  The outer water buffer is an economic option to shield the external radiative
backgrounds for liquid-scintillator neutrino detectors. However, the
consequential total reflection of scintillation light at the media boundary
introduces extra complexity to the detector optics. This paper develops a
precise detector-response model by investigating how total reflection
complicates photon propagation and degrades reconstruction. We first
parameterize the detector response by regression, providing an unbiased energy
and vertex reconstruction in the total reflection region while keeping the
number of parameters under control. From the experience of event degeneracy at
the Jinping prototype, we then identify the root cause as the multimodality in
the reconstruction likelihood function, determined by the refractive index of
the buffer, detector scale and PMT coverage. To avoid multimodality, we propose
a straightforward criterion based on the expected photo-electron-count ratios
between neighboring PMTs. The criterion will be used to ensure success in
future liquid-scintillator detectors by guaranteeing the effectiveness of event
reconstruction.
