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During experiment
This section describes support tasks that are typically requested from experimenters.
Mask calibration
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.
Note:
How to take data for the mask calibration:
Start a run without taking to disk and look in the S800 SpecTcl at the 2D xy position spectrum of (s800.fp.crdc1.x,s800.fp.crdc1.tac). If it starts to look like the spectrum shown below (the screenshot corresponds to mask 2, mask 1 looks inverted, in the sense of the triangle of dots pointing downwards), start taking data to disk until it has about the quality of the screenshot shown. If only a small fraction of the S800 focal plane is illuminated (dispersion-matched optics), consult with the S800 group to sweep the beam across the focal plane by changing the Brho of segment 8. The figure below shows the SpecTcl spectrum of the mask “shadow” measured in CRDC2 during a reaction setting.
Particle identification corrections
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:
In
SpecTcl GUI, click
Attach to File and select data file
run-xxxx-xx.evt in directory
/user/s800/stagearea/experiment/runxxxx, where xxxx stands for the run number
The correction process described above can be used for other ToF. SpecTcl includes a set of corrected ToF parameters:
s800.tof.obje2: Corrected ToF calculated from the OBJ-FP TAC
s800.tof.xfpe2: Corrected ToF calculated from the XFP-FP TAC
s800.tof.obje1: Corrected ToF calculated from the OBJ-FP Phillips TDC
s800.tof.xfpe1: Corrected ToF calculated from the XFP-FP Phillips TDC
Note that although the SpecTcl correction variables are shared between the ToFs from the TACs and Phillips TDC, they won't work simultaneously, i.e. the variables that work to “clean” the TAC-based PID spectrum won't work for the TDC-based PID spectrum
Handling detectors
Shimming OBJ scintillator
Shimming of the OBJ scintillator (or replacement) is required when the efficiency (measured by the experimenters) drops to about 85%. Please, refer to the operations guideline describing the procedure.