Minutes of the Tracker Technical Meeting October 11, 2000 Present: SLAC, Pisa, UCSC, Perugia, Trieste, Hytec Tom asked if Pisa had reviewed the MS Project schedule that he had sent. Sandro said that they could not open it, so Tom will send it again, this time in PDF format. Ossie reported that Si dummy detector wafers are waiting to get aluminized. The tungsten foils will clear purchasing today, for 5 mil (3.6% X0) and 18.5 mil (13.2% X0) foils. Hartmut said that a decision has been made by his committee to recommend 18% X0 foils for the back section, so it was decided that Ossie should stop the order and change the thick foils to that value. Ossie also reported that a ladder was constructed with 4 of the new Si wafers and will be used to check the straightness of the jig. Derrick reported that 3 strain gauges were mounted on one of the baby detectors of the UCSC test coupons and that the leakage current was the same after the procedure as before. He will be soldering on leads today. Eduardo asked Gwelen to send information on the leakage currents of those detectors as measured at UCSC. Gwelen requested that they set up the system at SLAC to monitor the leakage current separately in the two ladders. He will write up a test plan (temperature cycling ranges, ramp speed, etc.) and send it to SLAC. Gwelen reported on a little research he did to understand the turn-over of Roman's strain- measurement at high temperatures. He found from the manufacturer that the Scotchweld 2216 adhesive used to bond the Si to Al has a glass transition temperature of 13C, which could explain the turn-over and reduced strain at high temperature. Ossie commented that there may also be another explanation having to do with the instrumentation. Gwelen inferred from Roman's data that the stress at –70C was 35 ksi and at 50C only 3.5 ksi. That is to be compared with the average failure stress in the Hytec 3-point bend tests of about 30 ksi. Ossie said that he has learned that the glass transition temperature of epoxies is typically around the cure temperature, so that if we use a room-temperature cure (as we plan) then we can expect a room temperature glass transition. There was some discussion that this could be advantageous for reducing stress at the high temperature end and that, in any case, we don't want a transition near the upper end of our range (as that would allow a refreezing at that point and resulting higher stress at the low temperature end). There was some concern expressed about loss of alignment if the adhesive softens at the high end. It was decided that that is unlikely but would have to be tested. Guido remarked that for Agile they have had some success with bonding the tungsten to carbon-fiber using a silicon-based adhesive. No temperature cycling has yet been done, but when ready they will issue a report on this experience. Robert initiated a discussion on the tray dimensions. Erik said that Hytec is now working to the final dimensions, but he noted some disagreement between their drawings and BJ's grid drawings at the level of 0.5mm. He will send drawings to SLAC and UCSC. Pisa would also like to see them (to prepare their tray mockups). BJ will send his drawing of the hybrid dimensions to Hytec. Somebody asked about requirements on conductivity between the carbon-fiber and the aluminum core (i.e. will the adhesive leave the core floating?). Tom will look into the requirements. Guido will make a measurement on their tray prototypes. Robert said that Dave Nelson wants to allow the possibility of connecting the hybrid ground into the closeout. That could be done by way of a plate-through screw hole in the hybrid, aligned with an insert in the closeout (put in with conductive adhesive). Erik comments that it is urgent to start getting internal interface specification documents into place. Gwelen will work on writing up the specifications for the hybrid-tray interface. Sandro reported that the pitch-adapter prototype was a little delayed and will arrive next week. Hartmut has looked into the issues of radiation lengths in the tray material (see attachment). He reported a 0.1% X0 difference between using the IPC rules in the bias circuit versus a more aggressive design. There was some discussion of alternatives to Kapton for the bias circuit (FR4 or Kevlar). Ossie reported that we were reviewed by the safety oversight panel. They were happy overall with our manufacturing plan. One thing they commented on was to be careful not to make any large capacitors that could get charged up by the 100V bias. Nick reported on progress with electronics at Pisa. They now have the cable for their interface board and can communicate with the TEM. On the hybrid they found a bond broken on the controller chip, which is being repaired. Otherwise, they are ready to operate the GLAST electronics. They are ready to receive a full tray or two from UCSC. Wilko said that unstacking of the BTEM tower will begin today. Sandro reported that results from their thermal modeling work have been put on the web. They have first done a 2-D FEM analysis using ANSYS of the Si/Al configuration studied by Roman, assuming modulus and CTE parameters that are independent of temperature. The results agree well with Roman's measurements. Their next step will be to model in 2-D the full tray structure. Issues: 1. Glass transition temperature: will ladders maintain alignment at high temperature? 2. Material for bias circuit (Kapton, FR4, Kevlar). 3. Avoiding large capacitance in the mechanical structure, for safety reasons. Action Items: 1. Tom: send the schedule to Pisa in PDF format. 2. Ossie: change the Tungsten order to 18% X0 for the back layers. 3. Gwelen: send information on leakage currents of baby detectors to SLAC. 4. Gwelen: send a test plan for the baby detector ladders to SLAC. 5. Erik: send Hytec tray drawings to SLAC, UCSC, Pisa. 6. BJ: send hybrid dimensions and screw locations to Hytec. 7. Tom: look into requirements for electrical contact to the core. 8. Guido: measure electrical conductivity in Agile trays. 9. Gwelen: draft ICD for hybrid to tray interface.