Minutes of the Tracker Technical Meeting August 9, 2000 Present: SLAC, UCSC, Hytec, Guido (at CERN) Taka presented some results on depletion voltage measurement on STM detectors, obtained from measurements of body capacitance versus bias voltage. See the attached JPEG files. He found a variation from 60V to 160V. There is no understanding of why the variation is so large. It possibly could be a systematic error in the measurement. An example CV curve is included for one detector. Also included is a plot of depletion voltage versus leakage current, showing a possible correlation. Taka also has measured the capacitance between AC and DC pads on 600 strips. None of the strips was found to be bad. Ossie gave the status of the strain-gauge setup. The computer needed for this is being used by Taka's STM detector measurements, so they are still about 1 week from getting it set up. They are also waiting on news about repairs to the environmental chamber. Roman reported that he is working on close-form models (a la the Hytec work) for analytical studies of the payload stack. He is also working on getting properties of the adhesives from vendors to put into the model. SLAC has a sample of Nusil CV1500, which is between silicone and epoxy in terms of shear modulus. Ossie has spec sheets for the AI-Tech compliant epoxy, which has a potential problem of having its glass transition temperature within our range (at - 20C). One 50g sample costs $250. He has also been going through the NASA list and looking at all the vendors of compliant epoxies. Ossie pointed out that we need to obtain samples of tungsten to begin working with. He agreed to order some. It was decided that a 97% W alloy is probably a good thing to start with, as the highly pure metal is likely to be brittle and hard to work. Masa announced that he has a writeup on ideas for a tracker database. It can be found at http://scipp.ucsc.edu/groups/glast/system/tracker_database2.pdf. He had two goals: to work toward a common terminology and set of conventions for the tracker layout and to make a beginning proposal for the actual database. Robert agreed to contact R. Dubois to understand what is going on with respect to official GLAST database tools and to contact D. Marsh about procedures that we will need to follow for handling flight hardware. The latter will be important as early as March, when we will be receiving the pre-production run of detectors. Gwelen reported on work that he has been doing to arrive at a new size for the front-end readout chip. This is needed because work is already in progress to make a new chip layout. First he calculated a new board size, assuming 8.95 cm detectors with 200 micron gaps between ladders. The extension of the board beyond the detector plane (800 microns on each side) was assumed to be the same as on the BTEM. This gives a board length of 360.20 mm. He then figured out the spacing needed between chips, taking into account the clearance needed for the wire bonder, a minimum wire-bond length of 1.5mm, and IPC standards for trace spacing (which is 0.4mm for the HV traces). For chips with no traces in between he assumed that same minimum space as in the BTEM (0.5 mm). He then distributed the chips evenly into the remaining space and came up with a chip size of 13.9 mm (compared with 11.65 mm in the BTEM). He also looked at the distribution of input pads along the front edge of the chip and concluded that a pad width of 120 microns could be used (compared with 100 microns in the BTEM). This would make wire bonding easier, especially for angled bonds. He would like the make the length of the pads 200 microns, to allow a second bond on unused metal in case that repairs are needed. There has been a lot of desire expressed for going back to 200 micron ladder spacing, for ease of assembly, and no strong objections were given. So we decided to fix the ladder spacing to 200 microns, as in the BTEM (the NASA proposal assumed 100 microns). Martin expressed an urgent need to fix the tracker readout-cable width. There are still some issues with that interface that might force going to a wider connector, and IPC standards need to be applied to the cable layout. Robert will discuss this with Dave Nelson. Decisions: 1. The gap between detector ladders is fixed at 200 microns. 2. Maximum front-end chip length: 13.9 mm (TBR). 3. The PC board length is 360.2 mm (TBR). Issues: 1. The tracker readout cable width needs to be determined asap. Action Items: 1. Gwelen: produce drawings of the chip layout and write up his work on chip and board size. 2. Robert: talk to Dave Nelson about finalizing the tracker electrical interface and fixing the cable width. 3. Robert: talk to Richard Dubois about the database. 4. Robert: talk to Daren Marsh about procedures required for flight hardware. 5. Ossie: order samples of tungsten foil for both thin (2.5%) and thick (25%) converters.