The HARP Time Projection Chamber

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 $\sim$80{\AA} 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 and these act as photo-electron emitters. The photoelectrons drift inside the detector and produce the calibration signals. The technique allows to asses $E\times B$ distortions and to measure drift velocity, ion feedback and time stability in real time. The analog signals generated by the TPC front-end electronics have been characterized. Different methods to extract the amplitude and time of occurrence from the digitized signals have been studied and compared. Fast estimators, like the sum of all the discrete amplitude measurements inside one pulse, show a reconstructed amplitude and time of occurrence resolution comparable to those achieved by fitting the discrete amplitude measurements to the expected pulse shapes. The obtained relative resolutions ($\sim$5\%) are limited by the aliasing effect and can be improved by an order of magnitude (up to $\sim$0.3\%) by applying a correction to the estimators valu e which is constructed from the knowledge of the response of the preamplifier and the digitization procedure. An undesired cross-talk effect in the TPC front-end electronics has been found and traced to a capacitive coupling between the input and output of the preamplifiers. The cross-talk analog signals have an area which is, on average, $\sim$5\% of that of the inducing signal. However, because of the different pulse shape, after the digitization process, these cross-talk signals can produce areas which reach $\sim$25\% that of the original signal. A model has been identified, developed and tested to achieve a reduction of one order of magnitude in the cross-talk intensity.