Demonstration of Tl-208 background reduction for ZICOS experiment using topological information of Cherenkov lights

ZICOS is a future experiment for 0νββ of 96Zr nuclei. In order to achieve sensitivity over 1027 years, ZICOS will use tons of 96Zr, and need to remove 208Tl backgrounds as observed by KamLAND-Zen one order of magnitude. For this purpose, we have developed new technique to distinguish the signal and background using topology of Cherenkov light. We have measured directly this topology (averaged angle) using HUNI-ZICOS detector and the results clearly indicated topology as effective even 1MeV electron. Also the pulse shape discrimination for the extraction of PMT which receives Cherenkov lights in the liquid scintillator and the vertex reconstruction have been established. In order to confirm above technique, we have demonstrated using β - γ event such as 208Tl beta decay using 60Co source using UNI-ZICOS detector. The measurement is still on going due to March Earthquake in Japan, however, the  different averaged angle distribution will be obtained as expected by MC.