Cosmogenic 11C production and sensitivity of organic scintillator detectors to pep and CNO neutrinos -: art. no. 055805

Several possible background sources determine the detectability of pep and CNO solar neutrinos in organic liquid scintillator detectors. Among such sources, the cosmogenic C-11 nuclide plays a central role. C-11 is produced underground in reactions induced by the residual cosmic muon flux. Experimental data available for the effective cross section for C-11 by muons indicate that C-11 will be the dominant source of background for the observation of pep and CNO neutrinos. C-11 decays are expected to total a rate 2.5 (20) times higher than the combined rate of pep and CNO neutrinos in Borexino (KamLAND) in the energy window preferred for the pep measurement between 0.8 and 1.3 MeV. This study examines the production mechanism of C-11 by muon-induced showers in organic liquid scintillators with a novel approach: for the first time, we perform a detailed ab initio calculation of the production of a cosmogenic nuclide, C-11, taking into consideration all relevant production channels. Results of the calculation are compared with the effective cross sections measured by target experiments in muon beams. This article also discusses a technique for reduction of background from C-11 in organic liquid scintillator detectors, which allows to identify on a one-by-one basis and remove from the data set a large fraction of C-11 decays. The background reduction technique hinges on an idea proposed by Martin Deutsch, who suggested that a neutron must be ejected in every interaction producing a C-11 nuclide from C-12. C-11 events are tagged by a threefold coincidence with the parent muon track and the subsequent neutron capture on protons.