CNO and pep neutrino spectroscopy in Borexino:: Measurement of the deep-underground production of cosmogenic 11C in an organic liquid scintillator

Borexino is an experiment for low-energy neutrino spectroscopy at the Gran Sasso underground laboratories. It is designed to measure the monoenergetic Be-7 solar neutrino flux in real time, via neutrino-electron elastic scattering in an ultrapure organic liquid scintillator. Borexino has the potential to also detect neutrinos from the pep fusion process and the CNO cycle. For this measurement to be possible, radioactive contamination in the detector must be kept extremely low. Once sufficiently clean conditions are met, the main background source is C-11, produced in reactions induced by the residual cosmic muon flux on C-12. In the process, a free neutron is almost always produced. C-11 can be tagged on an event-by-event basis by looking at the threefold coincidence with the parent muon track and the subsequent neutron capture on protons. This coincidence method has been implemented on the Borexino Counting Test Facility data. We report on the first event-by-event identification of in situ muon-induced C-11 in a large underground scintillator detector. We measure a C-11 production rate of 0.130 +/- 0.026(stat) +/- 0.014(syst) day(-1) ton(-1), in agreement with predictions from both experimental studies performed with a muon beam on a scintillator target and ab initio estimations based on the C-11 producing nuclear reactions.