Speaker
Description
Borexino is a 280t liquid scintillator detector at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. Its main goal is the precision spectroscopy of solar neutrinos down to energies of ~190 keV and for this task it features a high radio-purity and a high light yield of ~10,000 scintillation photons per 1 MeV deposited energy. The solar neutrinos are measured by their elastic scattering off electrons which induce isotropically emitted scintillation photons that are detected by PMTs.
In this talk we present the first measurement of solar $^{7}$Be neutrinos using their associated Cherenkov photons in a high light yield scintillation detector. In Borexino electrons with E>160 keV produce Cherenkov photons but the ratio of Cherenkov photons from the neutrino scattered electrons is <1% for all PMT hits. A typical reconstruction of the event direction, as in water Cherenkov detectors, is therefore not possible. In this analysis we look instead at the integrated signal of all PMT hits for all detected events and correlate the position of the hit PMT relative to the reconstruced position of the event with the well known position of the Sun. The angular distribution of these correlated hits shows the statistical contribution of Cherenkov photons from the electrons that were scattered by solar neutrinos. In this analysis, the number of $^{7}$Be neutrinos is then inferred from the measured angle distribution, which is compared to probability density functions produced with the Geant4-based Borexino Monte Carlo simulation. To do that it is necessary to tune the MC Cherenkov time behaviour, which is possible in Borexino with the available $^{54}$Mn and $^{40}$K gamma calibration sources.
