First observation of 7Be solar neutrinos with KamLAND

Abstract

The international KamLAND collaboration operates a 1 kton liquid scintillation detector in the Kamioka mine in Gifu, Japan. KamLAND's main scientific results are the precision measurement of the solar Δm²₁₂ = 7.58 <super>+ +0.14</super>_{-0.13 stat}&plusmn; 0.15_syst and tan² θ₁₂ = 0.56<super>+0.10</super>_{-0.07 stat}<super>+0.10</super>_{-0.06 syst} utilizing reactor antinuetrinos and first evidence for the observation of geologically produced antineutrinos. In an effort to extend KamLAND's scientific reach, extensive research has been performed on preparing a spectroscopic measurement of ⁷Be solar neutrinos. This work provides the first inclusive analysis of KamLAND's backgrounds below 1 MeV. ⁸⁵Kr and ²¹⁰Pb, dissolved in KamLAND liquid scintillator, were found to be the dominant source of low energy backgrounds. The concentration of these ultra-trace contaminants were determined to be 10⁻²⁰ g/g. This is more then 6 orders of magnitude lower then commercially available ultra-pure liquids. To attain a signal-to-background ratio suitable for the detection of ⁷Be solar neutrinos, the concentration of these contaminants had to be reduced by 5 orders of magnitude. A comprehensive study of ²¹⁰Pb removal was undertaken over the course of this thesis. This work further covers techniques for the removal of ²²⁰Rn, ²²²Rn and their daughter nuclei from liquid scintillator at concentrations of 10⁻¹⁸ g/g . Purification techniques studied in this work include water extraction, isotope exchange, adsorption, and distillation. These laboratory studies guided the design and implementation of a large scale purification system in the Kamioka mine. The purification system's design and operation is discussed in detail as well as specific experiments devised to control scintillator quality and radio-purity. The purification system's effectiveness in removing radioactive trace impurities is analyzed in detail. The total scintillator purified over two years of operation was more then 4.6 ktons. It is shown here that the KamLAND collaboration has successfully reduced the ⁸⁵Kr specific activity of the scintillator by a factor of 2.6 x 10⁴ while ²¹⁰Bi was reduced by a factor 2 x 10³. Due to the success in reducing the intrinsic backgrounds through multiple purifications, this work provides the first evidence for a ⁷Be solar neutrino signal in KamLAND. The presented analysis covers 5.448 kton-days of exposure time. While the current work is not yet providing a robust measurement of the ⁷Be solar neutrino flux, the presence of ⁷Be solar neutrino is shown to be statistically preferred over a null hypothesis.