Neutron background measurements with a hybrid neutron detector at the Kuo-Sheng Reactor Neutrino Laboratory

We report in situ neutron backgroundmeasurements at the Kuo-Sheng Reactor Neutrino Laboratory (KSNL) by a hybrid neutrondetector (HND) with a data size of 33.8 days under identical shielding configurations as during the neutrino physics data taking. The HND consists of BC-501A liquid and BC-702 phosphor powder scintillation neutrondetectors, which is sensitive to both fast and thermal neutrons, respectively. Neutron-induced events for the two channels are identified and differentiated by pulse shape analysis, such that backgroundof both are simultaneously measured. The fast neutron fluxesare derived by an iterative unfolding algorithm. Neutroninduced backgroundin the germanium detector under the same fluxes, both due to cosmic-rays and ambient radioactivity, are derived and compared with the measurements. The results are valuable to backgroundunderstanding of the neutrino data at the KSNL. In particular, neutron-induced backgroundevents due to ambient radioactivity as well as from reactor operationare negligible compared to intrinsic cosmogenic activity and ambient $\gamma$-activity. The detector concept and analysis procedures are applicable to neutron backgroundcharacterization in similar rare-eventexperiments.