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Abstract
The Borexino Solar Neutrino Experiment will observe the monoenergetic (862 keV) 7Be neutrinos, produced in the solar reaction 7Be+e- →7 Li+νe. These neutrinos are the second most abundant species of solar neutrinos, with an expected flux at earth of 5 × 109/cm2/s. Using ν - e scattering in an aromatic liquid scintillator, Borexino will make the first real time measurement of the solar neutrino flux at energies less than 1 MeV.
In addition to checking Standard Solar Model and neutrino oscillation predictions at low energies, Borexino will test the MSW vacuum-matter transition, luminosity constraint, and non-standard theories such as mass varying neutrinos. The Borexino detector will also be sensitive to supernova neutrinos, geoneutrinos, reactor neutrinos, and pep solar neutrinos. The pep measurement will tightly constrain the primary pp solar neutrino flux whose energy is below the Borexino threshold.
With an expected rate of 35 events per day from solar 7Be neutrinos, the maximum tolerable background rate is one count per day. Removal of radioactive isotopes from the liquid scintillator is essential for the experiment's success and will be achieved with purification techniques including filtration, distillation, water extraction, nitrogen stripping, and silica gel adsorption. Results from small-scale purification efficiency tests are presented. Water extraction showed moderate but inadequate removal of 210Po which is a dominant background. Distillation reduced 210Po by a factor of more than 500.
Online purification involves cycling over 300 m3 of scintillator from the detector though the purification plants. Flow patterns within the detector that influence the purification efficiency were determined with numerical simulations. Poor flow in the prototype Counting Test Facility showed effectively stagnant volumes within the detector. These are not present in the larger Borexino detector.
Surface contamination in Borexino arises primarily from contact with contaminated liquids and the deposition of airborne radon progeny. Measurements of desorption rates showed that surface contaminants are transferred to the scintillator logarithmically with time. Partitioning constants between the scintillator and surfaces were measured and airborne deposition rate of radon progeny in a clean room environment are analyzed. The efficiency of various surface cleaning techniques was also tested.
