  The electro-magnetic calorimeter at the BaBar detector, part of the
asymmetric B Factory at SLAC, measures photons in the energy range from 20 MeV
to 8 GeV with good resolution. The calorimeter is calibrated at the low energy
end with 6.13 MeV photons obtained from a liquid source system. During the
calibration, a fluorine-rich liquid is activated via a neutron generator and
pumped past the front of the calorimeter's crystals. Decays that occur in front
of the crystals emit photons of well-defined energy, which are detected in the
crystals with the regular data acquisition system. The liquid source system
adds only very little material in front of the calorimeter, needs nearly no
maintenance, and allows operation at the switch of a key with minimal safety
hazards. The report describes the system, presents calibration results obtained
from its operation since 1999, shows the crystals' loss of light yield due to
radiation damage, and shares experiences gained over the years.
