Light output response of KamLAND liquid scintillator for protons and 12C nuclei

The light output responses for protons and carbon nuclei in the KamLAND liquid scintillator were precisely measured using a monochromatic neutron beam. The observed response for proton recoils is well described by Birks formula with the higher-order correction, for the recoil energy from 424 keV to 10.5 MeV. The response for carbon recoils is also well described by modifying the Birks formula with the nuclear energy loss terms, for the measured recoil energy from 171 keV to 2.2 MeV. The obtained light output responses were used to estimate the background energy spectrum of the C13(α,n)O16 reaction in the KamLAND. The systematic uncertainty of the energy scale in the expected background spectrum was further improved with the measured light output responses.