  We report in situ neutron background measurements at the Kuo-Sheng Reactor
Neutrino Laboratory (KSNL) by a hybrid neutron detector (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 neutron detectors, 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 background of
both are simultaneously measured. The fast neutron fluxes are derived by an
iterative unfolding algorithm. Neutron induced background in 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 background understanding of the neutrino data at the KSNL. In
particular, neutron-induced background events due to ambient radioactivity as
well as from reactor operation are negligible compared to intrinsic cosmogenic
activity and ambient $\gamma$-activity. The detector concept and analysis
procedures are applicable to neutron background characterization in similar
rare-event experiments.
