First measurement of the yield of ${}^8$He isotopes produced in liquid scintillator by cosmic-ray muons at Daya Bay

Abstract

Daya Bay presents the first measurement of cosmogenic ${}^8$He isotope production in liquid scintillator, using an innovative method for identifying cascade decays of ${}^8$He and its child isotope, ${}^8$Li. We also measure the production yield of ${}^9$Li isotopes using well-established methodology. The results, in units of 10${}^{-8}\hat{I}{¼}^{-1}$g${}^{-1}$cm${}^2$, are 0.307$\pm$0.042, 0.341$\pm$0.040, and 0.546$\pm$0.076 for ${}^8$He, and 6.73$\pm$0.73, 6.75$\pm$0.70, and 13.74$\pm$0.82 for ${}^9$Li at average muon energies of 63.9~GeV, 64.7~GeV, and 143.0~GeV, respectively. The measured production rate of ${}^8$He isotopes is more than an order of magnitude lower than any other measurement of cosmogenic isotope production. It replaces the results of previous attempts to determine the ratio of ${}^8$He to ${}^9$Li production that yielded a wide range of limits from 0 to 30\%. The results provide future liquid-scintillator-based experiments with improved ability to predict cosmogenic backgrounds.