Measuring cosmogenic 9Li background in a reactor neutrino experiment

Cosmogenic isotopes 9Li and 8He produced in the detector are the most problematic background in the reactor neutrino experiments designed to determine precisely the neutrino mixing angle θ 13 . The average time interval of cosmic-ray muons in the detector is often on the order of the lifetimes of the 9Li and 8He isotopes. We have developed a method for determining this kind of background from the distribution of time since last muon for muon rate up to about 20 Hz when the background-to-signal ratio is small, on the order of a few percents.