Forward Calorimeter Commissioning

Detector Commissioning Prior to Beam

A list of the tasks that should be completed prior to turning on beam in the hall appear below.

• Validate the compressed data taking mode of the Flash ADC, by reading out the FADC in the mode that provides time and pulse integral information for some run configuration (e.g. running with the LED monitoring system) and comparing that to a run in which the FADC is read out in raw mode and a software algorithm is used process the events.
• Test scaler mode of FADC: try to count the pulse rate of the internal pulser or monitoring system.
• Determine the FCAL occupancy as a function of the FADC pulse threshold. This will be related to the pedestal width and have implications on both the final event data size and sensitivity to small pulses.
• Examine the energy sum with bases off, bases on (no LED pulser), and bases on (with LED pulser). This will determine the baseline energy sum and suggest appropriate thresholds for triggering.
• Develop and test the CW-base control system. Necessary features:
  • Proper interlock and monitoring of environmental conditions in darkroom
  • Interface to EPICS control system
  • Simple interface for shift taking
  • Ability to read HV set points for each element from a configuration database
• Using the LED monitoring system perform an initial smoothing of the gains of the detector elements.
  • Make a reasonable assumption about the light intensity at the boundary. For example the intensity for elements on the outer edge may vary like the 1/distance from the edge of the Plexiglas pane and the intensity for elements around the beam line may be assumed to be constant.
  • Obtain an estimate of the light intensity for all elements subject to the boundary conditions above and the assumption that the intensity is smooth. (This can be done iteratively.)
  • From this light intensity determine a factor to multiply the gain for each element such the ratio of light intensity to pulse height is constant for all elements.
  • From PMT test data determine the fractional change in HV that produces this factor change in gain and update the HV constants.
  • This process may be iterated
  • It is anticipated that a final software gain correction will be needed for each element.
• Using the LED monitoring system determine the timing offsets for each element in the detector
  • For many events compare the average pulse time.
  • Determine a time offset for each block under the assumption that true time offsets for near-neighbors are zero.
  • Timing offsets will be HV dependent since the PMT transit time is significant. The timing offsets should be determined after the HV is adjusted to smooth gains.

Offline Calibration Constants and Sources

• Global alignment and element size: determined from survey data
• Channel gain: a multiplicative constant for each channel to smooth the gain that is determined after iterating on the HV set values
• Channel timing offset: an additive constant for each channel to equalize at high precision the times

Both the channel gain and timing offset can be determined from the LED monitoring system initially, as described above. Once we begin taking data with beam it will be necessary to reset these (and possibly adjust HV settings) based on the response of the detector to π⁰ events.