Absolute energy calibration with the neutron-activated liquid-source system at BABAR´s CsI(Tl) calorimeter

The electromagnetic 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. This paper 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.