Calibration and Monitoring for the Borexino Solar Neutrino Experiment Dissertation submitted to the Faculdade de Ciências da Universidade de Lisboa for the degree of Ph.D. in Physics by José Carvalho Maneira Advisors: Prof. Amélia Maio Prof. Gianpaolo Bellini (University of Milan) Lisbon, November 2001 To my parents and my brother. “The first question I ask myself when something doesn't seem to be beautiful is why do I think it's not beautiful. And very shortly you discover that there is no reason.” John Cage Summary One of the major open issues in Elementary Particle Physics today, the phenomenon of Neutrino Oscillations is a natural consequence of a non-zero neutrino mass and a non-diagonal leptonic mixing matrix. Even if the Standard Model extension to accommodate neutrino oscillations is relatively trivial, a non- zero neutrino mass and mixing is widely considered as a doorway for the Unified Theory of the fundamental interactions. In fact, Super Symmetry naturally explains small neutrino masses through the well-known “see-saw” mechanism. From the experimental point of view, neutrino oscillations are a privileged way of studying the neutrino mass spectrum, since small mass splittings can lead to large and measurable phase differences between interfering quantum-mechanical amplitudes. This is particularly true for Solar Neutrino Experiments, since the large distance between source and detector (1.5´1011 m) allows for a good sensitivity to very small mass differences (down to about 10-11 eV2), not available with the present accelerator and reactor experiments. In fact, the first indication for the possibility of Neutrino Oscillations came from the first measurements of Solar Neutrinos, more than thirty years ago. This hypothesis has been increasingly strengthened by the addition of successive results, from measurements of both Solar and Atmospheric Neutrinos. The experimental constraints on the neutrino mass and mixing parameters have been obtained through the combined analysis of the complementary results from experiments with different potentialities. The Borexino experiment is conceived according to this perspective, since it is aimed at the real-time measurement of the 7Be neutrino flux, a high rate (46 events/day), low energy (0.86 MeV) component of the solar neutrino spectrum for which only indirect, integrated measurements are available up to now. The Physics potential of such a measurement relies on the fact that the 7Be component is monoenergetic so the oscillation patterns are not smoothed by averaging over the energy spectrum and on its low energy, where the presently allowed oscillation solutions predict large rate differences. v The main goal of the Borexino experiment, presently in its final installation phase at the Gran Sasso underground laboratory (LNGS) in Italy, is the detection of 7Be solar neutrinos, requiring both a low energy threshold (250 keV) and a very low level of background from natural radioactivity in the detector materials (10-16 g/g of 238U and 232Th). In order to achieve this, the active detector target will be made of liquid scintillator, pseudocumene with PPO as fluor. However, the use of the scintillation technique limits the data analysis, since it does not allow any directional signature for neutrino events, and their identification is statistical, based on the energy spectrum shape analysis. Therefore, the challenging experimental measurement in Borexino requires precise energy and time calibrations, an accurate control of the background rejection cuts and excellent detector stability. The aim of the work presented in this thesis was to supply the Borexino experiment with the calibration tools necessary to interpret the data and extract the Physics of Neutrino Oscillations. The priority was given to the design, testing, construction and installation of the different calibration systems, but a significant effort was also dedicated to the development of software tools for the energy spectrum analysis and 7Be neutrino identification. The calibration and monitoring systems, face the same challenge that drives some of the most crucial aspects of the Borexino design: to reach and maintain the lowest radioactivity levels ever achieved in any large scale underground detector. This fact limits the mass of any permanent calibration devices in the detector and the risk of radioactive contamination puts significant constraints on the access to the active scintillator volume and on the use of radioactive sources. The strategy followed was to develop non-invasive monitoring systems based on the coupling of optical fibers to lasers and a system to position a radioactive g source (228Th - 200 mCi) in an external region. The calibration and monitoring operations will be scheduled according to the radioactive contamination risk, each step being carried vi out only if the required accuracy was not obtained with the preceding ones. Naturally, the calibration with internal radioactive sources, and in particular, long- lived ones, is the last step in the operational schedule. This thesis is organized in six Chapters. Chapter 1 introduces the Physics of Solar Neutrinos and Neutrino Oscillations, while Chapter 2 describes the goals and design of the Borexino experiment. The design and tests of the Calibration and Monitoring systems are presented in Chapters 3 to 5, while Chapter 6 discusses the sensitivity of Borexino to 7Be Solar Neutrino Physics. The work of the author includes Chapters 3, 4 and 6 and half of Chapter 5. Chapter 3 presents the system for time and charge calibration of the 2200 photomultipliers (PMTs) that detect the light produced in the liquid scintillator. The system is based on the illumination of all the 2200 PMTs with a common light source -- a diode laser -- through a two-stage fiber splitting system, so that each PMT is illuminated by one fiber. We present the requirements, guidelines and design of the system, as well as the results from a prototype of the complete optical chain, built and operated in realistic conditions. From these results and further tests of the final system, now almost fully installed, we expect that all the Borexino PMTs will be illuminated with a low intensity, fast, light pulse, allowing a charge calibration at the single-photoelectron level and a time calibration with a 0.5 ns accuracy. These will be relevant, for an accurate energy measurement and event position reconstruction respectively. Chapter 4 presents the two systems for the optical calibration of the scintillator and buffer liquid. The first of these systems is internal to the active volume and uses absorption and re-emission of ultraviolet laser pulses in the scintillator to measure its optical properties and mimic charged particle scintillation events. We describe the goals and design of the system, as well as the results of preliminary tests with a single photoelectron counting setup. We confirmed that, if the wavelength is low enough (below 300 nm), it will excite primarily the solvent and the fast component of the scintillation decay time is the same as for charged vii particles. This allows us to use laser sources to measure the time distribution of the actual scintillation pulses, necessary for position reconstruction and external background rejection. The second optical calibration system is external to the active volume and it will use a set of light beams in different directions in order to study the stability of the attenuation/scattering length of the buffer liquid and scintillator, that has a strong impact on the efficiency of the internal background rejection cut. The feasibility of this method, in which laser beams are carried to the detector by optical fibers, was tested in the Two Liquid Test Tank (TLTT), a 7 m3 tank built in LNGS for a long-term test of 50 PMTs in realistic conditions, i.e., immersed in the pseudocumene. The time distributions of PMT signals from different zones were used to distinguish between direct, reflected and scattered light and obtain control parameters. Independent measurements of the pseudocumene attenuation length confirmed the sensitivity of these parameters to the transparency changes of the liquid. The design and configuration of the light beam for the final system, now installed, was optimized by means of further tests and Monte Carlo simulations, so we expect to have a good accuracy for the attenuation length monitoring. Chapter 5 presents the systems for calibrations with internal and external radioactive sources. We present the hardware design and the Monte Carlo feasibility studies for a system based on a radioactive g source placed in a region outside the active detector volume, in order to minimize any detector contamination risk. In order to be sensitive to the ± 3.5 % yearly variation of the solar neutrino flux, we need to monitor the counting rate in one of the internal shells of the scintillator volume with an accuracy of 1 %. From our calculations and source insertion design, this can be accomplished with a 200 mCi 228Th source in a short time (a 2 hour run). For completion, we present also the internal source system, designed by our collaborators. Chapter 6 presents a study of the Borexino sensitivity to 7Be solar neutrinos, focusing on the last step of the data processing, that is the energy spectrum viii analysis, since the identification of the 7Be neutrino signal is ultimately done by the recognition of its characteristic “Compton-like” edge. In the energy spectrum study, analytic functions based on the theoretical cross-sections and the detector resolution were used to describe both the neutrino and the background energy distributions. The goal of this strategy is to complement the detailed description of all the expected shapes with a flexible approach that allows the method to better adapt itself to unexpected and not easily reproducible effects. The expected data were generated by Monte Carlo simulations. We studied the sensitivity to 7Be neutrinos for different oscillation scenarios, and its dependence on the internal background rate.