between the surface spin wave and the backward volume spin wave modes. Measurements were also conducted in the Permalloy microstrips at zero external magnetic field, showing evidence that propagating spin waves can be observed by exploiting the effects of shape anisotropy. Finally, preliminary measurements were done on CoNi microstrips with perpendicular anisotropy. A magnetic signal was detected, however further investigation will be needed to determine the exact origin of the observed signal and to definitively answer the question as to whether or not BLS can be used to measure spin waves in perpendicularly magnetized films. Overall, the experiments and results presented in this thesis show that BLS is a useful tool for measuring spin wave properties in magnetic thin films.
Open Access
Calibration of the Pierre Auger Observatory fluorescence detectors and the effect on measurements
(Colorado State University. Libraries, 2015) Gookin, Ben, author; Harton, John, advisor; Toki, Walter, committee member; Buchanan, Kristen, committee member; Menoni, Carmen, committee member
The Pierre Auger Observatory is a high-energy cosmic ray observatory located in Malargue, Mendoza, Argentina. It is used to probe the highest energy particles in the Universe, with energies greater than 10¹⁸ eV, which strike the Earth constantly. The observatory uses two techniques to observe the air shower initiated by a cosmic ray: a surface detector composed of an array of more than 1600 water Cherenkov tanks covering 3000 km², and 27 nitrogen fluorescence telescopes overlooking this array. The Cherenkov detectors run all the time and therefore have high statistics on the air showers. The fluorescence detectors run only on clear moonless nights, but observe the longitudinal development of the air shower and make a calorimetric measure of its energy. The energy measurement from the the fluorescence detectors is used to cross calibrate the surface detectors, and makes the measurements made by the Auger Observatory surface detector highly model-independent. The calibration of the fluorescence detectors is then of the utmost importance to the measurements of the Observatory. Described here are the methods of the absolute and multi-wavelength calibration of the fluorescence detectors, and improvements in each leading to a reduction in calibration uncertainties to 4% and 3.5%, respectively. Also presented here are the effects of introducing a new, and more detailed, multi-wavelength calibration on the fluorescence detector energy estimation and the depth of the air shower maximum measurement, leading to a change of 1±0.03% in the absolute energy scale at 10¹⁸ eV, and a negligible change in the measurement on shower maximum.
Open Access
Comparative analysis of Cu(InGa)Se2 solar cells
(Colorado State University. Libraries, 2016) Counts, Kahl, author; Sites, James, advisor; Sampath, W. S., committee member; de la Venta, Jose, committee member
Cu(InGa)Se2, often abbreviated CIGS, photovoltaics have proven to be a commercially viable solar-energy conversion technology. Diverse processes have been employed in the manufacture, with varying end products, most resulting in high efficiency. A collaborative project was undertaken with several CIGS labs and industrial partners to explore the different electrical and spatial characteristics of CIGS solar cells relative to one another. Characterization methods utilized include, current-voltage measurements, quantum efficiency, capacitance-frequency and capacitance-voltage, electroluminescence, light-beam-induced current and Auger profling. Specific parameters for each cell were extracted from the measurements. Together the methods used are a tool for understanding device performance and optimization. Efforts were made to identify strengths, similarities and differences and to connect processing details with observed characteristics.
Open Access
Computational modeling of low-density ultracold plasmas
(Colorado State University. Libraries, 2017) Witte, Craig, author; Roberts, Jacob L., advisor; Eykholt, Richard, committee member; Kruger, David, committee member; Sambur, Justin, committee member
In this dissertation I describe a number of different computational investigations which I have undertaken during my time at Colorado State University. Perhaps the most significant of my accomplishments was the development of a general molecular dynamic model that simulates a wide variety of physical phenomena in ultracold plasmas (UCPs). This model formed the basis of most of the numerical investigations discussed in this thesis. The model utilized the massively parallel architecture of GPUs to achieve significant computing speed increases (up to 2 orders of magnitude) above traditional single core computing. This increased computing power allowed for each particle in an actual UCP experimental system to be explicitly modeled in simulations. By using this model, I was able to undertake a number of theoretical investigations into ultracold plasma systems. Chief among these was our lab's investigation of electron center-of-mass damping, in which the molecular dynamics model was an essential tool in interpreting the results of the experiment. Originally, it was assumed that this damping would solely be a function of electron-ion collisions. However, the model was able to identify an additional collisionless damping mechanism that was determined to be significant in the first iteration of our experiment. To mitigate this collisionless damping, the model was used to find a new parameter range where this mechanism was negligible. In this new parameter range, the model was an integral part in verifying the achievement of a record low measured UCP electron temperature of 1.57 ± 0.28K and a record high electron strong coupling parameter, Γ, of 0.35 ± 0.08. Additionally, the model, along with experimental measurements, was used to verify the breakdown of the standard weak coupling approximation for Coulomb collisions. The general molecular dynamics model was also used in other contexts. These included the modeling of both the formation process of ultracold plasmas and the thermalization of the electron component of an ultracold plasma. Our modeling of UCP formation is still in its infancy, and there is still much outstanding work. However, we have already discovered a previously unreported electron heating mechanism that arises from an external electric field being applied during UCP formation. Thermalization modeling showed that the ion density distribution plays a role in the thermalization of electrons in ultracold plasma, a consideration not typically included in plasma modeling. A Gaussian ion density distribution was shown to lead to a slightly faster electron thermalization rate than an equivalent uniform ion density distribution as a result of collisionless effects. Three distinct phases of UCP electron thermalization during formation were identified. Finally, the dissertation will describe additional computational investigations that preceded the general molecular dynamics model. These include simulations of ultracold plasma ion expansion driven by non-neutrality, as well as an investigation into electron evaporation. To test the effects of non-neutrality on ion expansion, a numerical model was developed that used the King model of the electron to describe the electron distribution for an arbitrary charge imbalance. The model found that increased non-neutrality of the plasma led to the rapid expansion of ions on the plasma exterior, which in turn led to a sharp ion cliff-like spatial structure. Additionally, this rapid expansion led to additional cooling of the electron component of the plasma. The evaporation modeling was used to test the underlying assumptions of previously developed analytical expression for charged particle evaporation. The model used Monte Carlo techniques to simulate the collisions and the evaporation process. The model found that neither of the underlying assumption of the charged particle evaporation expressions held true for typical ultracold plasma parameters and provides a route for computations in spite of the breakdown of these two typical assumptions.
Open Access
Constraints on the galactic magnetic field with two-point cumulative autocorrelation function
(Colorado State University. Libraries, 2012) Petrov, Yevgeniy, author; Harton, John L., advisor; Mostafá, Miguel A., committee member; Berger, Bruce, committee member; Burns, Patrick J., committee member
The fact that ultra high energy cosmic rays are charged particles complicates identication of their sources due to deflections by the intervening cosmic magnetic fields. The information about the fields is encoded in the amount of deflection experienced by a charged particle. Unfortunately, the positions of sources are unknown as is the structure of the magnetic field. However, it is possible to deduce the most favorable galactic magnetic field by examining the parameter space of different models of the galactic magnetic field. The method presented in this work is valid under some plausible assumptions, such as extragalactic origin of the UHECR, pure protonic composition above 50 EeV and sufficiently weak randomly oriented galactic and extragalactic components of the magnetic field. I use a two point cumulative autocorrelation function combined with the backtracking method to find regions in the parameter space that are compatible with statistically significant clustering on the extragalactic sky. This approach is independent of any catalog of sources. The ratio between the number of pairs within a certain angular window at the Earth sky and at the extragalactic sky after backtracking serves to indicate focusing or de-focusing properties of a particular field configuration. The results suggest that among several tested fields, the Harari-Mollerach-Roulet model with a bi-symmetric spiral and even vertical symmetry favors clustering of arrival directions at the extragalactic sky with the probability of 2.5% being from an isotropic distribution. Addition of the toroidal halo field improves clustering for the Harari-Mollerach-Roulet field for both bi-symmetric and axisymmetric spirals with even vertical symmetry, and the isotropic probabilities are 2.5% and 5.3% correspondingly. The bi-symmetric and axisymmetric spirals with odd vertical symmetry are disfavored, as well as the models with annular structure.
Open Access
Cosmogenic background rejection for the sterile neutrino search with the Short-Baseline Neutrino Program far detector
(Colorado State University. Libraries, 2020) Hilgenberg, Christopher M., author; Wilson, Robert J., advisor; Harton, John, committee member; Toki, Walter, committee member; Zhu, Wen, committee member
As the Short-Baseline Neutrino Program Far Detector, the ICARUS liquid argon time-projection chamber will operate at shallow depth and therefore be exposed to the full surface flux of cosmic rays. This poses a problematic background to the sterile neutrino search. A direct way to reject this background is to surround the cryostat with a detector capable of tagging incident cosmic muons with high efficiency, the Cosmic Ray Tagging System (CRT). I have worked to develop the CRT detector hardware and the simulation and reconstruction software. This system, currently undergoing installation, is approximately 25% commissioned and taking data. I have developed a powerful method for cosmogenic background rejection utilizing the CRT and Photon Detection System. Results from a simulation based study, informed by CRT data, demonstrate the power of the technique in rejecting cosmogenic events with little adverse impact on the neutrino sample.