BCAL Readout Segmentation Task group Meeting: Jun 23, 2011

Meeting Info

Time:

• 11:30 JLab
• 9:30 Regina

Location:

• CC F326-327

Vid-con:

• ESNet 8542554

Agenda

Review action items from previous meeting

• Review Andrei's proposed procedure for calculating sampling fraction functions (Elton)

  ?

• Plot Nshower per event for fine and course data samples (Andrei)

  Plot

• Send calibration scripts with instructions to David (Andrei)

  ?

• Implement time distribution of signals at photodetectors (David)

  See today's slides

• Verify noise term is zero in resolution fits (David)

  No progress

• Investigate effect of TRMS threshold used to split clusters in KLOE code (David)

  No Progress

New Agenda items

• BCAL Signal Timing Distributions (David L.)

Minutes

Participants

David L., Andrei S., Matt S.

Review action items from previous meeting

Last weeks action item list was reviewed. No progress on several of the items.

• Andrei showed two plots indicating that only about half of the events reconstructed with the course segmentation had only 1 shower. Most had more. In contrast, the fine segmentation resulted in most events having only 1 shower.
  • This needs to be understood in order to have any meaningful comparison between course and fine segmentation schemes
  • The cause of this may be related to the long standing splitoff issue.

• Andrei commented on the status of the scripts used to generate BCAL calibrations such as were presented last week.
  • He is currently adding comments and instructions so they may be used and understood by others
  • More automation is being added so dependence on things such as statistics will be removed where possible
  • System is being made more robust with warning messages when unusual conditions are encountered

BCAL Signal Timing Distributions

Dave presented some slides related to an ongoing effort to understand the details of the signal timing distribution at the photodetectors. The work concentrates on using DMCTrajectoryPoint objects which record details about every call to gustep. These are readily available without any modification of the existing code.

Slide 2

Slide showing the 12^o trajectory of particles and distribution of z-location of individual steps forming a shower.

• Matt noted that the BCAL should start at R=64.3cm whereas in the picture until close to R=75cm

Slides 4 & 5

These two slides indicated the z-projection of shower particle steps taken in the BCAL and that most of the energy is deposited in steps that are <2cm long.

• Andrei noted that GEANT can skip over an intermediate volume in a single step. While that particular problem tends to occur for very thin volumes, it may be worth checking the step's projection in the x-y plane to verify that it never steps over a cell
• Dave agreed to check this

Slide 6

• A back-of-envelope calculation was shown indicating that most steps should correspond to about a 200ps spread in time per step
• Andrei noted that nucleons may have a much lower β leading to a larger spread in time.
• Dave recalled negligible contribution to the shower from nucleons, but agreed to double check this as it is apparently inconsistent with the results of the detailed simulation
• A model of the electronic pulse shape was made having properties consistent with the specs for the BCAL SiPMs
  • Dave noted that the model was based on specs that were given as upper limits, but that it may not be at all accurate.
  • An e-mail has been sent to Fernando and we're waiting for a response

Slide 7

Plots of the signal distribution in time arriving at the upstream photodetectors were shown along with the results of convoluting the distributions with the electronic pulse shape modeled.

There was some disagreement on whether the shaping due to the electronics would improve or hurt the timing resolution. It was a brief academic discussion however as this will be tested directly by applying a threshold and looking at the jitter of the time at which the signal crosses the threshold.

• Andrei noted that the fluctuations in the signal will result in a shallower rise time for some pulses, skewing the time at which it crosses the threshold.
• The resolution of the discriminator will also contribute by projecting out larger time error for shallower slopes
• Dark hits are not yet included and may significantly change the timing distributions. (These will be included.)

Action Items

• Reproduce Nshower plots but with dark hits disabled (Andrei)
• Send calibration scripts with instructions to David (Andrei)
• Check on step size in X-Y plane and total step size to verify steps don't jump cells (David)
• Check proton and neutron contribution to BCAL showers (David)
• Continue signal timing distribution studies (David)