GlueX TOF Meeting, September 15, 2011
- 1 Location, Time
- 2 Agenda
- 3 Communication
- 4 Minutes
Thursday, September 15, 2011
10:00 am EDT
JLab: CEBAF Center, Room F326
- Minutes from the last meeting
- Prototype Status
- Detector dimensions, light guide dimensions
- PMT base design, rates, currents, amplification
- Attempts to improve timing resolution 
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- FSU: Paul Eugenio, Sasha Ostrovidov
- JLab: Chuck Hutton, Mark Ito (chair), Simon Taylor, Tim Whitlatch, Beni Zihlmann
We talked about the meeting with Hamamatsu representatives scheduled for tomorrow here at JLab, mainly to discuss SiPM's. Paul has been talking to Andrew Allen of Hamamatsu who will be there and asked him some of the questions on current limits that we have been discussing. Beni and Mark will meet with the visitors over lunch to discuss these issues as well.
Review of minutes from last meeting
We went over the minutes from the September 1st meeting.
- Mark will circulate a new draft of the scintillator specifications this afternoon.
Detector dimensions, light guide dimensions
We discussed many of the issues that have been raised over the past few weeks. Chuck provided a set of drawings of the current design to guide discussion.
- Andrew Allen mentioned to Paul that we may be able to get the μ-metal shield extended beyond the photocathode, given the quantity we are ordering. Andrew is looking into it.
- Tim said there was still a couple of Gauss at the photocathode with the off-the-shelf dimensions.
- We decided to have a 1 cm straight section of the light guide where it joins to the scintillator. This helps with the non-square problem (see below) That way we can support both sides of the glue joint.
- If this new straight section is made too long, then the light guide cannot be machined out of the currently used Lucite cylinder.
- Tim reported that there are constraints on the geometry from the FCAL and the FCAL support structure. If the light guides could be made shorter, or be bent less, that would help.
- Paul remarked the the in-board edge of the iron sleeve can be trimmed (corner cut off) to help with the interference between neighboring PMT's.
- Tim reported that the iron shielding can be obtained with a smaller diameter and still accept the PMT's.
- Non-square problem: when the thickness of wrapping is included the array is no longer square and the ends of the scintillator would be inside the outside edge of the scintillators of the other plane. We will make the scintillator a bit longer (from 252 cm to 254 cm or so) to avoid this as well as adding a straight section to the light guides.
- Paul raised the possibility of needing extra Tedlar wrapping, beyond the current design, to insure a good light shield around the VM2000 wrapping should it be necessary to buy the VM2000 in small sheets. If that is the case the VM2000 would be in sections butted up against each other and each joint would constitute a thin spot in the light-tight-ening. The extra layer of Tedlar changes the total thickness of wrapping from 1.34 cm to 1.79 cm. We thought that an external layer on the upstream and downstream faces might suffice and avoid another Tedlar around each counter.
- We decided on a 3 mm gap between planes of the TOF to reduce stress on the mechanical design process.
- There is 2 cm between the TOF and FCAL presently. Tim suggested we open that up to 3 cm or so.
Tim and Chuck will work on a optimization of these parameters and of the light guide shape and send around a proposal. Tentatively we will have a special meeting, this Thursday at 9 am, with the mechanical folks at FSU to discuss the proposal.
Tim reminded us of where we are supposed to be, soon:
- PMT's procured in two weeks
- support structure, buy in the Spring
- by next May testing scintillators and light guides
- procure scintillator Nov. 2
This will be tough to make. We do not have a contract yet, so JLab may have to buy the scintillator if we want to keep on the schedule.
PMT base design, rates, currents, amplification
Beni pointed us to his wiki page where he studies the implications of the rate on the TOF on PMT lifetime. If he assumes a maximum current of 100 μA, then the inner three full-length counters on each side and the half-length counters will have to be run at lower gain and an amplifier used on the PMT output. In addition the voltage divider would have to be modified so allow sufficient voltage on the first few dynodes.
- Paul pointed out that the rates may have been calculated for 107 photon flux. If so we may be able to avoid the special arrangement until we go to full luminosity.
- There was some controversy about the number used as the constraint on the acceptable rate. We will ask the visitors from Hamamatsu more on this tomorrow.
Strange shape of Delta(t) distribution in the short trigger counters
Sasha presented recent work trying to understand the longitudinal position distribution in the trigger counters he is using in the cosmic test set up. See his page for details. He experimented with various discriminator thresholds and with back termination of the signal cables.