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					Introduction


   The Gamma-ray Large Area Space Telescope (GLAST) is an international space
mission, scheduled for launch in 2007. It will study the cosmos in the energy range
10keV-300 GeV, the upper end of which is one of the poorly observed regions of the
celestial electromagnetic spectrum. GLAST will have an imaging gamma-ray telescope, -
the Large Area Telescope (LAT) - vastly more capable than the instruments previously
flown (factor 30 or more advance in sensitivity), as well as a secondary instrument to
augment the study of gamma-ray bursts.
   The GLAST experiment is a NASA mission with a strong contribution of the high-
energy physics community that has the responsibility of the project and of the
construction of the LAT. NASA team up with the U.S. Department of Energy and
institutions in France, Germany, Japan, Italy and Sweden. The LAT is composed by three
main subsystems: the Anti-Coincidence Detector (ACD), the Electro-magnetic
Calorimeter and the Silicon Tracker. The construction of the tracker is the result of the
collaboration among US (Stanford Linear Accelerator Centre- SLAC, Santa Cruz Institute
of Particle Physics – Santa Cruz Ca), Japan (University of Hiroshima) and Italy (Agenzia
Spaziale Italiana-ASI and Istituto Nazionale di Fisica Nucleare-INFN). The US
partnership has the responsibility of the mechanical project and it provides a fraction of
the silicon detectors and all the electrical components. Japan shares the silicon detectors
project and procurement. The Italian partnership procures part of the silicon sensors and it
is responsible for the whole integration and test of the tracker.
        The silicon tracker detects the gamma rays through their conversion in electron-
positron pairs and tracks their trajectories whose vertex points towards the incoming
direction of the photons. It is the largest tracker ever built for a space application and
constitutes one of the largest scale applications of the silicon strip technology (83m2 of
sensors). Modularity is the key word that drove its design/its design projetcproject. The
silicon tracker is composed by a 4X4 array of identical modules, which are the tracker
towers, plus a spare flight tower and a ground based calibration tower (18 towers). Each
tower is the stack-up of 19 trays closed by 4 carbon fiber sidewalls (342 trays). The trays
are square composite panels that support the silicon detectors and the relative readout
electronics. Four Silicon Strip Detectors (SSD) joint together form a ladder, a silicon strip
Introduction


detector 36cm long, 9cm wide with 384 strips 228m spaced (2592 ladders, 10368
SSDs, 1 million readout channels).
         The construction of the silicon tracker demands for several challenging
requirements. In order to achieve the best angular sensitivity, the relative SSDs alignment
must be of the strip pitch order. This implies very tight mechanical tolerances, of the
0.1mm order in dimensions, planarity and squareness of the trays. These are not
monolithic drilled metal pieces, but composite structures obtained by the gluing of
several parts with poor mechanical properties. The trays shape also determines the shape
of the towers, which have an allowed clearance of 0.35mm only.
         The trays and the sidewalls mechanical properties guarantee the survival of the
delicate equipment to the launch loads; moreover, all the sub-assemblies have to be in
compliance with the space environment conditions. These space payload environmental
requirements must be verified by a detailed test plan. In compliance with the NASA
GSFC (Goddard Space Flight Center) GEVS (General Environmental Verification
Specification), The the test plan must guarantee, in compliance with the NASA GSFC
(Goddard       Space   Flight   Center)   GEVS     (General    Environmental     Verification
Specification) , from the simpler part up to the tracker tower level, the survival of the
tracker to the launch loads and to the space environment conditions, from its simplest part
up to the tracker tower level. All the tracker parts, structural materials, detectors, and
electronic boards, are expensive, customized, hard to replace parts., tThus, the spare parts
are limited to 5%-10% only. In order Tto avoid failures at the tower level, the test plan
must verify the tolerances, the detection performance and the compliance to with the
environmental requirements of the space mission of 100% of the parts.
         The INFN laboratories alone could not address the large numbers involved in this
experiment and the quality requirements for a space production. The INFN took
advantage from the collaboration of a qualified pool of industries with a powerful
merging of the relative competences.
         Hereafter there is a summary of the main targets to be met in order to build the
silicon tracker:
        Definition of a construction strategy able to build the towers with the required
       tolerances, in a reliable way, and in time within the LAT schedule. We have added
       to the original design a set of mechanical references to the trays sides that drive all
       the assembly phases avoiding the errors pileup errors. The present document will
       demonstrate that    athat a very good alignment precision of the parts does not


                                               4
Introduction


       guarantee the final result, unless there is a good assembly strategywithout a good
       assembly strategy very good alignment precision of the parts does not guarantee the
       final result.
        Production of the composite panels and of the towers sidewalls with very tight
       tolerances. We designed the assembly molds and the assembly procedure to meet
       this requirement with high yield and reliability. Coordinate Measuring Machine
       (CMM) measurement of 100% of the parts with centesimal precision withand a daily
       monitoring accompanies accompany the whole production.
        Check and certification of each composite part manufacturing and of each bonding
       interface are required. The composite structures of the trays must be stiff enough to
       survive to the launch and must have no hidden defects that could result in a
       degradation of the SSDs performance in the space environment. Two instruments
       have been developed: 1) the constructor did not received the tray specs only, but
       also detailed agreed fabrication procedure 2) definition and implementation of a test
       plan, testing 100% of the gluing actions. In particular a custom Non Destructive
       Inspection (NDI) based on the Electron Speckle Pattern Interferometry (ESPI), ) –
       i.e. an optical computerized sub-micrometric inspection system, -has been developed
       and qualified. Thanks to By means of the ESPI, we can fully determine the stiffness
       and the absence of small area defects of the tray’s structure in a complete and
       securesafe way.
        Design and productions of the tools in the required number, definitions of the
       procedure and of the working conditions (class 100.00 clean rooms, Electro-Static
       Discharge protected procedure) ) in order to handle and to assemble with extreme
       precision the very delicate payload at all levelsany step of the process: SSDs
       handling and test, ladder assembly and test, ladders and electronics assembly onto
       the trays, payload completed trays test and shipping, tower assembly, test, handling
       and shipping.
        Storing of all the data in an INFN custom electronic database. The database and
       the accurate knowledge of the procedure allow the complete monitor of the
       production, and fast quick and effective actions /response(?) when problems are
       encountered.
        Coordination of the activities (4 industrial sites and 4 INFN labs all around Italy)
       assuring ensuring everywhere the needed throughput to meet the LAT schedule
       (production of the silicon tracker in 1 year only).

                                                5
Introduction


   . This thesis expounds in details all the construction phases of the flight LAT silicon
   tracker, with particular emphasis on the mechanical aspects. The qualification program
   (a non functional Engineering Model tower, the EM tower, and a reduced scale
   functional tower with 4 trays only, the min-tower) and the INFN contributions to
   identify the final solutions are mentioned
   The main Industrial Partners involved in the tracker construction and test, are:
              PLYFORM s.r.l. Varallo Pombia (NO): tray mechanical structure and tower
               sidewalls.
              G&A Engineering s.r.l Oricola (AQ): ladder assembly, ladder and electronics
               implementation over the trays
              MIPOT s.r.l., Cormons (GO): ladder assembly;
              Alenia Spazio, Roma: towers space environmental (vibe and thermo-vacuum)
               test facilities
   These industries are ISO9001 certified. The involved INFN laboratories (Bari, Pisa,
Perugia-Terni, Roma II) developed and put in place a computerized Quality Assurance
system with effective tracking of all the items and with registering and management of
the Non Conformities Reports (NCR).
   The section below represents an abstract for this thesis. Hereafter there is the plan of
the thesis with the contents of each chapter
   Chapter 1 briefly describes the architecture of the GLAST Satellite, its functional
concept, the gamma-ray astronomy history and the Italian activities in the tracker
construction.
   Chapter         2 describes the general working conditions and quality assurance
implementation for the construction and test of the flight parts.
   Chapter 3 describes the assembly of the mechanical trays, the main issues faced during
the flight production, the manufacturing procedure and the mechanical and metrological
tests performed on the trays and the results are shown.
   Chapter 4 describes the ESPI testing system, that is the NDI system developed for the
tray stiffness evaluation and the bonding defects detection. It introduces tThe testing
system is introduced, withand the consider the reasons considerations that drove to this
choice. This new NDI system has been qualified with Finite Element Models (FEM)
analyses and experimental tests on a tray with know (known) defects. The results of the
ESPI tests over the produced trays are shown and are analyzed with sensitivity studies
performed by means of/via the of FEM simulations.



                                                6
Introduction


   Chapter 5 describes the sidewalls construction process and the tests required for the
acceptance..
   Chapter 6 introduces the silicon detectors and their integration in ladders,          It
explaining explains the detector properties and the electrical performance; an overview
about the results of the mechanical and the electrical tests are is given.
   Chapter 7 describes the integration of the detectors and of the electronics onto the
mechanical trays, the tools designed for the detectors and the electronics handling, and
the integration procedure. explaining in detail tThe several assembly steps are illustrated
in details.


   Chapter 8 describes the tower assembly strategies, and the assembly and handling
tools. A brief introduction is given about the preliminary approach for the trays stacking
and the improvements obtained with thanks to the INFN procedure. Moreover the CMM
measurements and the EMI (Electro Magnetic Interference) requirements are introduced,
ending with some alignment and functional test results of the flight production are at the
end of the chapter/ conclude this chapter.r….
   Chapters 9 describes the procedures for the tower vibration and thermo vacuum tests,
in conformity with the GEVS, . with a short presentation of the rResults are shortly
described.
   The cConclusions summarize the main achievements of this very successful activity.




                                              7
Introduction



Personal contributions.



      I was involved in several main activities:
      Da un punto di vista formale puoi anche andare a capo ed eliminare I due punti,
oppure togliere la maiuscola e mantenere I due punti.
       Tray assembly: I did the complete upgrade upgraded of the existing tools design,
        included the design of the tools for the Bottom tray and the Top tray assembly, then
        I also    also definition defined of all the CMM measurement procedures, the
        definition of the test plan, the definition and finalization of the procedures for all
        the assemblies and sub assemblies regarding the flight tray and sidewalls
        production.
       Ladder assembly and electronic integration over the tray: I upgraded the prototype
        tools and I did the engineered design of the tools for positioning and bonding the
        ladders over the trays, the tools for the ladder alignment, and the others handling
        and service tools.
       Tower assembly: I designed the tool to assembly the tower and the tool to rotate it;
        I participated to the EM tower and to the mini tower assembly exercises which have
        been the roadmap to define for the definition of the preliminary detailed tower
        assembly procedure and to define the required upgrades of the tools.
       ESPI: I researched and developed a NDI system for the mechanical tray testing that
        has been a challenging and successfully activity. I was the responsible for tThe
        qualification of the ESPI test as a workmanship acceptance test for flight items, the
        relative procedure and documentations were my responsibility. and I wrote the
        relative procedure.
       Engineering Model activities: During the Engineering Model construction, I have
        been widely involved in the trays Trays vibration Vibration tests in Centrotecnica,
        in the thermal Thermal (termali?) tests in Terni, in the Vibration and Thermo
        vacuum tests of the EM tower in Alenia, and in the preliminary design activities
        related to the facility MGSE tooling design. These activities tested and qualified all
        the space environmental tests on flight items.


                                                   8
Introduction


     Flight trays and sidewalls production: I lead the flight trays and sidewalls
      production, more than 100 missions (170 days) spent in the Plyform branch. I was
      in charge of managing managing the optimization and, the trays logistic, and
      pushing the schedule, CMM testing, participating to the quality inspections/
      controls(?), troubleshooting and root cause analysis, monitoring all the processes
      and the final acceptance.




                                           9
Introduction


   Table 1. List of the collaborations with external institutes and industries

                                                              Construction of all the composite parts of
      Plyform               Varallo Pombia (NO)
                                                         the tracker (trays and sidewalls)


       G&A                                                   Ladders assembly and payload assembly,
                                Oricola (AQ)
  Engineering                                            construction of all the precision tools.

     Galli &                                                 Trays inserts machining and mechanical
                                   Lucca
    Morelli                                              tools.
     Iacomelli                                               Water jet cut of the Tungsten tiles
                            Massa e Cozzile (PT)
Metalmeccanica                                           alignment tool.

                                                             Water jet cut and electron erosion cut of
       B.L.G.                  Vergiate (VA)
                                                         the Tungsten tiles

    AlphaSinerg                                             Machining and CMM measurement of the
                                  Rho (MI)
       y                                                 Carbon-Carbon closeouts.
    Centrotecnic
                                   Milano                    Vibe tests on shaker of the trays
       a

       ENEA                    Frascati (RM)                 Development of the tray ESPI test


     Metalscan             Saint Remy (Francia)              Development of the tray ultrasonic test


      M.i.p.o.t.               Cormons (GO)                  Ladders assembly

                                                            Institute responsible for the delivery of the
      S.L.A.C.          Palo Alto, California (U.S.A.)
                                                         LAt to NASA

       Hytec          Los Alamos, New Mexico (U.S.A.)        Project of the silicon towers mechanics

       Alenia
                                  L’Aquila                   Production of a tray prototype
     Spazio

       Alenia                                               Vibe test and thermo vacuum tests of the
                                   Roma
     Spazio                                              EM tower and of the flight towers

     Universita’                                             Thermo vacuum test of the mechanical
                            Roma – Tor Vergata
   di Roma II                                            trays

      Gramoni               Varallo Pombia (NO)              Trays final machining

     Sede INFN
di Perugia presso
  Università dei                    Terni                    Tray thermal cycles in climatic chambers
   Materiali di
      Terni




                                                   10
Introduction




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