During experiment

This section describes support tasks that are typically requested during the course of an experiment. Some of them, like Mask calibration, need to be coordinated with the experimenters.

• Mask calibration
• Particle identification corrections
• Efficiency of OBJ Scintillator

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.

• Unreacted beam does not provide good conditions for performing the mask calibration, but sometimes it provides the only option

• To cover mask
  • Detune non-dispersive y quad (ratio=0.5) in spectrograph to spread beam in y
  • Sweep beam across dispersive direction using spectrograph dipole

• Set the trigger to S800 singles and downscale factor 1

<del>* On PanelMate, open page 4 in S800DRIV</del> 

• On Diag_Main.opi, open Device Drives tab. The mask drive controls are at the bottom of this page in the Focal Plane section, namely RBT_BTS34:MSK_D2692 (Mask #1) and RBT_BTS34:MSK_D2703 (Mask #2).

• Click on Mask #1 (the upstream mask) and hit the “insert” button. This puts the mask in front of CRDC1

• 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.

• Remove Mask #1 by pressing “Out”.

• Repeat the procedure with CRDC2 by inserting Mask #2 (the downstream mask) on the same diagnostics page.

• Mask pattern

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 resolution of the PID. This correction is supposed to be done “offline”, i.e. using a data file recorded in the stagearea:

The SpecTcl parameters s800.tof.objcorr1, s800.tof.objcorr2, and s800.tof.objcorr3 correspond to the “raw” parameters s800.tof.obj, s800.tof.tac_obj, and s800.tof.mtdc_obj corrected from the dependences described above

The SpecTcl parameters s800.tof.xfpcorr1, s800.tof.xfpcorr2, and s800.tof.xfpcorr3 correspond to the “raw” parameters s800.tof.xfp, s800.tof.tac_xfp, and s800.tof.mtdc_xfp corrected from the dependences described above

The process described below describes to improve the resolution of the PID based on the OBJ-FP ToF from the ORTEC TAC. The correction process for the PID spectra based on other ToFs is similar

• Open the Variables page in SpecTcl and make sure that the variables s800.tof.obj2Correction1 and s800.tof.obj2Correction2 are zero. (The figure below shows the Variables page with the final values of the correction variables)

• 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

• Looking at the spectrum PID:OBJCORR2_TRACK.AFP (figure below, top), change the value of the variable s800.tof.obj2Correction1
  • Enter a new value (typically in the range 500-1000)
  • Click Set
  • Re-scan the data file
  • Keep trying different values, until the dependence is eliminated. The figure below (bottom) shows the corrected spectrum (for this particular case, s800.tof.obj2Correction1 = 450)

• Repeat the process looking now at the spectrum PID:OBJCORR2_CRDC1.X (figure below, top)
  • Enter a new value for the variable s800.tof.obj2Correction2 (typically in the range 0.01-0.1)
  • Click Set
  • Re-scan the data file
  • Keep trying different values, until the dependence is eliminated. The figure below (bottom) shows the corrected spectrum (for this particular case, s800.tof.obj2Correction2 = 0.05)

• The figure below shows the spectrum PID:OBJCORR2_IC_SUM after the right corrections were included in SpecTcl

• The correction process described above can be used for other ToF. SpecTcl includes a set of corrected ToF parameters and variables for the OBJ-FP and XFP-FP ToFs taken from the Phillips TDC, TACs, and MTDC:
  • s800.tof.objcorr1, s800.tof.obj1Correction1, s800.tof.obj1Correction2: Corrected parameter and correction variables for the OBJ-FP ToF taken from the Phillips TDC
  • s800.tof.objcorr2, s800.tof.obj2Correction1, s800.tof.obj2Correction2: Corrected parameter and correction variables for the OBJ-FP ToF taken from the ORTEC TAC
  • s800.tof.objcorr3, s800.tof.obj3Correction1, s800.tof.obj3Correction2: Corrected parameter and correction variables for the OBJ-FP ToF taken from the MTDC
  • s800.tof.xfpcorr1, s800.tof.xfp1Correction1, s800.tof.xfp1Correction2: Corrected parameter and correction variables for the XFP-FP ToF taken from the Phillips TDC
  • s800.tof.xfpcorr2, s800.tof.xfp2Correction1, s800.tof.xfp2Correction2: Corrected parameter and correction variables for the XFP-FP ToF taken from the ORTEC TAC
  • s800.tof.xfpcorr3, s800.tof.xfp3Correction1, s800.tof.xfp3Correction2: Corrected parameter and correction variables for the XFP-FP ToF taken from the MTDC

At relatively high rates (~0.5 MHz), the OBJ scintillator will be gradually damaged. Experimenters should check the efficiency of this detector continuously during the course of the experiment. Whenever the rates drop below ~90%, they may request the Device/Beam Physicist to shim the detector.

Before shimming, make sure that the thresholds are set properly (not too high) (refer to section “Object Scintillator Setup during XDT”

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.