This work sets out to be a description of the initiatives utilizing the Borexino liquid scintillator neutrino observatory to perform the first direct, high-precision, wideband solar neutrino spectroscopy measurement of the the solar neutrino spectrum's main components, as well as its next-generation short-baseline source program (SOX). Its original scope revolved around the creation of a O(MCi) ⁵¹Cr source to be inserted under the detector, intended to explore the small region of the anomaly-favored sin²θ₁₄/Δm₁₄² phase space where sterile neutrinos may lie -or otherwise unambiguously measure or disprove signs of anomalous oscillatory behavior in low L/E electron-neutrinos and antineutrinos. Investigating the feasibility and optimization of producing such a large amount of ⁵¹Cr for the source, by irradiating chromium material in a high-flux reactor, required extensive simulative work with the MCNP-5 neutronics code.
With the switch of pace toward a ¹⁴Ce-¹⁴⁴Pr electron-antineutrino source, this work was re-oriented toward the efforts to re-calibrate the detector after the 2009-10 campaign, improving and expanding upon it by the introduction of new source fabrication techniques, a source positioning LED device, and a re-evaluation of the objectives sought after, fitting the needs of Borexino's Phase 2 priorities. Indeed, the detector's unprecedented and record-setting background levels are tightening its requirement for background stability. Aiming to reduce fluctuations in 210Po levels that remain problematic in Borexino's quest to lower the upper limit of the solar CNO neutrino flux (or even measure it), among other components, a new Temperature Monitoring and Management System was deployed and associated tools necessary to fully utilize it were developed as part of the present work. Computational Fluid Dynamics (CFD) simulations in 2D and 3D, conductive and fully convective, were also developed in collaboration with Dr Riccardo Mereu of Milan's Polytechnic Institute in order to model, characterize and ultimately predict the subtle fluid currents (around 10⁻⁷) m/s) that may be of concern for the required background stability. A brief discussion of the recent >5sigma measurement of geo-neutrinos by Borexino, a complementary part of the work for this thesis, is presented as well. / Ph. D. / Neutrinos are one of the few distinct probes we have to observe the large-scale features of the world around us. Together with photons, some charged leptons, and the dense conglomerates of quarks and gluons we call atomic nuclei, they are the only fundamental particles with large enough a range to travel from distant objects to reach us. In contrast with any of the other particles though, they are the only ones solely bound by one of the fundamental non-gravitational forces alone: the weak nuclear force. This makes them the most penetrating radiation currently known, but consequently the one whose detectable effects are the subtlest. Observing our environment through neutrinos, we transcend the boundaries imposed by electromagnetic interactions: most objects, even astronomical ones, are no longer opaque, but mostly transparent. Further, the only luminous objects (in neutrinos) are those where some nuclear reactions take place, which release them.
Being the only known particle with such properties, they can be used as an extraordinarily versatile tool to observe and understand Nature in its most fundamental and far-reaching realm –in addition to opening a whole new array of possible future practical uses, some closer to feasibility than others, that our early and still incomplete understanding of neutrinos barely scratches the surface of: from ultra-penetrating communication links to tomographies of whole planets, targeted quickened radioactive decay (for example, for medical purposes) or unshieldable monitoring of nuclear material for non-proliferation purposes.
The Borexino neutrino observatory is a detector located under the Gran Sasso massif in central Italy, whose objective is to precisely measure the neutrinos emitted in nuclear processes such as the nuclear fusion chains powering the our Sun. In fact, the main subject of study for Borexino is solar neutrinos, which travel mostly unimpeded at close to the speed of light from their generation areas inside the Sun to Earth. Three known types of neutrinos exist, defined depending on the way they interact with matter, producing other particles. These types intermix among themselves, as the neutrino propagates through space, and the behavior of this transmutation (known technically as <i>oscillation</i>) is influenced by areas with high density of matter, such as the Sun itself. The study of neutrino fluxes and their oscillation is a very recent topic of research, and one which offers a crucial handle on physical processes in Nature that we still do not know of, or do not fully understand: the so-called <i>Beyond Standard Model</i> processes. The Sun is the largest neutrino emitter at low energies we can detect from Earth, and the study of the precise amounts it produces is crucial to understand how it (and, by extension, other similar stars) work in detail. Furthermore, just like with light, the Sun produces neutrinos at different energies (akin to sunlight’s different colors, or wavelengths), depending on the nuclear reaction which produced them –and their relative contributions can be separated by Borexino. All solar neutrino components (except two very faint ones) have been directly observed by Borexino, many for the first time, with varying levels of precision, since it started operations in 2007, by disentangling against the backgrounds the very feeble contribution of the <200 neutrinos that leave a signal every day in Borexino.
Borexino’s unique sensitivity lies in its unprecedented and extreme radiopurity: the levels of radioactive elements in its innermost, most pristine materials it is composed of are much below typical natural values, and have been steadily improving for the last 10 years, in some cases to record-setting lows. The data accumulation since 2012, when a purification campaign brought down the background levels dramatically, has yielded extremely high-quality datasets. However, since Borexino’s active material is liquid, small temperature upsets in the detector’s environment have caused fluid shifts that brought less radiopure material into the cleanest area. This dissertation details the work devoted to monitoring, managing, stabilizing and improving Borexino’s temperatures with the aim to reduce background fluctuations that are obstructing efforts to measure the CNO solar neutrino component, which has never been observed before, having a small contribution in our Sun, but holds the key to understanding how many other stars work (in particular, larger ones, where the CNO process is much more dominant), as well as having profound implications on our own.
The insulation of Borexino’s exterior, paired with the precise determination of its exterior and interior temperatures, has also enabled the development of Computational Fluid Dynamics (CFD) simulations that shed light into how the fluids inside Borexino respond to recorded past temperature changes, or to possible thermal distributions in the future –which is crucial to limit the penetration of backgrounds that may hinder a CNO measurement, or the improvement in the precision with which other previously-detected solar neutrino components can be measured. Furthermore, in order to reach such accurate results, a careful calibration of the detector is needed, so that its signals can be correctly interpreted: this dissertation explains the upgrade and improvement work carried out in order to perform a new calibration campaign in 2017, after the very successful one completed at the beginning of Borexino’s life in 2009-10. Additionally, this dissertation explains the latest measurement, to a high statistical precision, of other naturallyoccurring type of neutrinos: geo-neutrinos, that is, those emitted by radioactive components inside Earth, which allow us to understand the composition, evolution and thermal power output of our planet.
Finally, this dissertation details the simulation work performed for the SOX-A experiment, which intends to utilize a man-made neutrino source, located in a tunnel under the detector, as a way to understand these particles’ oscillatory behavior, which may have shown characteristics we do not yet understand in the past. One of such hypothesis is the existence of practically undetectable, <i>sterile</i> neutrinos, into which the three known types can oscillate –effectively making a given neutrino flux appear weaker than expected. The SOX source would serve as a reference "candle" which could be probed for deficits in the flux of the known neutrinos (those Borexino can detect) as a smoking gun for the existence of others. In particular, the creation in ORNL’s HFIR of a high-intensity neutrino source based on radioactive <sup>51</sup>Cr is discussed here. Confirming or disproving the existence of these type of unknown oscillatory behavior is critical for the understanding the core framework of Particle Physics, as well as expand its frontiers to unexplored areas we know exist, but have not yet been probed.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/73581 |
| Date | 06 December 2016 |
| Creators | Bravo Berguno, David |
| Contributors | Physics, Vogelaar, R. Bruce, Pitt, Mark L., Sharpe, Eric R., Heremans, Jean J., Takeuchi, Tatsu |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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