Development of a Liquid Scintillator and of Data ... - Borexino - Infn

Abstract
The first chapter contains a short overview of the theory of solar neutrinos, their production
inside the sun and their detection with present and future experiments. The implications of the
discrepancy between the predicted and the measured count rates that all experiments to date
have seen is discussed, followed by a presentation of the different possible solutions to the
solar neutrino problem. The most likely solution, neutrino oscillations, will be discussed in
detail.
The second chapter introduces the physics goals of the BOREXINO experiment. The design
of the detector and its ancillary systems are described. The different types of background are
discussed and the importance of their effective suppression is underlined. In chapter 3 the
design and results from the first data taking period of the Counting Test Facility (CTF) are
presented, and the main design improvements of the upgraded CTF2 are shortly discussed.
In chapter 4 a newly developed reconstruction method for scintillation events for both BORE-
XINO and the CTF is presented. This method is based on a comparison of each real event with
simulated events from known positions, and includes a model that describes the generation
and propagation of the scintillation photons in the detector. Promising results are obtained
with Monte Carlo simulations and confirmed by the analysis of real data from the CTF.
Chapter 5 discusses the scintillation processes which allow particle identification, and the
application of neural networks to this problem. Results obtained with Monte Carlo simulations
and CTF data show the capabilities of this method.
Chapter 6 is devoted to the development of an alternative liquid scintillator for the BORE-
XINO experiment, namely PXE. This scintillator offers the advantage of a higher density (and
therefore higher target mass) compared to the solution chosen for BOREXINO (pseudocumene
based scintillator). The results of several tests and measurements in laboratory and in the CTF
are presented. The optical properties (light yield, fluorescence time, absorption and reemission),
which are essential for position reconstruction and «¬-discrimination, are comparable
to those of pseudocumene. The radiopurity level obtained with a newly adopted purification
method using a silica gel column was for U and Th below the sensitivity limit of the neutron
activation analysis. Due to background problems, however, it was not possible to confirm
these results by the measurements in the CTF.
The last chapter describes the calibration measurements in the CTF2 that I performed. With
these measurements the position- and energy-dependent detector response could be tested.
The spatial and energy resolution were determined as a function of position and energy, and
systematic effects due to the position reconstruction could be excluded.
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