CosmogenicC11production and sensitivity of organic scintillator detectors topepand CNO neutrinos

Abstract

Several possible background sources determine the detectability of pep and CNO solar neutrinos in organic liquid scintillator detectors. Among such sources, the cosmogenic 11 C nuclide plays a central role.11 C is produced underground in reactions induced by the residual cosmic muon flux. Experimental data available for the effective cross section for 11 C by muons indicate that 11 C will be the dominant source of background for the observation of pep and CNO neutrinos.11 C 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 11 C 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, 11 C, 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 paper also discusses a technique for reduction of background from 11 C 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 11 C 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 11 C nuclide from 12 C. 11 C events are tagged by a three-fold coincidence with the parent muon track and the subsequent neutron capture on protons.

Direct Measurement of theBe7Solar Neutrino Flux with 192 Days of Borexino Data

“…This is 65% larger than extrapolated from activation on µ beams at the CERN NA54 facility [20] and 45% larger than calculated in Ref. [21].…”

“…Weights for pp and pep neutrinos are fixed to the values expected from the Standard Solar Model (SSM) [18] and from a recent determination of sin 2 2θ 12 =0.87 and ∆m 2 12 =7.6×10 −5 eV 2 [6], which correspond to the Large Mixing Angle (LMA) scenario of solar neutrino oscillation via the MSW effect [19]. The spectra for CNO neutrinos and 210 Bi are almost degenerate and cannot be distinguished prior to removal of the 11 C background [20,21]: we use a single component whose weight is a free parameter. Two independent analysis codes report consistent spectra and results, shown in Fig.…”

Cosmogenic activation of materials

“…39, 40. It is worth noting that from all the directly observed radionuclides, only 125 Sb is considered to be a potential relevant background for a multi-ton scale dark matter search.…”

“…125 taking into account all relevant production channels. Evaluation of cross sections from different sources combining data and calculations was made and a FLUKA simulation of monoenergetic muons in Borexino liquid scintillator (trimethilbenzene) was run to derive rates and paths of secondary particles; then, combining this information on the secondaries with the cross sections the individual and total production rates were derived for different muon energies.…”

Solar neutrino oscillations and the recent results of Borexino and SNO

“…However, an event-by-event subtraction of this background is possible by a triple-coincidence using the initial muon, the knock-out neutron from 12 C and the 11 C decay signal [15]. The 11 C reduction might also allow the direct detection of CNO-ν's.…”