The KamLAND Outer Detector

Abstract

The Kamioka Liquid Scintillator Anti-Neutrino Detector (KamLAND) consists of a one kiloton liquid scintillator Inner Detector (ID) and a three kiloton water u[C]erenkov Outer Detector (OD). The goal of KamLAND is to determine whether the flux and energy of electron anti-neutrinos generated by Japanese nuclear power reactors is consistent with the hypothesis that neutrinos have mass. The size and location of KamLAND allow for the first time a terrestrial test of the validity of the Large Mixing Angle solution to the Solar Neutrino Anomaly.

The anti-neutrinos are detected in the ID by a coincidence signal associated with the inverse beta decay reaction on a proton, followed by the subsequent capture of the neutron by another proton. The function of the OD is to tag cosmic ray muons and to suppress muon-induced neutron events in the ID which could otherwise be confused with real anti-neutrino events.

The Triangle Universities Nuclear Laboratory entered into the KamLAND collaboration to oversee the design, construction, testing, and operation of the OD. The OD consists of 225 twenty inch photomultiplier tubes (PMTs) arranged in four sections in a Tyvek lined cavity. The design of the detector is described, along with the testing procedures that were performed to determine the PMT operating characteristics. Simulations were performed to determine the muon-tagging efficiency of the OD as a function of the trigger conditions for an event. The neutron background caused by untagged muons was calculated for the current complement of PMTs. The efficiency of the OD was calculated to be 99.5\% and the untagged neutron background was calculated to be $1.3pm0.4$ for the 145.1 days of data-taking included in the first KamLAND result. The minimum value for the muon-tagging efficiency for the OD to be viable was determined to be 94.8\%.