Delayed coincidence with a day-scale window for tagging 232Th series isotopes in KamLAND

Decays of Th-232 series isotopes such as (212)Bi(212)po and Tl-208 form background events for underground physics experiments. In this study, we performed new delayed coincidence with a day-scale window using data of KamLAND, a 1 kton liquid scintillator detector. We firstly searched for a coincidence of Rn-220(_216) po (T-1/2 = 0.15 s), prompt coincidence (PC), and then searched for events of (BiPo)-Bi-212-Po-212 or Tl-208 decays within 2 day from PC. It yields similar to 80% tagging efficiency with modest similar to 10%, deadtime despite the requirement of long time correlation thanks to low accidental coincidence rate of PC. This method will be newly introduced to analyses of solar neutrino measurement at 3 MeV region with KamLAND and neutrinoless double-beta decay search with KamLAND-Zen to reduce the backgrounds.