Nov 30, 2010 Calibration/Monitoring
Teleconference Time: 09:30 CST (Regina) / 17:30 GMT+2 (Athens)
- Direct Polycom-Polycom meeting
- Monitoring R&D issues for the relative gain calibration and monitoring system for the calorimeters and possibly other detector systems.
- Calibration Specifications Table for BCAL, FCAL, TOF and START
- Oct 26, 2010 Calibration/Monitoring minutes
- Nov 23, 2010 Calibration/Monitoring minutes
- Go over goals/objectives of Monitoring system and review past discussions.
- Discuss LED-fibre-light guide concept.
Attendees: Christina, George V., Pavlos (Athens), George L., Zisis (Regina)
- Linearity: Elton mentioned possible tests for the SiPM linearity.
- Each array has around 50,000 pixels which at 25% QE correspond to 200,000 photons. A significant loss of linearity will undoubtedly be associate with a serious degradation of the unit that should clearly manifest itself in its overall operation.
- Linearity should be tested on the bench, prior to installing SiPMs on the modules. After that, there is no need to monitor it, however, the LEDs can be driven at different biases if we so desire. Linearity checks will probably not be done well with only 3x or 4x the light anyway. Much more would be needed.
- Breadth of fibre spectrum would complicate extraction of linearity.
- We need to decide whether online monitoring of linearity is needed.
- Mechanical: Mechanical issues of the fibre concept are a serious concern in many areas:
- Mounting/installing. Careful calculations of fibre lengths are required. The radius of curvature of each fibre must stay fixed once installed or the light losses vary.
- Maintenance. Any breaks will be hard to replace. The light guide will need to be drilled in situ, hole cleaned and fibre epoxied. All in situ once the BCAL is installed.
- Cost is significant for either single fibres or fused ones with a single/common input and many (16) outputs.
- Illumination: Initial tests in Athens and Regina showed that illuminating both ends of a cell with either LED is not possible due to the large dynamic range; most likely one LED would illuminate the far SiPM, and vice versa. Another idea is to use a combination of UV and blue LEDs, the first to stimulate the fibres in the BCAL and illuminate the opposite side, the latter to illuminate the near side. This offers redundancy.
- Coupling: George V. compared an air gap up to 1.5mm between LG and PMT versus an optical Si cookie there. The Si cookie improves light collection by 15%. Useful, particularly for near-threshold light.
- Time frame: Athens is eager to move forward with a Construction MOU and the remaining R&D on LEDs types, board final design, etc.