Abstract

Daya Bay presents the first measurement of cosmogenic He8 isotope production in liquid scintillator, using an innovative method for identifying cascade decays of He8 and its child isotope, Li8. We also measure the production yield of Li9 isotopes using well-established methodology. The results, in units of 10−8μ−1g−1 cm2, are 0.307±0.042, 0.341±0.040, and 0.546±0.076 for He8, and 6.73±0.73, 6.75±0.70, and 13.74±0.82 for Li9 at average muon energies of 63.9 GeV, 64.7 GeV, and 143.0 GeV, respectively. The measured production rate of He8 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 He8 to Li9 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.Received 8 February 2024Revised 28 March 2024Accepted 30 May 2024DOI:https://doi.org/10.1103/PhysRevD.110.L011101© 2024 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNeutrino oscillationsTechniquesNeutrino detectionParticles & FieldsNuclear Physics