Yields and production rates of cosmogenic 9Li and 8He measured with the Double Chooz near and far detectors

The yields and production rates of the radioisotopes $^9$Li and $^8$He created by cosmic muon spallationon $^{12}$C, have been measured by the two detectors of the Double Choozexperiment. The identical detectors are located at separate sites and depths, which means they are subject to different muonspectra. The near (far) detector has an overburdenof $\sim$120 m.w.e. ($\sim$300 m.w.e.) corresponding to a mean muonenergy of $32.1\pm2.0\,\mathrm{GeV}$ ($63.7\pm5.5\,\mathrm{GeV}$). Comparing the data to a detailed simulation of the $^9$Li and $^8$He decays, the contribution of the $^8$He radioisotope at both detectors is found to be compatible with zero. The observed $^9$Li yields in the near and far detectors are $5.51\pm0.51$ and $7.90\pm0.51$, respectively, in units of $10^{-8}\mu ^{-1} \mathrm{g^{-1} cm^{2} }$. The shallow overburdensof the near and far detectors give a unique insight when combined with measurements by KamLAND and Borexinoto give the first multi--experiment, data driven relationship between the $^9$Li yield and the mean muonenergy according to the power law $Y = Y_0( / 1\,\mathrm{GeV})^{\ overline{\alpha}}$, giving $\ overline{\alpha}=0.72\pm0.06$ and $Y_0=(0.43\pm0.11)\times 10^{-8}\mu ^{-1} \mathrm{g^{-1} cm^{2}}$. This relationship gives future liquid scintillator based experiments the ability to predict their cosmogenic $^9$Li background rates.