226Ra in the nylon scintillator vessel as a background source in the Solar Neutrino Experiment BOREXINO

Pauli postulated the existence of neutrino in the year 1931. This new particle has been introduced in order to solve the problem of energy and momentum loss in nuclear b decay. It has been proposed as a massless and chargeless particle traveling at the speed of light. Cowan and Reines observed neutrino first time in 1956 and since that time its study plays a great role in elementary particle physics and astrophysics. The most important source of these particles seems to be the Sun. It produces constantly a high flux (1010 n/cm2/s) of low energy (up to 18 MeV) neutrinos in thermonuclear reactions taking place in its center. Neutrinos and energy production in the Sun describes in details the Standard Solar Model (SSM). Up to now, five different collaborations have reported detection of solar neutrinos. The main result that they came to is the record of a flux of two to free times lower than expected according to the SSM. The discrepancy between the theoretical end experimental results is known as the Solar Neutrino Problem (SNP). Presently a few new projects aiming detection of solar neutrinos are under development or construction. They are characterized by real time and high statistics data taking, low energy thresholds and ability to distinguish different types of neutrino interactions (e.g. via charged or neutral current). These detectors are called “second generation detectors” – experiments run up to date exemplify the “first generation” detectors. One of the new projects is being realized by the BOREXINO collaboration in the Laboratori Nazionali del Gran Sasso (LNGS) of the Instituto Nazionale di Fisica Nucleare (INFN). BOREXINO is a 300-ton unsegmented liquid scintillator detector, which aims to measure mono-energetic 7Be neutrinos in real time through elastic neutrinoelectron scattering. Expected signal according to the SSM vary between 9 and 53 events per day depending on the neutrino oscillation scenario. Very low counting rate requires very low background of the detector. For this reason ultra-low levels of 238U, 232Th, 40K, 222Rn, 14C and other radioactive isotopes are requested in construction materials and in the scintillator. Background from cosmic rays is reduced by a 3500 m w.e. shield of dolomite rock over the laboratory. The possibility to achieve project’s specifications of radiopurity for a full-scale detector has been shown with the small prototype called Counting Test Facility