Heidelberg, Univ., Diss., 2005. / Computerdatei im Fernzugriff.

München, Techn. Univ., Diss., 2005. / Computerdatei im Fernzugriff.

Techn. University, Diss., 2005--München.

Techn. University, Diss., 2006--München.

München, Techn. University, Diss., 2005.

University, Diss., 2005--Heidelberg.

Universiẗat, Diss., 2003--Heidelberg.

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.

Le détecteur Borexino, situé au laboratoire souterrain du Gran Sasso (LNGS), mesure les neutrinos solaires depuis 10 ans. Parmi les neutrinos solaires, le spectre continu du 8B jusqu’à 17 MeV permet de tester la zone de transition de l’effet de résonance dans la matière dit effet MSW. Cette nouvelle analyse augmente d’un ordre de grandeur la statistique par rapport à la précédente mesure de Borexino publiée en 2011. Pour ce faire, l’ensemble du volume scintillant a été inclus dans l’analyse, aucune coupure géométrique n’ayant été effectué au dessus de 5 MeV. Cela a permis l’identification d’un nouveau bruit de fond non pris en compte précédemment. L’ensemble des bruits de fond au dessus de 3 MeV est maintenant compris et la composante neutrino peut-être extraite d’un fit radial du détecteur. Afin de tester l’existence d’un neutrino stérile léger, une source de 3-5,5 PBq de 144Ce sera installée sous Borexino au début de l’année 2018 pour un an et demi de prise de données : c’est l’expérience CeSOX. Cette source est produite par PA MAYAK par purification de combustible nucléaire usagé, par conséquent les potentiels contaminants radioactifs sont très nombreux et peu contraints. Pour tester l’hypothèse stérile, une mesure en flux, une mesure en forme et une mesure combinée seront effectuées dans l’ensemble du détecteur Borexino. Ces mesures sont fortement dépendantes de la connaissance intime de la source (composition, forme du spectre beta du 144Ce, énergie moyenne 144Ce et 144Pr). A cette fin, un spectromètre gamma a été spécifiquement étalonné et entièrement simulé au CEA, Saclay. De même un spectromètre beta a été dessiné, assemblé, simulé et est en cours d’étalonnage. Finalement, des mesures de spectrométrie alpha et de masse seront réalisés sur des échantillons représentatifs envoyés au CEA, Saclay afin de contraindre au mieux la composition de la source de 144Ce de SOX. / Located in the Gran Sasso underground laboratory (LNGS), Borexino measures solar neutrinos for 10 years. Among solar neutrinos, 8B continuous spectrum (up to 17 MeV) enables to test the transition zone between vacuum and matter regime of the MSW effect. This new measurement increases by one order of magnitude the exposure with respect to previous Borexino publication. To do so, the entire active volume is considered in this analysis above 5 MeV. A new background has been identified and a radial fit is done above 3 and 5 MeV enabling to extract the neutrino component. Existence of a light sterile neutrino would have important consequences on astrophysics and cosmology. SOX is the only experiment aiming at testing this hypothesis using a punctual radioactive source. A 3-5.5 PBq 144Ce source is actually under production and will be positioned under Borexino in 2018. Precise knowledge of the source is one of the main challenge of this experiment, based on rate and shape neutrino measurement. Two critical parameters are the heat released by the source for activity measurement and the expected neutrino spectrum in the detector. We first describe the SOX experiment insisting on 144Ce source production. Then, we focus on Saclay installations dedicated to constrain radioactive contamination inside the source using representative samples. Alpha, gamma and mass spectroscopy calibration and simulation are discussed and competitive constrains are derived. A status on 144Ce beta shape measurements is done as well as presentation of future measurement.

A real time low energy spectral measurement of the neutrinos coming from the Sun will give us a greater understanding of energy production in the Sun, and the mechanisms of neutrino mixing. We will, for the first time, measure the solar neutrino spectrum for all solar neutrinos <2MeV in particular pp, Be and CNO neutrinos, be able to compare the solar photon derived energy luminosity (Lï §) to the solar neutrino derived energy luminosity (Lï ®) independent of any solar model, explore dark energy with respect to mass varying neutrinos, and explore CNO abundances in the Sun. These measurements require new technology in Indium loaded scintillators and large scale detector designs, namely increased spatial resolution through a novel scintillation lattice. I will present the advances we are making to these fields at Virginia Tech as well as neutrino science and the physics of the LENS detector. / Ph. D.