Precision Background Stability and Response Calibration in Borexino: Prospects for Wideband, Precision Solar Neutrino Spectroscopy and BSM Neutrino Oscillometry Through a Deeper Detector Understanding

Autor: Bravo Berguno, David
Rok vydání: 2016
Předmět:
Druh dokumentu: Dissertation
Popis: 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 oscillation) 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 Beyond Standard Model 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 sterile 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 51Cr 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.
Databáze: Networked Digital Library of Theses & Dissertations
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