Performance evaluation of the KamLAND2 prototype detector

To achieve the sensitivity improvements moving from the KamLAND-Zen experiment to the next-generation KamLAND2-Zen experiment, we constructed the KamLAND2 prototype detector to preliminarily evaluate the proposed improvements to increase the light yield, including the instal-lation of high quantum efficiency photomultiplier tubes, light-collection mirrors, and high emission liquid scintillator.The performance evaluation using this detector showed that the installation of light-collection mirrors alone can increase the light yield by a factor 1.7 to 2.6.Furthermore, the observed light yield showed remarkable stability within the margin of error over a long period of about one year.On the other hand, hardware problems and insufficient tuning of the simulation were also identified.