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PHD-Thesis-Chapter-1
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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 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,
Introduction
9cm wide with 384 strips 228m 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 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.
4
Introduction
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
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 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
5
Introduction
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.
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 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
acceptance.
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.
6
Introduction
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.
7
Introduction
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.
8
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
9
Introduction
10
