Refurbishment of KamLAND outer detector

KamLAND, a large size liquid scintillator detector for anti-neutrino detection, has observed the reactor neutrino oscillation and geo-neutrinos since its operation started in 2002. The outer detector (OD) is a water Cherenkov detector with 3,200 m³ of water and 225 Kamiokande-PMTs serving as the veto counter against the cosmic-ray muons. Muon tagging efficiency of the OD has been decreasing since 2010 because of a gradual increase of PMT failures, which results in the increase of muon-induced backgrounds. We have carried out a refurbishment of the OD from January to April in 2016 by replacing the PMTs and improving light collection in the less-sensitive equator region. As a result, we achieved a recovery of the OD detection efficiency of muons equal to 99.8%.