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CRDC mask calibrations are used to relate channel numbers and true physical distances in mm for the position-sensitive cathode readout drift chambers. A mask with a distinct and well-determined pattern of slits and holes can be inserted remotely in front each CRDC. The particles passing through the holes leave the pattern of the mask in the position spectrum of the detectors. First-order polynomials are used for the calibration. In x direction, where the position is determined from the charge induced in the pads, the slope of the first-order polynomial is fixed by the geometry of the detector to 2.54 mm/pad. The offset has to be set to center the “beam hole” at 0 mm. Since the drift time depends on the drift voltage, the gas composition and pressure, the y slope will change from experiment to experiment.
In a typical reaction setting, the dependences of ToF on the dispersive angle and position make it hard to resolve the particle-identification spectrum. As an example, the figure below shows the PID SpecTcl spectrum PID:TAC.OBJ_IC.SUM using the OBJ-FP ToF from the ORTEC TAC:
The following process describes how to improve the PID resolution. This correction is supposed to be done “offline”, i.e. using a data file recorded in the stagearea:
A drop in efficiency of the OBJ scintillator might indicate that the detectors needs to be shimmed. The OBJ detector has a total of six shimming positions, three on each side.
Shimming of the OBJ scintillator is required when the efficiency (measured by the experimenters) drops to about 85%. Please, refer to the operations guideline describing the procedure.
The replacement of the OBJ scintillator is described in this guideline.