The yields and production rates of the radioisotopes 9Li and 8He created by cosmic muon spallation on 12C, have been measured by the two detectors of the Double Chooz experiment. The identical detectors are located at separate sites and depths, which means that they are subject to different muon spectra. The near (far) detector has an overburden of ∼120 m.w.e. (∼300 m.w.e.) corresponding to a mean muon energy of 32.1 ± 2.0 GeV (63.7 ± 5.5 GeV). Comparing the data to a detailed simulation of the 9Li and 8He decays, the contribution of the 8He radioisotope at both detectors is found to be compatible with zero. The observed 9Li yields in the near and far detectors are 5.51 ± 0.51 and 7.90 ± 0.51, respectively, in units of 10−8μ−1g−1cm2. The shallow overburdens of the near and far detectors give a unique insight when combined with measurements by KamLAND and Borexino to give the first multi-experiment, data driven relationship between the 9Li yield and the mean muon energy according to the power law Y=Y0(〈Eμ〉/1GeV)α¯, giving α¯=0.72±0.06 and Y0 = (0.43 ± 0.11) × 10−8μ−1g−1cm2. This relationship gives future liquid scintillator based experiments the ability to predict their cosmogenic 9Li background rates.[Figure not available: see fulltext.].
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