  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 theta13. 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.
