PHD Thesis Chapter 1 - DOC by asafwewe


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   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 project. 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 detector 36cm long,

9cm wide with 384 strips 228m spaced (2592 ladders, 10368 SSDs, 1 million readout
        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 test plan must guarantee 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 boardsare
expensive, customized, hard to replace parts. Thus, spare parts are 5%-10% only. In order
to avoid failures at the tower level, the test plan must verify the tolerances, the detection
performance and the compliance 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 pileup errors. The present document will demonstrate
     that a very good alignment precision of the parts does not guarantee the final result,
     unless there is a good assembly strategy.


   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 and daily
     monitoring 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 ESPI, we can determine stiffness and absence of small area
     defects of the tray’s structure in a complete and safe way.
   Design and production 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 assemble with extreme
     precision the very delicate payload at any 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
   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 quick and effective actions when problems are encountered.
   Coordination of the activities (4 industrial sites and 4 INFN labs all around Italy)
     ensuring everywhere the needed throughput to meet the LAT schedule (production
     of the silicon tracker in 1 year only).
    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
              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.
   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
   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 the testing
system and consider the reasons 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 known defects. The results of the ESPI tests over the produced trays are shown and
are analyzed with sensitivity studies performed by means of the FEM simulations.
   Chapter 5 describes the sidewalls construction process and the tests required for the
   Chapter 6 introduces the silicon detectors and their integration in ladders, It explains
the detector properties and the electrical performance; an overview about the results of
the mechanical and electrical tests 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. The 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 thanks to the INFN procedure. Moreover the CMM
measurements and the EMI (Electro Magnetic Interference) requirements are introduced,
alignment and functional test results of the flight production conclude this chapter.
   Chapters 9 describes the procedures for the tower vibration and thermo vacuum tests,
in conformity with the GEVS. Results are shortly described.
   Conclusions summarize the main achievements of this very successful activity.


Personal contributions

      I was involved in several main activities:
       Tray assembly: I upgraded the existing tools design, included the design of the
        tools for the Bottom tray and the Top tray assembly, then I also defined all CMM
        measurement procedures, test plan, and finalization of the procedures for all the
        assemblies and sub assemblies regarding 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 for the definition of the preliminary detailed tower assembly
        procedure and 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. The qualification of the ESPI test
        as a workmanship acceptance test for flight items, the relative procedure and
        documentations were my responsibility.
       Engineering Model activities: During the Engineering Model construction, I have
        been widely involved in Trays Vibration tests in Centrotecnica, in Thermal tests in
        Terni, in 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.
       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 optimization and trays logistic, and pushing the schedule,
        CMM testing, participating to the quality inspections, troubleshooting and root
        cause analysis, monitoring all the processes and the final acceptance.


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
    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.
                                   Milano                    Vibe tests on shaker of the trays

       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

                                  L’Aquila                   Production of a tray prototype

       Alenia                                               Vibe test and thermo vacuum tests of the
     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
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