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A walk through the LAT Tracker towers alignment

A walk through the LAT Tracker towers alignment. Nicola Omodei INFN-Pisa. cucina italiana I – tower ingredients. 38 C-fiber facesheets. 576 SSD 55K channels 228 m m pitch. 19 Al honeycomb. 38 C-C MCM closeouts. 38 C-C structural closeouts. 36 Multi-Chip Modules. 4 C-fiber sidewalls.

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A walk through the LAT Tracker towers alignment

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  1. A walk through the LAT Tracker towers alignment Nicola Omodei INFN-Pisa

  2. cucina italiana I – tower ingredients 38 C-fiber facesheets • 576 SSD • 55K channels • 228 mm pitch 19 Al honeycomb 38 C-C MCM closeouts 38 C-C structural closeouts 36 Multi-Chip Modules 4 C-fiber sidewalls 8 kapton flex cables 36 kapton bias circuits ~ 1000 screws (several types) 192 3% X0 W tiles 64 12% X0 W tiles glue, paint, tape ……

  3. cucina italiana II the course and the flavour

  4. Silicon Strip Detectors – japanese devotion Alignment of strips wrt wafer cut: displacement of each cross from nominal distance is measured for both axis under the INFN microscope/CMM 8.95 cm SSD reference crosses Any displacement has an RMS ~ 2.5 mm Requirement of maximum displacement < 20mm comfortably met by each sensor 8.95 cm

  5. Ladders assembly and metrology • The excellent SSD alignment allow a simple and cost-effective approach to ladder construction: • silicon wafers are wet with glue on the edge and positioned in the assembly jig • each wafer is pushed against the adjacent wafer and against a reference wall machined to high precision • once in place the wafers are held in position with vacuum and cured at RT • No need for lengthy and expensive CMM alignment • 8 reference crosses position are measured and their displacement from the best-fit line is plotted • All RMS are < 2 mm and the maximum displacement is typically ~ 5mm

  6. Trays assembly strategy • Trays are made by gluing composite materials that cannot be machined to extremely high precision • The reference pins for aligning the tray to the assembly jig are redundant • the position of each pin is measured • for each tray the best 3 pins are chosen to define the tray position in the assembly jig reference frame • Ladders are referred to their assembly jig and handled with a tool that is in turn referred to the tray assembly jig Ladder handling tool tray Ladder assembly jig Positioning and gluing of ladders on tray tray assembly jig (reference frame)

  7. Flight tray metrology • The ladders displacement (X/Y/Z) on trays has an RMS ~ 20mm • SSD inside ladders are controlled to a much better precision • The silicon layer can be considered as a single, monolithic plane and its position in the tower can be controlled offline, at 1st order, with 3 parameters (Dx/y, DZ, Df)

  8. Tower assembly and metrology • Trays are slit in position with a crane and stacked into the assembly jig (top to bottom and alternatively rotated at 90o) • Each tray position is referred to the assembly jig (absolute reference frame)  displacements in XY or Z are averaged out and do not sum Grid simulator The complete tower is measured under the INFN CMM CMM head

  9. Tower metrology • Typical tower stay-clear verification • All towers are well within specs • Nominal position • Stay-clear (375 mm from nominal in all directions)

  10. Tower metrology with CR • Cosmic Rays can be used to study the trays alignment • Vertical shift, horizontal shift and rotation around z. • Cuts on events can be used to study the alignment of the single ladders: the results is that the silicon plane (4 ladders) is a monolithic structure (within 20µm): at this stage disalignments are only shifts and rotation of a tray wrt the others! • Iterative procedure: • Starting from a MC ideal geometry • One iteration. For each event: • Fit a reference track for each view (2 fits) • Cut on the chi square to reduce the cpu time (35% events selected) • For each plane, refit the track with clusters removed on both planes of the tray (36 fits)

  11. real position θ X4 X3 X2 X1 X0 ideal position res = x + z · cot(θ) horizontal displacement: 157m vertical displacement: 81m Residuals vs. slope Horizontal and vertical alignment • Aligns: • horizontal ( to strips) • vertical

  12. X Y, ideal position Y, real position rotation around z: 0.54mrad Residuals vs. position in other view A straight line fits well all 4 SSDs in all 4 ladders  all 4 ladders are well aligned  tracks do not see independent sensors

  13. Iterative alignment procedure 25 k 50 k 75 k 100 k ~ 2 hrs data taking MC geometry Aligned geometry • Minimizing the residuals: iteration procedure, till • Either the new is identical to the old geometry • Or the new geometry is identical to any in the sequence of geometries generated during the iteration (there can be periodic sequences of n geometries). • The precision reached depends on the number of events analyzed

  14. Coordinate shifts X Shipment to SLAC Sidewalls assy TV test Vibe test The tower jigs aligns the pins that have been used to align the ladders (the best reference) When the pins on the alignment wall of the tower jig are removed, the sidewalls partially realign the trays to their external dimensions with a max alignment loss ~0.1mm Sidewall removal

  15. Comparison between Pisa and SLAC runs No relevant differences before and after the shipment 20 µm

  16. Back to the ideal world of MonteCarlo Residual from real data after offline corrections: rms = 137 mm Residual distribution from MC data : rms = 124 mm • all effects from ladder, tray, tower integration are minimized through careful assembly • such misalignments are controlled in the offline with few, small parameters: 1728 constants for all 576 layers with 3 parameters each, rather than a maximum of 6 parameters for all 9216 SSD (>55K constants!) • what remains after the correction is very similar to MC data, where only intrinsic tracking fluctuations hold (single hit resolution and multiple scattering)

  17. Conclusion • Alignment and geometry extensively measured for all assembly and test phases • Many different components are integrated into the tracker: • the excellent intrinsic precision of base components (SSD) is fully exploited and maintained in higher level components (e.g. ladders) • assembly procedures overcome limitations from specific materials (composites, glue) and deliver integrated units (trays, towers) with monolithic performance (this is also valid for electrical properties) • The design, the precision of components and assembly is such that the relative small misalignments and rotations of layers in a tower can be controlled by simple offline corrections based on few parameters • The same performance homogeneity hold among all tracker towers and is crucial for a smooth integration with the other subsystems and a good calibration of the LAT, with positive impact on the science performance This is the same difference between 200ml of olive oil, 1 egg, 1 spoonful of lemon, salt and a good, creamy mayonnaise!

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