The HARP TPC laser calibration system

A novel apparatus for the calibration of the HARP Time Projection Chamber has been designed, developed and built. The apparatus consists of a large number of point-like photo-electron sources located at precise positions inside the detector volume. The photo-electron sources are optical quartz fibers on which one end is coated with an aluminum layer of ∼80 A thickness and are held in place on the high-voltage membrane. The fibers are used to guide UV laser light pulses that generate photoelectrons on the fiber tips acting as photo-electron emitters. The photo-electrons drift inside the detector and produce the calibration signals. The technique allows to assess ExB distortions and to measure drift velocity, ion feedback and time stability in real time