P UBLISHED BY IOP P UBLISHING AND SISSAR ECEIVED : April 11, 2008
ACCEPTED : October 7, 2008
P UBLISHED : October 30, 2008

The integration and engineering of the ATLAS
SemiConductor Tracker Barrel–1–2008 JINST 3 P10006

A. Abdesselam,w P.P. Allport,n C. Anastopoulos,ac B. Anderson,ah L. Andricek,sF. Anghinolfi,g R. Apsimon,aa T. Atkinson,r D.J. Attree,ah N. Austin,n A. Bangert,sG. Barbier,k P. Barclay,aa A.J. Barr,w L.E. Batchelor,aa R.L. Bates,al J.R. Batley, f
G.A. Beck, p P.J. Bell,q W.H. Bell,al A. Belymam, p J. Benes,z P. Benes,z J. Bernabeu,a f
S. Bethke,s J.P. Bizzell,aa J. Blocki,i J. Bohm,x C.N. Booth,ac E.V. Bouhova-Thacker,mO. Brandt,w T.J. Brodbeck,m Z. Broklova,y J. Broz,y P.A. Bruckman de Renstrom,w,1
S. Burdin,n C.M. Buttar,al J.M. Butterworth,ah E. Capocci,aa C. Carpentieri, j
A.A. Carter, p J. R. Carter, f A. Catinaccio,g J.R. Catmore,m M. Chamizo llatas,kD.G. Charlton,d A. Cheplakov,al A. Chilingarov,m S. Chouridou,ab D. Chren,zM.L. Chu,ad V. Cindro,o A. Ciocio,b J.V. Civera,a f A. Clark,k A.P. Colijn,a j
M.J. Costa,a f D. Costanzo,ac J. Cox,w C. Dabinett,aa W. Dabrowski,h J. Dalmau, p
K.M. Danielsen,v S. D’Auria,al I. Dawson,ac P. de Jong,a j M.D. Dehchar,w
B. Demirköz,w P. Dervan,n S. Diez Cornell,a f S.D. Dixon,ac E. Dobson,w Z. Dolezal,yM. Donega,k M. D’Onofrio,k O. Dorholt,v M. Doubrava,z J.D. Dowell,d Z. Drasal,yI.P. Duerdoth,q R. Duxfield,ac M. Dwuznik,h S. Eckert, j L.M. Eklund,g R. Ely,bC. Escobar,a f V. Fadeyev,ab D. Fasching,ag F. Fawzi,r L. Feld, j D. Ferguson,ag
P. Ferrari,g D. Ferrere,k J. Fopma,w P. Ford,aa R. Fortin,g J.M. Foster,q H. Fox, jT.J. Fraser,ah J. Freestone,q R.S. French,ac J. Fuster,a f S. Gadomski,k B.J. Gallop,aa
M. Galuska,z F.C. Gannaway, p C. Garcia,a f J.E. Garcia Navarro,a f N. Ghodbane,sM.D. Gibson,aa S.M. Gibson,w T. Göttfert,s S. Gonzalez,ag S. Gonzalez-Sevilla,a f
M.J. Goodrick, f G. Gorfine,ak A. Gorišek,o E. Gornicki,i A. Greenall,n D. Greenfield,aa
A.A. Grillo,ab J. Grosse-Knetter,g C. Haber,b R. Härtel,s K. Hanagaki,uT. Hansl-Kozanecka,ab K. Hara,ae M. Harris,aa F.G. Hartjes,a j D. Hauff,s B. M. Hawes,wT. Hayler,aa S.J. Haywood,aa F.E.W. Heinemann,w R.C.W. Henderson,m N.P. Hessey,a j
A. Hicheur,aa J.C. Hill, f M.C. Hodgkinson,ac P. Hodgson,ac T.I. Hollins,d A. Holmes,w
R. Holt,aa T. Holy,z T. Horazdovsky,z S. Hou,ad D.F. Howell,w G. Hughes,m T. Huse ,v
M. Ibbotson,q Y. Ikegami,l C. Issever,w K. Jakobs, j J. Jakubek,z R.C. Jared,ag
P. Jarron,g L.G. Johansen,a P. Johansson,ac A. Jones,aa M. Jones,w R.W.L. Jones,mT.J. Jones,n T.W. Jones,ah D. Joos, j J. Joseph,ag P. Jovanovic,d J. Jusko,z O. Jusko,zJ. Kaplon,g M. Karagöz Ünel,w V. Kartvelishvili,m N. Kerschen,ac C. Ketterer, j
A.G. Kholodenko,ai S.H. Kim,ae S. Kluth,s P. Kodys,y E. Koffeman,a j Z. Kohout,zT. Kohriki,l T. Kondo,l S. Koperny,h V. Koukol,z V. Kral,z G. Kramberger,o P. Kubik,ya University of Bergen, Department for Physics and Technology,

Allegaten 55, NO - 5007 Bergen, Norway
Lawrence Berkeley National Laboratory and University of California, Physics Division,
MS50B-6227, 1 Cyclotron Road, Berkeley, CA 94720, United States of Americac INFN Genova and Università di Genova, Dipartimento di Fisica,
via Dodecaneso 33, IT - 16146 Genova, Italyd School of Physics and Astronomy, University of Birmingham,
Edgbaston, Birmingham B15 2TT, United Kingdom
e Kyoto University of Education, 1 Fukakusa, Fujimori,fushimi-ku, Kyoto-shi, JP - Kyoto 612-8522, Japanf Cavendish Laboratory, University of Cambridge,
J J Thomson Avenue, Cambridge CB3 0HE, United Kingdomg CERN, CH - 1211 Geneva 23, Switzerlandb–2–

2008 JINST 3 P10006

N. Kundu,w C. Lacasta,a f V.R. Lacuesta,a f W. Lau,w S.-C. Lee,ad R.P. Lefevre,kK.J.C. Leney,n C.G. Lester, f Z. Liang,ad M. Limper,a j S.W. Lindsay,n V. Linhart,zA.J. Lintern,aa G. Llosa Llacer,a f C. Lockett,aa F.K. Loebinger,q M. Lozano Fantoba,a f
I. Ludwig, j J. Ludwig, j G. Lutz,s J. Lynn,w M. Maaßen, j D. Macina,k A. Macpherson,gC. Macwaters,aa C.A. Magrath,a j P. Malecki,i I. Mandić,o M. Mangin-Brinet,k S. Marti i
Garcia,a f G.F. Martinez-McKinney,ab T. Maruyama,ae J. Matheson,aa S.J. McMahon,aa
T.J. McMahon,d J. Meinhardt, j B.R. Mellado Garcia,ag I. Messmer, j B. Mikulec,kM. Mikuž,o S. Mima,t M. Minano,a f J. Mistry, p V.A. Mitsou,a f P. Modesto,a f S. Moed,kB. Mohn,a R.M. Moles Valls,a f G. F. Moorhead,r J. Morin, p A.K. Morley,rM-C. Morone,k J. Morris, p M.C. Morrissey,aa H.G. Moser,s A. Moszczynski,i
A.J.M. Muijs,a j W.J. Murray,aa K. Nagai,z Y. Nagai,ae D. Naito,t K. Nakamura,aeI. Nakano,t C. Nelson,aa A. Nichols,aa R. Nicholson,ac R. B. Nickerson,w R. Nisius,sM. Olcese,c M. Olivo Gomez,s V. O’Shea,al B. Ottewell,w O. Oye,a E. Paganis,acM.J. Palmer, f M.A. Parker, f U. Parzefall, j S. Pataraia,s J.R. Pater,q G. Pellegrini,a f
H. Pernegger,g∗ E. Perrin,k A.W. Phillips, f P.W. Phillips,aa K. Poltorak,h S. Pospisil,zT. Pritchard,g K. Prokofiev,ac P.N. Ratoff,m P. Reznicek,y V.N. Riadovikov,ai
R.H. Richter,s A. Robichaud-Veronneau,k D. Robinson, f S. Roe,g K. Runge, j
H.F. Sadrozinski,ab J. Sanchez,a f H. Sandaker ,a J. Santander,a f D. Scheirich,yJ. Schieck,s K. Sedlak,w A. Seiden,ab A. Sfyrla,k T. Slavicek,z T.J. Sloan,m B. Smith,aa
M. Smizanska,m S.W. Snow,q M. Solar,z A.O. Solberg,a B. Sopko,z V. Sopko,zL. Sospedra Suay,a f E. Spencer,ab H. Spieler,b E. Stanecka,i S. Stapnes,v J. Stastny,xI. Stekl,z M. Stodulski,i A. Stradling,ag B. Stugu,a P. Sutcliffe,n R.R. Szczygiel,iR. Takashima,e R. Tanaka,t G.J. Tappern,aa J. Tarrant,aa G. N. Taylor,r P.K. Teng,adS. Terada,l R.J. Thompson,q M. Titov, j D.R. Tovey,ac S.N. Tovey,r A. Tricoli,aa
M. Turala,i M. Tyndel,aa F. Ukegawa,ae M. Ullan Comes,a f Y. Unno,l V. Vacek,zS. Valkar,y E. Van der Kraaij,a j T. Vickey,ag G.H.A. Viehhauser,w E.G. Villani,aa
A.P. Vorobiev,ai J. H. Vossebeld,n V. Vrba,x T. Vu Anh,k R.S. Wallny,g C.P. Ward, f
R. Wastie,w P. Webb,aa M. Webel, j M. Weber,aa A. R. Weidberg,w P.M. Weilhammer,gC. Weiser, j P.S. Wells,g P. Werneke,a j M.J. White, f M. Wiesmann,s I. Wilhelm,yI. Wilmut,aa J.A. Wilson,d M.W. Wolter,i S. L. Wu,ag H.Z. Zhuac and A. Zseneikh Faculty of Physics and Applied Computer Science of the AGH-University of Science andTechnology, (FPACS, AGH-UST), al. Mickiewicza 30, PL-30059 Kraków, Poland
The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences,ul. Radzikowskiego 152, PL - 31342 Kraków, Poland
j Physikalisches Institut, Universitaet Freiburg,
Hermann-Herder Str. 3, D - 79104 Freiburg i.Br., Germanyk Université de Genève, Section de Physique,
24 rue Ernest Ansermet, CH - 1211 Genève 4, Switzerlandl KEK, High Energy Accelerator Research Organization,
1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japanm Physics Department, Lancaster University, Lancaster LA1 4YB, United Kingdom
n Oliver Lodge Laboratory, University of Liverpool,
P.O. Box 147, Oxford Street, Liverpool L69 3BX, United Kingdomo Jožef Stefan Institute and Department of Physics, University of Ljubljana,
SI-1001 Ljubljana, Slovenia
p Department of Physics, Queen Mary, University of London,
Mile End Road, London E1 4NS, United Kingdom
q School of Physics and Astronomy, University of Manchester,Manchester M13 9PL, United KingdomrSchool of Physics, University of Melbourne, Parkville, Victoria 3010, Australias Max-Planck-Institut für Physik, (Werner-Heisenberg-Institut),
Föhringer Ring 6, 80805 München, Germanyt Okayama University, Faculty of Science, Tsushimanaka 3-1-1, Okayama 700-8530, Japanu Graduate School of Science, Osaka University,
Machikaneyama-machi 1-1, Toyonaka, Osaka 560-0043, Japanv Department of Physics, University of Oslo, Blindern, NO - 0316 Oslo 3, Norway
w Department of Physics, Oxford University, Denys Wilkinson Building,
Keble Road, Oxford OX1 3RH, United Kingdom
x Institute of Physics, Academy of Sciences of the Czech Republic,
Na Slovance 2, CZ - 18221 Praha 8, Czech Republic
y Charles University in Prague, Faculty of Mathematics and Physics, Institute of Particle andNuclear Physics, V Holesovickach 2, CZ - 18000 Praha 8, Czech Republicz Czech Technical University in Prague, Zikova 4, CZ - 166 35 Praha 6, Czech Republicaa Rutherford Appleton Laboratory, Science and Technology Facilities Council, Harwell Scienceand Innovation Campus, Didcot OX11 0QX, United Kingdom
ab University of California Santa Cruz, Santa Cruz Institute for Particle Physics (SCIPP),
Santa Cruz, CA 95064, U.S.A.
ac University of Sheffield, Department of Physics & Astronomy,
Hounsfield Road, Sheffield S3 7RH, United Kingdom
ad Insitute of Physics, Academia Sinica, Taipei 11529, Taiwan
ae University of Tsukuba, Institute of Pure and Applied Sciences,1-1-1 Tennoudai, Tsukuba-shi, Ibaraki 305-8571, Japana f Instituto de Fisica Corpuscular (IFIC), Centro Mixt Univ. de Valencia - CSIC, Apdo. 22085, ES- 46071 Valencia; and Instituto de Microelectronica de Barcelona (IMB-CNM), Dept. de Fisica
Atomica, Molecular y Nuclear, Univ. de Valencia, Spaini2008 JINST 3 P10006–3–

ag University of Wisconsin, Department of Physics, Madison, WI 53706, United States of America
ah Department of Physics and Astronomy, University College London, United Kingdom
ai

Institute for High Energy Physics, Protvino, Russia

a j NIKHEF, Amsterdam, The Netherlands
ak Bergische Universität Wuppertal, Wuppertal, Germany
al University of Glasgow, Department of Physics and Astronomy,
1

Glasgow G12 8QQ, United Kingdom
On leave of absence from Cracow
E-mail: heinz.pernegger@cern.ch

K EYWORDS : Particle tracking detectors; Solid state detectors; Detector design and construction
technologies and materials; Large detector systems for particle and astroparticle physics.

∗ Corresponding author.

c 2008 IOP Publishing Ltd and SISSA

http://www.iop.org/EJ/jinst/

2008 JINST 3 P10006

A BSTRACT: The ATLAS SemiConductor Tracker (SCT) was built in three sections: a barrel andtwo end-caps. This paper describes the design, construction and final integration of the barrel
section. The barrel is constructed around four nested cylinders that provide a stable and accurate
support structure for the 2112 silicon modules and their associated services. The emphasis of thispaper is directed at the aspects of engineering design that turned a concept into a fully-functioning
detector, as well as the integration and testing of large sub-sections of the final SCT barrel detector.
The paper follows the chronology of the construction. The main steps of the assembly are described with the results of intermediate tests. The barrel service components were developed
and fabricated in parallel so that a flow of detector modules, cooling loops, opto-harnesses andFrequency-Scanning-Interferometry (FSI) alignment structures could be assembled onto the fourcylinders. Once finished, each cylinder was conveyed to the next site for the mounting of modules
to form a complete single barrel. Extensive electrical and thermal function tests were carried out on
the completed single barrels. In the next stage, the four single barrels and thermal enclosures werecombined into the complete SCT barrel detector so that it could be integrated with the Transition
Radiation Tracker (TRT) barrel to form the central part of the ATLAS inner detector. Finally, thecompleted SCT barrel was tested together with the TRT barrel in noise tests and using cosmic rays.

Contents
Introduction

2

2.

Material budget and barrel radiation length contributions

5

3.

Assembly of single barrels
3.1 Barrel cylinders and module support
3.1.1 Design of the barrel cylinder
3.1.2 Manufacturing of the cylinders
3.1.3 Preparation of cylinders and measurements
3.1.4 Design and integration of module supports on cylinder
3.2 Barrel cooling
3.2.1 Description of the SCT cooling system
3.2.2 Design of the cooling-loops
3.2.3 Production of cooling-loops, capillaries and interconnects
3.3 Power supply and readout services
3.3.1 Electrical and optical services
3.3.2 Mechanical and thermal interfaces
3.4 Integration of harnesses and cooling-loops
3.4.1 Harness to barrel integration
3.4.2 Cooling-loop to barrel integration
3.4.3 Design of the cooling-loop to bulkhead connection
3.4.4 Design and production of the heat exchangers
3.5 integration of the FSI system
3.6 Module to barrel assembly
3.7 Transport of the barrels
3.7.1 Requirements
3.7.2 Designs of the transportation containers
3.7.3 Transport
3.8 Barrel thermal enclosure and grounding
3.8.1 Overview and requirements
3.8.2 Outer thermal enclosure
3.8.3 Side panels
3.8.4 Inner thermal enclosure cylinder
3.8.5 Bulkhead
3.8.6 End panels and heaters
3.8.7 Grounding and shielding
3.8.8 Production techniques and experience

8
9
9
9
10
11
14
14
17
17
19
19
19
20
20
21
21
22
22
24
26
26
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26
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27
28
30
30
31
31
32
33

–1–

2008 JINST 3 P10006

1.

Assembly of the four barrels and integration to TRT
4.1 Four-barrel assembly
4.1.1 Description of barrel assembly
4.1.2 ESD precautions
4.1.3 Integration with the thermal enclosure and sealing
4.1.4 Description of the procedure
4.1.5 Barrel interlinks
4.1.6 The Intermediate Service Support Structure (ISSS)
4.1.7 Tests done during four-barrel assembly
4.1.8 Photogrammetry survey
4.2 Integration into the TRT
4.2.1 Support requirements of the SCT inside the TRT
4.2.2 Integration procedure
4.2.3 Relative alignment between SCT and TRT

33
33
33
34
34
34
37
39
40
41
41
41
41
43

5.

Tests of single barrels
5.1 Test setup for single barrels
5.1.1 Detector control system
5.1.2 Detector readout system
5.1.3 Test procedure
5.2 Results of electrical tests on single barrels
5.3 Results of thermal tests on single barrels

44
44
45
46
46
47
49

6.

Final combined tests before installation
6.1 Operation of the SCT inside the TRT
6.2 Results of barrel calibration tests inside the TRT
6.3 Noise and cosmic-ray tests with the TRT
6.3.1 Noise tests
6.3.2 Cosmic-ray tests

52
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57
58
58

7.

Summary60

A. Acronyms

61

1. Introduction
The ATLAS [1] SemiConductor Tracker (SCT) is part of the ATLAS inner detector system [2,3], which provides charged-particle tracking in the centre of the ATLAS experiment. The innerdetector system (ID) consists of a hybrid-pixel detector in its centre, followed by the SCT silicon
strip detector and the gas straw-tube transition radiation tracker (TRT) surrounding the SCT. A
typical track will generate three measurements in the pixel detector, traverse eight silicon strip–2–2008 JINST 3 P10006

4.

Table 1. Layout parameters of the SCT barrel detector.

34.4
4 barrels
3.2
2112
23
17
580
|η | .1.3
299 < R < 514

detectors (to give four space-points) and 36 straw tubes. The ID sub-detectors act together toprovide tracking of charged particles for momentum measurement as well as primary and secondary
vertex reconstruction in a pseudorapidity range of |η | < 2.5.1
The entire tracker is surrounded by a solenoid, which provides a magnetic field of 2 T. The
SCT consists of four concentric barrels for tracking at mid-rapidity and two end-caps which extend
tracking to the forward region. Each end-cap is comprised of nine disks surrounding the beam axis.
The overall layout parameters for the barrel are summarised in table 1, with a schematic drawingof the ATLAS inner detector showing the SCT barrels and end-caps in figure 1. The basic building
blocks of the SCT are the silicon strip modules for barrel and end-caps [4, 5]. Each barrel module
consists of four silicon sensors [6]: two sensors on each side are daisy-chained to give 768 strips
of approximately 12 cm length and with a strip pitch of 80 µ m. A second pair of identical sensors
is glued back-to-back to the first pair at a stereo angle of 40 mrad around a thermo-mechanical baseboard [7] to provide space-points. The module strips are read out by 12 radiation-hard ABCD3TA
front-end readout chips [8]. The readout ASIC’s are integrated on a multi-layer Cu-polyimide flex
circuit [9] and are connected to the silicon strips by wire bonding. The flex circuit is mounted
above the silicon sensors of the barrel modules and at the end of the silicon sensors of the endcap modules. The hit information is “binary”: only the channel address of a hit strip is read out
optically from each module side to the counting room; no analogue information of the signal isprovided during normal running. The intrinsic spatial resolution in the sensor plane orthogonal
to the strip direction is 23 µ m.2 Using the hit information from each module side, a space-point
is constructed. The effective design measurement precision of the space-point measured per SCT
layer is 17 µ m in the Rφ -direction and 580 µ m along the z-direction. The silicon sensors aremaintained at a temperature around −7◦ C to minimise the effect of radiation damage. Mechanicaland thermal performance parameters of barrel modules are summarised in [4].
The support structure of the SCT barrel consists of four concentric carbon-fibre reinforced
plastic (CFRP) light-weight sandwich cylinders with integrated flanges at both ends. The fourcylinders have 32, 40, 48 and 56 rows of 12 modules for barrels 3, 4, 5 and 6 respectively, withbarrel 3 being the innermost barrel and barrel 6 the outermost. The modules are fastened directly
1 The pseudorapidity is defined as η = − ln [tan(θ /2)], where θ denote the relative angle to the beam axis.
2 The intrinsic resolution with binary readout is given through the strip pitch divided byover this intrinsic resolution can be expected for hits with charge sharing.

–3–√
12. A slight improvement

2008 JINST 3 P10006

silicon area [m2 ]number of layers
number of channel [106 ]number of modules
intrinsic design resolution in sensor plane [µ m]effective space-point Rφ resolution [µ m]effective space-point z resolution [µ m]
pseudorapidity coverageradial coverage [mm]

via special brackets to machined pads bonded to the outside surface of the cylinder. The cylinders
also offer support for the cooling circuits (see section 3.2), the harnesses servicing each module(see section 3.3) and the frequency scanning interferometry (FSI) position-monitoring system (seesection 3.5). The full SCT barrel is enclosed in a thermal enclosure (TE) (see section 3.8), which
separates the cold, ambient environment of the SCT from the room-temperature environment of the
TRT. The TE also acts as a Faraday cage for the SCT barrel.
Every component of the SCT has to comply with stringent radiation hardness requirements.
For the expected 10 years’ lifetime for silicon components, this is estimated to be 2×1014 1-MeV
equivalent neutrons/cm2 for non-ionising energy loss (NIEL) and 100 kGy(Si) of ionising energy
loss. As the non-ionising energy loss is dominated by charged hadrons (pions and protons) at
the radii of the SCT, many radiation-hardness tests were carried out at the CERN PS accelerator
up to a fluence of 3×1014 24-GeV protons/cm2 . Furthermore, all components are required to be
non-magnetic as the SCT is located in a 2 T magnetic field.
The integration of the SCT barrel detector from single modules to the completed detector readyfor installation within the ATLAS experiment has the following major steps:
1. Preparation of the four barrel support cylinders.
2. Assembly of services for cooling, FSI, powering and readout to the barrel support cylinder.
3. Mounting of modules to each barrel.

–4–2008 JINST 3 P10006

Figure 1. Schematic drawing of one of the quadrants of the ATLAS inner detector and SCT barrel andend-cap detectors.

4. Test of each complete single barrel.
5. Assembly of the four single barrels into each other and the thermal enclosure (TE).
6. Insertion of the completed SCT barrel into the TRT barrel.
7. Final test of the SCT barrel inside the TRT barrel and a combined test SCT + TRT withcosmic rays.

2. Material budget and barrel radiation length contributions
The performance requirements of the ATLAS inner detector are more stringent than any tracking
detector built so far for operation at a hadron collider. The conflicting requirements of good mechanical stability, low mass and, at the same time, high detector granularity with its associatedon-detector electronics, readout services and cooling within a small detector volume, make theoverall weight and material budget (in terms of radiation length X0 and interaction length λI ) muchlarger than for previous tracking detectors.
A detailed simulation of the ID has been made. Figure 3 shows a map of generated photon
conversion vertices in the ID volume for electrons with pT ≥ 0.5 GeV from photons originating at
the primary vertex in minimum bias events. The figure shows the location of conversions, integrated
over azimuth, with their radial (R) and axial (z) coordinate. The integration over the top half ofthe detector is displayed as “positive” radius, the integration over the bottom half is displayed as“negative” radius. The figure corresponds to a sample of 500,000 simulated events. The SCT
barrel and disk layers are clearly recognisable. It should be noted that many structural elements,for example the end-plates of both the barrel and end-cap, are azimuthally discrete and have beenmodelled accurately.
3 Rutherford Appleton Laboratory, Science and Technology Facilities Council, Didcot, OX11 0QX, UK.

–5–2008 JINST 3 P10006

Details of assembly steps (1), (2) and (3) are summarised in section 3; steps (5) and (6) insection 4. The testing carried out in steps (4) and (7) is described in sections 5 and 6 respectively.
Figure 2 shows a schematic flow chart of the assembly and integration process for the SCT barrel
detector. An explanation of commonly used acronyms is given in appendix A.
The barrel support structures were prepared at the University of Geneva. The individual CFRP
cylinders complete with module supports were sent to RAL3 for service assembly to each barrel.
From there, the cylinder was transported to Oxford University where modules were mounted toeach cylinder to complete the single barrels. The single barrels where then transported to the CERN
inner detector integration facility located in the ATLAS surface building, CERN-SR1. Detailed
acceptance tests on single barrels were carried out in Oxford before transport and at CERN-SR1
after transport. After the completion of the tests, the four SCT barrels were assembled into eachother and the thermal enclosure in CERN-SR1. The last integration step in CERN-SR1 was theinsertion of the completed SCT barrel into the completed TRT barrel, and a combined test andcosmic-ray run with the SCT and TRT barrels. Following the final test of the combined SCT and
TRT barrel in the inner detector integration facility, the two sub-detector sections were transported
together to the experimental cavern. Once in the cavern, they were installed in the cryostat of the
ATLAS liquid-argon calorimeter [2] as one physical object ready for cabling.

ModuleModuleClusterModuleCluster
BarrelClusterModuleProduction
Clusters

Barrel Support
CylinderPreparation

FSI to barrel
support cylinders

Mountingof opto/electricalservices tobarrel cylinder

Single barrel
Acceptance Test

Cooling loop
to barrel cylinder

BarrelIntegration withTRT Barrel

Four-Barrel
assembly

Final Test of
SCT barrel with
TRT barrel

Radius (mm)Figure 2. Flow chart of the assembly and integration process for the SCT barrel detector.

1000

500

0

-500

-1000
-3000

-2000

-1000

0

1000

2000

3000
z (mm)

Figure 3. Mapping of photon conversions as a function of z and radius, integrated over φ , for the ID. The
mapping has been made from 500,000 simulated minimum bias events using ∼90,000 conversion electronsof pT ≥ 0.5 GeV originating from primary photons.

Figures 4 and 5 show the integrated X0 and λI traversed by a straight track as a function of |η |at the exit of the ID envelope. The SCT contribution is shown in this figure. A striking feature is
the onset of material from non-active services and structures at the interface of the SCT barrel and–6–2008 JINST 3 P10006

Module toBarrelAssemblyInteraction length (λ )0Radiation length (X )2.5
Ext. Services
TRT
SCT
Pixel
Beam-pipe

2
1.5

0.7
Ext. Services
TRT
SCT
Pixel
Beam-pipe

0.6
0.5
0.4
0.3

1

0.2
0.5
0.1
0
0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0
0

5
|η|

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5
|η|

Interaction length (λ )0Radiation length (X )

0.9
0.8
0.7
0.6

External
Supports/other
CablesCooling
Electronics
Active

0.2

0.18
0.16
0.14
0.12

0.5
0.4

0.1

0.08

0.3

0.06

0.2

0.04

0.1
0
0

External
Supports/other
CablesCooling
Electronics
Active

0.02
0.5

1

1.5

2

2.5

0
0

3
|η|

0.5

1

1.5

2

2.5

3
|η|

Figure 5. Material contribution (X0 , λI ) of the SCT barrel and end-caps. The distribution is shown as afunction of |η |, averaged over φ . The breakdown shows the contribution of different SCT components.

end-cap regions. This includes cooling connections at the end of the SCT barrel, and SCT barrel
services extending radially to the PP1 patch-panel connection box on the cryostat and then along
the cryostat wall. A large fraction of the service and structural material is external to the active ID
envelope, therefore preserving the tracking performance.
The consequences of the material’s overall radiation length and interaction length in the Inner
Detector are quite serious:a) Many electrons lose most of their energy through Bremsstrahlung before reaching the electromagnetic calorimeter.
b) Approximately 40% of photons convert into an electron-positron pair before reaching theliquid-argon cryostat and the electromagnetic calorimeter.
c) Even in the case of low-energy charged pions, a significant fraction will undergo an inelastic
hadronic interaction inside the inner detector volume.
For incident tracks at |η |=0, the total X0 of the SCT barrel is 8.6%. When averaged over theactive area, the total X0 of the barrel layers becomes 11.8% (2.96% per layer including 1.33% for–7–2008 JINST 3 P10006

Figure 4. Material distribution (X0 , λI ) at the exit of the ID envelope, including the services and thermal
enclosure. The distribution is shown as a function of |η |, averaged over φ . The breakdown indicates thecontribution of external services, individual sub-detectors including the SCT, and services in their active
volume.

Table 2. Bottom-up measurement of individual components of the SCT barrels, before and after detector
installation. The pre-integration weight includes modules, the barrel structure and services excluding thecooling fluid, and off-detector services including the cooling capillaries, the thermal enclosure and the barrel
interlinks. The on-barrel laser alignment, estimated as 2 kg, is not included. The post-integration weight
includes services up to the PP1 patch panels. In principle, the simulations should agree with the postintegration weight.
Component
Barrel 3
9.578

Total
52.26

5.710

8.175

9.102

10.738

33.73

3.068

3.864

4.692

5.510

17.14
-2.708

5.186

5.575

7.786

9.085

Off-barrel services and structure
Estimated pre-integration weight (kg)
Estimated post-integration weight (kg)
Measured weight (kg)Simulation post-integration weight (kg)27.81
128.2358.44
186.7±5222±6201±20222

modules, 1.15% for services and 0.48% for the support structure). The module budget includes
sensors (0.61%), hybrids (0.22%), module extension flexible circuit and baseboards. Services include the electrical services, cooling blocks and pipes. Off-barrel or off-disk services, especially at
the interface between the barrels and end-caps, and items such as the thermal enclosures, are notincluded.
A check has been made on the assumed SCT material by comparing its measured weight witha bottom-up inventory of the SCT components. Table 2 shows the status of that comparison. Thereis good agreement between the measured and estimated weights of the barrel sub-detectors andtheir services.

3. Assembly of single barrels
This section summarises the assembly of single barrels, starting with the preparation of barrel
support cylinders, the integration of services on the cylinder and finally the mounting of SCT
barrel modules on the cylinder to complete the single SCT barrels.

–8–2008 JINST 3 P10006

Modules
CylinderFlanges
Brackets
CoolingServices
Removecoolant
Electrical
Services
Total

Weight (kg) fromcomponent measurements
Barrel 4 Barrel 5 Barrel 6
11.87514.25016.625

3.1 Barrel cylinders and module support
3.1.1 Design of the barrel cylinder

3.1.2 Manufacturing of the cylinders
After completion of the design, a prototype cylinder was produced. This work was subcontracted to
CASA.6 The prototype was completed in 1998 and submitted to thermal expansion measurementsin the manufacturer’s plant. Further tests were made at the University of Geneva to develop thetechniques to bond and machine the PEEK module bracket attachment (MBA) pads designed toreceive the module-mounting brackets. In parallel, for the fabrication of the four barrel cylinders,specifications and production drawings were issued, reviewed, and possible manufacturers werequalified with criteria described in the CERN market survey MS2434/PPE. After a production
readiness review, a call for tender was made and finally the contract was awarded to Astrium.7
This company used the proposed carbon fibre resin system and subcontracted the machining to a
specialised company, WEKON.8 Machining was done in a temperature-controlled workshop. The
preparatory steps were as follows:• The lay-up of the three layers of unidirectional (UD) prepreg forming the inner-cylinder
facing on a mandrel.
• The addition of preformed honeycomb panels to it, spliced at the junction of honeycomb
panels and at both ends of the cylinder.
• The addition of three layers of UD prepreg forming the outer facing.
4 YLA Inc, 94510 Benicia, CA, USA.
5 Ultracor Inc, 94551 Livermore CA, USA; previously YLA Cellular Products, USA.
6 CASA Alvarez Division Espacio 28022, Madrid, Spain.
7 EADS Astrium GmbH - Friedrichshafen, D-88090 Immenstaad, Germany.
8 WEKON Heinz Schnedler GmbH, D-61185 Rosbach, Germany.

–9–2008 JINST 3 P10006

Highly demanding specifications in term of precision, dimensional stability and material budget [3]led to the choice of carbon fibre reinforced plastic (CFRP) sandwich structures. Simulations andtests were performed to optimise the structural design and to evaluate different concepts to support
the detector modules. In parallel, candidate materials were investigated and tested. Finally the decision was made to mount the modules directly onto four concentric cylindrical structures without
intermediate local support. For the cylinders themselves, the choice was to use a sandwich-type
construction with very thin skins over a CFRP honeycomb core. Each cylinder was reinforced
with a CFRP flange at each end; this also allowed the implementation of the fixation points forconnecting the four cylinders with radial CFRP interlinks. The fibre-resin system chosen is basedon a cyanate ester resin (RS-3) and high-modulus carbon fibres (YSH50A). This product was delivered in 3K unidirectional prepreg form from YLA.4 The CFRP honeycomb chosen is Ultracor
UCF-83-1/4-3.0 from Ultracor.5 To achieve high mounting precision, PEEK pads were bonded tothe outside surface of the CFRP cylinders and machined, thus offering precise mounting points forthe brackets supporting the modules (see section 3.1.4) and opto components. The inner surface ofthe cylinder is equipped with frequency scanning interferometry (FSI) components.

• The addition of thin metal sheets and vacuum bags.
The cylinder was then cured in an autoclave at 180◦ C. After removal from the mandrel, the cylinder
length was machined and the geometry of each cylinder measured. In parallel, the flanges werelaminated on a special mandrel and cured in an autoclave. Bending and ultrasonic tests were
performed to verify the integrity of the sandwich, and the flanges were bonded to the cylinders.
This was followed by another bending test and the machining of the end faces of the flanges. The
module-bracket attachment pads were positioned with guides and bonded to the outer skin of thecylinder. After curing, the final machining of the pads was performed and the cylinders were readyfor the final metrology before acceptance. This was done by Messtronik.9 Finally, the first cylinder(barrel 3) was delivered to CERN in October 2002 and the last one (barrel 4) in March 2003.
Figure 6 shows the CFRP cylinder during milling (left) and during the bonding of module bracket
pads (right).
During manufacture and machining, the following problems were encountered: The flanges
turned out to be far more labour intensive to manufacture than expected and during machining, it
was difficult to fasten the cylinders onto the machine without inducing distortions. Finally, a few
pads were broken during machining and had to be re-bonded in situ on the milling machine.
3.1.3 Preparation of cylinders and measurements
Following delivery of each cylinder, the excess of adhesive was removed and cleaned. The functionality of the service clips integrated into the bonded pads, which later hold the opto and electricalservices in place, was verified. A dry assembly of three of the four cylinders together with temporary interlinks was performed to cross-check their assembly (figure 7). For schedule reasons, thefour cylinders were never available together at the same place before their final assembly at CERN.
With barrels 3 and 6 already at RAL for service assembly, it was discovered that a few modulebracket pads had become de-bonded. It was decided to reinforce all pads with miniature M1
stainless-steel screws, bolting the pads through the CFRP sandwich into a special light PEEK
mushroom plug. After pad reinforcement, the cylinders were equipped with the module-support
9 Messtronik Weisser GmbH Prüflabor, D-78112 St. Georgen, Germany.

– 10 –

2008 JINST 3 P10006

Figure 6. Machining of the delivered cylinder (left) and bonding on module bracket pads to barrel 3 (right).

bracketry and some FSI components on their inside surface before being packed in containers andshipped to RAL for the continuation of the work.
3.1.4 Design and integration of module supports on cylinder
Hermetic coverage of the barrel surfaces with sensors is achieved by mounting modules with overlapping detector edges. To achieve overlaps in φ and reduce the effects of the Lorentz angle, themodules are inclined by 11◦ in barrels 3 and 4 and 11.5◦ in barrels 5 and 6 with respect to thetangential plane. Figure 8 shows the module inclination and overlap between adjacent rows ofmodules. The module in figure 8 is shown while being inserted into its final position: it has been
inserted into the third mounting point (PEEK spring clamp) at the left edge but has not yet madecontact with the cooling block.
Overlap of modules along the z-axis (the axial direction of the cylinder) was achieved bystaggering the modules in the radial direction by 2.8 mm, by using high or low brackets for alternate
rings of modules. The module overlap in the z-direction was calculated to have full overlap for
tracks with pT > 1 GeV originating from within 2σ of the nominal interaction point. Figure 9shows a mechanical-grade module used for test assembly and mounted onto a module-support
bracket on a cylinder. The module is positioned by two mounting bushes in one bracket (topof figure 9) and a third (sliding) mounting point on the bracket in the adjacent row (bottom of
figure 9).
The choice was made to support each module via a CFRP bracket precisely located and bolted
onto PEEK pads bonded to the cylinders. A further third point was added to offer a stiffer and more

– 11 –

2008 JINST 3 P10006

Figure 7. Test assembly of three completed cylinders.

Figure 9. Mechanical-grade module mounted on a support bracket.

precise fixation of the other side of the module. Several options were considered for the bracket
production. The limited volume available, the small clearances and specific requirement of eachbarrel required eight different bracket types (two per barrel) with a total of 2112 brackets. Finally,a plastic molded version was abandoned in favour of a CFRP-laminated, parylene-coated bracket,
shown in figure 10.

– 12 –

2008 JINST 3 P10006

Figure 8. Cross-section of the insertion of a module (dark) into a barrel. Existing parts of the barrel(brackets, services and mounted modules) are in a lighter grey.

Figure 11. Tools and jigs for bracket production: the six molds, each allowing the lamination of three shells,
and the contouring and finishing jigs to support the bracket during machining.

These CFRP brackets offered the benefit of better performance at lower mass but were complicated to produce. After a few trials, a 5-ply molded laminate of T300/RS-3C 1K PW fabric prepreg(from YLA) in a 0◦ /45◦ /0◦ /45◦ /0◦ lay-up was chosen.
After the bracket design and production drawings were completed the production method wasdeveloped in collaboration with Composite Design.10 Prepregs were cut to shape with templates
and laid up in the molds. Silicon pressure parts were added and the mold was then vacuum-bagged
and the laminate cured in autoclave at 180◦ C. All the tools and jigs for the different operations of
the bracket machining (figure 11) were developed and produced using 3D CAD transfer between
the design office and the workshop. The shells were later machined to become brackets and sent to
10 Composite Design, CH-1026 Echandens, Switzerland.

– 13 –

2008 JINST 3 P10006

Figure 10. Module-support brackets during manufacture.

COMELEC11 to be parylene coated. Then the brackets were equipped with small components, as
shown in figure 12, and they were mounted in position by means of screws and centring bushes on
the barrels. Figure 13 shows a completed cylinder with module supports ready to be shipped forservices mounting.
3.2 Barrel cooling
3.2.1 Description of the SCT cooling system
The ATLAS pixel and SCT detectors are cooled with an evaporative fluorocarbon system, designed to minimise the material in the detector volume, maintain a low temperature on the detector
and keep the inlet and outlet tubes at room temperature. The selected refrigerant, C3 F8 , is nonflammable, non-conductive and radiation resistant [12, 13]. It has a good heat-transfer coefficient
11 COMELEC, CH-2300 La Chaux de Fonds, Switzerland.

– 14 –

2008 JINST 3 P10006

Figure 12. Bracket and small components for fastening modules and services onto the SCT cylinder.

and a small vapour specific volume allowing minimal tube sizes. The total cooling power requiredis 62.4 kW for the pixel and SCT detectors. The scope of the evaporative cooling system is to coolthe detectors, maintaining the maximum temperature of the silicon sensors at −7◦ C by operating
the cooling system at a minimum evaporation temperature of −25◦ C. This requirement applies
to the fully irradiated sensors and electronics and allows the effect of the radiation damage to beminimised and the detector to survive at least ten years of operation. During the initial operation,
when the detectors are only mildly irradiated, this requirement is less stringent and the acceptabletemperature may be significantly higher than −7◦ C. Testing in CERN-SR1 was carried out withcooling-pipe temperatures around +13◦ C and these same conditions will be used for the initial
operation in ATLAS. Details of the cooling plant and its control systems can be found in [14] anda schematic view of the system is given in figure 14. The technical specifications for the innerdetector cooling are available in [15].
The barrel is divided into 44 almost identical cooling-loops. Each loop cools 48 modules, so
for example, barrel 6 supports 14 loops cooling 672 modules. Figure 15 shows a barrel coolingloop with its two inlets on the left (at the top and the bottom) and the exhaust connector in between
the inlet connectors. The incoming coolant arrives at the detector under high pressure at room
temperature. The coolant to each loop is pre-cooled by the passage through a heat exchanger(HEX). This facilitates the flow of heat from the incoming warm liquid to the colder, returning
vapour/liquid mixture. Pre-cooling allows for increased cooling capacity at a given coolant mass

– 15 –

2008 JINST 3 P10006

Figure 13. Barrel support cylinder complete with module support brackets.

safetyvalve
safetyvalve
pneumatic
valve

4x11
barrel
inlet
lines

PR

Sub-cooler

Detector
Loop withmodulesmixed water

Heat exchanger
heatermanualvalve

gas line from SCT
barrelDummy
Load(inactive
duringdetector
operation)

4 Haugcompressors

condenser

SCT endcap A
SCT endcap C
Pixelmanualvalve

vacuummixed water
vacuumpump

Pneumatic lines to BPR and PR

Pneumatic
valve

UX15

air
compressor

e-p
conve
rter

USA15
PLC

Figure 14. Internal and external parts of the inner detector evaporative cooling system in a simplified
schematic.

Figure 15. Drawing of a barrel cooling-loop.

flow. The inlet splits into two 0.76 mm inner-diameter annealed cupro-nickel (Cu-Ni) capillaries.
Each capillary has been trimmed to give an identical mass flow at a standard pressure (∼5.5 g/sec
at absolute pressure of 13.0 bar(a)).12 The final trimmed length ranges from 650 to 750 mm, with
12 In the following pressure values are given as absolute pressures denoted as “bar(a)”.

– 16 –

2008 JINST 3 P10006

BPR

pneumaticvalve

SCT endcap A
SCT endcap C
Pixel

2 capillaries

4x11
gas returnlines

liquidtankmanualvalve

liquid line to
SCT barrel

a mean value of just below 720 mm. The two capillaries on each loop are connected throughcustom-made Cu-Ni connectors using a 0.6 mm indium-wire compression-seal. The returning
liquid/vapour mixture is removed via a single exhaust manifold, again sealed with an indium-wiregasket of 0.8 mm diameter wire. The back-pressure on the exhaust can be set remotely with apressure regulator to allow the adjustment of the cooling temperature.
3.2.2 Design of the cooling-loops

3.2.3 Production of cooling-loops, capillaries and interconnects
The cooling-loops were assembled from identical “6-block sections”. These were fabricated byfirst annealing the pipe and then leak testing. The pipe was then bent in a series of machined jigs
to provide three module cooling-block locations aligned to one (upper) module plane and a further
three positions set to the lower module plane. The aluminium alloy14 cooling blocks were made13 ASTM E1382-97.
14 BS 1470 / 5083.

– 17 –

2008 JINST 3 P10006

The cooling-loops on the barrel transport the coolant along the module row underneath the module
mounting surface. At each module location, a cooling block is attached to the cooling-loop with
an optimised cooling contact surface between the pipe and module. The coolant enters the loop on
one barrel end, follows the module row to the other barrel end, where the pipe turns around to theadjacent module row and flows in the adjacent module row back to the barrel end where it entered.
The candidate tube materials were aluminium alloy, austenitic stainless steel and cupro-nickel(Cu/Ni 70:30). The final decision was based on a desire to reduce the total material budget, toavoid risks of corrosion in the cooling components and to provide a relatively simple and flexible
assembly technique. To withstand 25 bar without permanent deformation, aluminium would require a wall thickness of around 250 µ m. A Cu-Ni pipe offering the same radiation length would
have a wall thickness of only about 40 µ m. For stainless steel, the number would rise to 50 µ m.
Cu-Ni was selected because it offered excellent corrosion resistance and ease of fabrication. A
wall thickness of 70 µ m was chosen for the final production, despite the penalty of an increased
radiation-length budget. This was because leaks were discovered in early, thinner samples of thismaterial when they were tested after the annealing process. The origin of these leaks was traced
to the grain growth that is observed in copper-based alloys undergoing annealing. The recommendations from the relevant ASTM code13 led to the conclusion that at least three grain thicknesses
were needed in any tube wall. In the worst case, this implied a wall thickness of 70 µ m.
The cooling-loops were manufactured from non-magnetic material with a high degree of resistance to both corrosion and radiation damage. The circuits were designed for an evaporation
temperature of −25◦ C, corresponding to an evaporation pressure of 1.67 bar(a). The minimumpressure in the liquid part of the circuit is driven by the need to operate above the saturation point.
At 40◦ C the saturation pressure is 12.8 bar(a); 13 bar(a) is then set as the minimum pressure in
the inlet circuit at the inlet of the capillary. The additional pressure drop between the plant andthe capillaries is around 3 bar. Therefore, the maximum pressure at the outlet of the liquid pumpof the cooling plant is 16 bar. This defines the proof-pressure test required for the downstream
components. It is generally accepted that a proof-pressure test >1.5 times the maximum applicablepressure should be applied for at least 30 seconds. The figure used was rounded up to 25 bar(a).

as split halves and were plated with Ni/Cu/Au (3 µ m, 12 µ m, 0.5 µ m respectively). They were
soft-soldered using a tin-lead solder15 and a suitable flux.16

Each module block served the dual purpose of cooling both the module and the associatedopto-flex, as described in section 3.3. The dimensions of the surface of the cooling block used forthe thermal contact to the module are 8 mm by 62 mm. As mentioned earlier, the thermal interface
between both the module and opto-flex and the cooling block is a sliding one, by nature of its design. To allow for a sliding contact between the cooling block and the module, thermal grease wasapplied to the cooling block.17 It was essential to test that this displacement did not have a detrimental effect on the module-position stability. This was answered by mounting mechanically andthermally accurate, non-functional modules on a prototype sector of an ATLAS SCT support cylinder complete with brackets, opto harnesses and a cooling-loop. The entire structure was mounted
inside a large freezer so that the cooling-loop could be operated in an ambient temperature ofaround −7◦ C. Then the mechanical modules were mounted with precision cubes whose displacements could be measured by capacitive probes. The largest displacements (10-70 µ m) observed
were in the R-direction (radial) and were caused, principally, by the capacitive probe support. In
the z- and φ -directions, the displacements were always <10 µ m.
After each 6-block section was completed, the cooling-block faces were re-machined to eliminate any distortions caused by soldering. The assembly of a complete loop involved mounting eight
6-block sections into a jig and connecting them with short Cu-Ni pipes whose ends were swaged toallow insertion of the 6-block sections. Two “U-bends” were added in a similar way and each joint
was soldered to complete the loop. The assembly jig ensured that the cooling-block faces maintained their planarity and positional accuracy. The exhaust manifolds were fabricated using thesame alloy and plating as the cooling blocks. Figure 15 shows the drawing of a completed coolingloop. In many cases, the manifolds had to be designed to access a space among the other barrel
services. The final step was to solder the exhaust manifolds and inlet connectors into place and to
attach Litz wire ground connections to each exhaust and U-bend. These Litz wires would be soldered to the cooling reference disks (CRD). The CRD serves as a common grounding plane at thebarrel ends and is made from 25 µ m copper-plated polyimide. During the four-barrel integration,
the cooling-loop Litz wires were soldered to the CRD to reference electrically the cooling-loops toground.
15 BS 219 Sn/Pb 60:40.
16 Castolin Eutectic 197c.
17 Dow Corning DC340– 18 –

2008 JINST 3 P10006

The control of the height difference between the upper and lower module cooling blocks is
critical to maintaining good thermal contact between the module and the block. This was achieved
by machining the step height to the correct value after the upper and lower parts of the block were
fixed to the pipe. The step height was checked in a jig with glass microscope slides set with theappropriate step height acting as modules. A small amount of dilute ink was smeared onto the slides
and the 6-block cooling-loop section was offered from below. In the case of good step heights theloop is held by surface tension. Any gaps or twists in the loop are easily seen as a void in theotherwise uniform layer of ink.

3.3 Power supply and readout services
3.3.1 Electrical and optical services

3.3.2 Mechanical and thermal interfaces
The vertical clearance from the opto-package to the neighbouring module is only 1.39 mm. This
clearance is critical to avoid damage to exposed wire bonds on the modules. Therefore the thicknesses were measured at several stages during the assembly of the opto-package and opto-flex
circuit.
In order to minimise the pick-up by the module of any electrical noise on the cooling block, ashunt-shield flexible circuit was used over the cooling block. To optimise the capacitive shielding, asmall overhang of 1 mm was added to the shunt shield. This also had the benefit of providing extra
protection against HV breakdown between the module and the cooling block. The shunt shield wasmade from 18 µ m copper on 25 µ m polyimide and was connected to the analogue ground of themodule by a 4-pin electrical connector. The option of changing the grounding scheme to connect
the cooling block to the module analogue ground was made available by a tab which could beconnected to the cooling block by an M1.6 aluminium screw; however, this connection was neverrequired. The thermal impedance of the shunt shield has to be minimised in order not to increasesignificantly the temperature difference between the cooling block and the module. To this end, theshunt shields were attached to the cooling blocks with 20 µ m of boron-nitride loaded epoxy.
The power consumption of the opto-electronics for one module is 360 mW. In order to avoid
heating the silicon detectors, the opto-flex has to be in thermal contact with the cooling block.
However, when the detector is cooled down, the cooling blocks move by distances of up to 1 mm.
Therefore, the thermal connection was made using thermally-conductive grease18 to allow for asliding grease joint. A specially designed PEEK clip was used to ensure that the opto-flex kept
good thermal contact with the cooling block. In order to optimise the thermal impedance from the
18 Dow Corning DC340– 19 –

2008 JINST 3 P10006

The electrical power distribution has to provide all the low and high voltages and DC control
lines to operate the SCT modules and the associated opto-electronics. Also there are two temperature sensing signals for each module, carried on the electrical distribution system back to thetemperature-sensing circuitry in the power supply units. The SCT power supply and distribution
systems are described in [10]. The optical links read out the data from each module and provide thetiming, trigger and control (TTC) data to each module. The SCT optical links are described in [11].
In order to design a hermetic detector with a minimal area of silicon, there is very limited space
available for the optical and electrical services and their contribution to the total detector radiation
length needs to be minimised. Due to these severe space and material constraints, the electrical
power distribution from the local patch panels (PPB1) to each barrel detector module is incorporated within harnesses together with the opto transmission. In order to reduce the radiation-length
budget, the electrical power is distributed using low-mass tapes (LMT’s) made of 50 µ m thick aluminium conductors on polyimide. The LMT’s are connected to the modules with copper-polyimide
opto-flex circuits [11]. Furthermore, the heat dissipated by the on-detector opto-electronics has tobe transported efficiently to the cooling system so as to avoid excess heating of the silicon detectors.

opto-electronics to the cooling block, an aluminium-nitride (AlN) ceramic stiffener was glued on
the opto-flex underneath the opto-electronics. In addition, thermal vias were used to enhance thethermal conduction from the opto-electronics to the AlN stiffener.
Figure 16 shows part of the assembled barrel 6 with harnesses and cooling-loops mounted
to the support cylinder. On the right edge of the photograph the service cage can be seen, which
supports the patch panels during the assembly.
3.4 Integration of harnesses and cooling-loops
3.4.1 Harness to barrel integration
To minimise the number of connection cycles made with final patch-panel connectors and to protect the patch panel itself, a temporary patch panel (TPP) was connected to the final patch panel andtogether they were positioned in an aluminium support. Therefore, any test involving the connection to the harness, e.g. subsequent module tests, only used the temporary patch panels for outsideconnection.
Prior to installation, the electrical continuity and proper function of a complete harness, servicing six modules in a row, was tested as described in [11]. The harness was positioned on thebarrel and secured with PEEK service clamps which lock into the module bracket pads. Fibres
were carefully routed to the barrel ends, respecting their minimal bending-radius, and secured withPEEK fibre clips. After installation, the harness was retested. As a final check mechanical “enve-– 20 –

2008 JINST 3 P10006

Figure 16. Part of the assembled barrel 6 before module mounting.

lope” modules were mounted on each location to verify the clearances. The clearance at the end of
the barrel, allowing for all the tolerances, was calculated to be 1.8 mm. Therefore, the thickness of
the LMT’s and the total height of the stacks of six LMT’s were checked during the assembly.
The weight of the patch panels at the end of the harness was supported by a temporary service
support-cage. Excess lengths of fibres and LMT’s were stored in dedicated slots of the service
support-cage. Patch panels were positioned such that all connectors on patch panels and fibres
were accessible for future tests of the single barrels.
3.4.2 Cooling-loop to barrel integration

3.4.3 Design of the cooling-loop to bulkhead connection
Each cooling-loop terminates near the barrel end. To connect the cooling-loop exhaust to the connector in the thermal enclosure (TE) bulkhead (described in section 3.8.5), intermediate pipes,
called “spiders”, were designed. Each spider connects one cooling-loop exhaust to the TE bulkhead.
Calculations were carried out to estimate the maximum force that could be transmitted to theexhaust-manifold flange by exhaust spiders during a temperature change of 50◦ C (e.g. from −25◦ C
to +25◦ C). The exhaust spiders are “L-shape” copper pipes of 8 mm outer diameter and 200 µ m– 21 –

2008 JINST 3 P10006

The first cooling-loop assembly step was to fit the barrel-end cooling loop. Mounting brackets werefitted to the barrel flange using countersunk screws. For barrel 6, a combined bearing and bracket
was used. Visual inspection was carried out to check that the mounting holes were clear and thatthe brackets were square to the barrel surface. This inspection also ensured that all harnesses
were correctly located under their clamps. Next, the cooling-loop installation frame guides were
mounted on the barrel ends and a check made to ensure that the cooling-loop installation frame
could be correctly located into them. A cooling-loop installation frame was required during loop
manipulation because the cooling-loops are not self-supporting.
All loops had been helium leak-tested and cleaned during the final stages of the quality control
procedures. The next step was to transfer the cooling-loop from the assembly jig into a final
assembly fixture. The frame was lowered over the loop and transverse members were fitted tocomplete the frame and provide additional torsional rigidity. The cooling-loop tubes were locatedin the recesses in the tooling. Screws securing the loop at the manifold end were removed and acheck made to ensure all Litz wires and shunt-shield tails were clear.
The loop was then lifted out of the assembly frame and transported to the barrel on the installation frame. The frame was located on guides and lowered to an adjustable stop. After checking forpotential conflicts, the frame was lowered using a fine adjustment. When in place, the mechanicalgrade modules were screwed to the module brackets. The inlet and exhaust manifolds were secured
to brackets at the end of the barrel and PEEK clips were inserted to secure the opto-flex to the underside of the cooling block. PEEK module clips were also added to apply pressure to the cooling
blocks and the mechanical-grade modules, thus trapping the cooling-loop in its final position. The
ground connector on the shunt shields was also connected to the opto-flex. The Litz wires wereattached to the loop U-bends. The transverse members were unclipped and the support frame wasremoved. A final step was the removal of the barrel-end guides. Figure 16 shows a cooling-loop
after installation on the barrel.

wall thickness. At each end is a custom connector to mate with the exhaust manifold and the HEX.
Different geometrical layouts were considered:• The spider clamped at both ends: resulted in a maximum force of ∼18 kgf.
• The spider hinged at both ends: resulted in a maximum force of ∼1.05 kgf.
• The spider hinged at the bulkhead and clamped at the flange: resulted in a maximum forceof ∼1.4 kgf. It was this arrangement that was followed in the final assembly.

3.4.4 Design and production of the heat exchangers
As described in section 3.2.1, the C3 F8 fluid is pre-cooled. This is done by delivering the fluid ina pipe through the outgoing exhaust vapour in a recuperative heat exchanger (HEX). After passage
through the HEX, the exhaust vapour temperature remains significantly below the prevailing dew
point (about 11◦ C). Therefore, a heater is connected in series with each heat exchanger to raise thetemperature of the returning coolant and evaporate any remaining liquid coolant. The demountablejoint used between the HEX and the heater has to be both leak-tight and electrically isolating.
Both the inlet and exhaust pipes are connected with breakable but non-isolating couplings at thethermal enclosure (TE) bulkhead. The inlet pipe enters the HEX volume, passes through the colder
exhaust vapour and emerges close to the TE bulkhead, where the incoming flow is split into thetwo capillaries for each loop. The entire HEX is made from Cu-Ni tube and is of a soft-soldered
construction. The heater is made of stainless steel and is fitted with polyimide-insulated cables
for both power and temperature sensors. The HEX’s are encased in pre-formed thermal insulation
within their cable trays on the wall of the liquid-argon calorimeter cryostat [2]. A filter is inserted
between the end of the inlet pipe and beginning of the capillaries to remove particles that could
block a capillary. The 44 (22 per side) HEX/heater assemblies are arranged in bunches of either six
or five against the liquid-argon cryostat wall.
3.5 integration of the FSI system
The relative positions of the silicon modules must be determined to <12 µ m in Rφ so as not todegrade the tracking performance by >20% [3]. As it is impossible to guarantee the short-term
mechanical stability of the detector, the SCT has been equipped with an optical alignment system– 22 –

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In the case of two rigidly clamped ends, “force filtering” would be induced by stressing the
spider above its yield point. Furthermore, it was ensured that the spiders were fully annealed
after fabrication. In this case, the maximum yield strength falls to 6 kgf/mm2 , which limits the
maximum force to ∼3.7 kgf. The change of length of spiders is small (100-200 µ m) and the force
drops rapidly if one end moves slightly, which led to the choice of a mount with one hinged and
one fixed end.
To check these calculations, the yield point on two of the final barrel 6 manifolds was measured. Although the yield point is difficult to assess for copper due to the inevitable work-hardening
caused by the measurement, the results for these two samples showed a yield point of ∼4.5 kgf/mm2 .
Further “filtering” of the force was obtained by clamping the spiders at the bulkhead in a way thatallowed them lateral movement of up to 80 µ m.

that can monitor micron-scale motion of the detector in real-time. The alignment system consistsof 842 interferometers built into the SCT and arranged as a geodetic grid of distance measurements
between nodes. The technique of frequency scanning interferometry [16] has been developed tomeasure simultaneously the interferometer lengths to a precision of <1 µ m. The positions of
the 3D grid nodes can be reconstructed to <5 µ m in the critical directions [17, 18]. Repeatedmeasurements of the FSI grid follow time-dependent, correlated displacements of silicon modules.
The FSI system offers access to the short-timescale, low spatial-frequency detector deformations.
In the SCT barrel, the FSI components are attached to the inner surfaces of the four concentric
CFRP cylinders that support the silicon modules. The precise shape of each cylinder is monitoredby an inner barrel grid, which consists of 32 grid nodes connected by 80 grid-line interferometers(GLI’s), as shown in figure 17. 48 GLI’s employ a mirror to measure radially inwards to certain
modules of the adjacent smaller barrel, to help constrain radial deformations. Two barrel-flange
grids, each of 72 GLI’s, also link the four single barrels in the transverse plane at either end of the
SCT barrel. The combined total of 512 grid lines permits complex, subtle deformations of the SCT
barrel to be determined.
Each grid line is a fibre-coupled interferometer, which is formed between two components(described in more detail in [18]). The main component is called a “quill”. It is manufactured
from two parallel delivery and return fibres in a PEEK support, and is attached to a fused silica– 23 –

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Figure 17. Part of the FSI geodetic grid inside an SCT barrel support cylinder. The grid-line lengths are
simultaneously measured to a precision of <1 µ m. Further radial lines (not shown) directly monitor silicon
modules on the adjacent barrel.

beam-splitter. The quill illuminates a distant retro-reflector. The distance between a quill in one
jewel and a retro-reflector in another jewel is measured by FSI. The jewels were placed in a jig that
replicated the grid geometry in ATLAS, so that the GLI’s could be pre-aligned and verified. The
quill pointing was adjusted to maximise the interference fringe visibility, and then Araldite 2011
was applied to fix the alignment. The jewels were installed into precisely machined slots in PEEK
inserts on the SCT support cylinders. This method ensured pointing alignment was maintained towell within the 2.5◦ divergence of the laser beam.
The barrel-flange FSI components were glued directly to the SCT barrel flange during the fourbarrel assembly (see figure 18) using temporary jigs. The installation proceeded in three stages,
as each pair of adjacent barrels was brought together during four-barrel assembly. A PEEK cover
was added to protect the FSI components before unfolding the services from the barrels. Also theservices were prevented from obscuring the lines of sight by protective tubes. Thermistors wereattached to each inner-barrel grid node to monitor the ambient gas temperature near the light pathso that refractive index corrections can be applied.
3.6 Module to barrel assembly
To achieve reliable assembly and cope with the tight clearances between modules, a robotic assembly procedure [19] was chosen. To be able to work on two barrel assemblies in parallel, twoassembly stations with one robot each were used simultaneously in the assembly cleanroom.
The robot had to 1) pick up the module from a protective box, 2) insert the module and position
it precisely on its support bracket, and 3) insert and tighten the M1.6 mounting screws. It also had tobe capable of reversing this sequence if a module had to be removed. Prior to mounting modules,
the robot surveyed the mounting points on the barrel optically to achieve the location precision.
With the layout of modules on the barrel, it was natural to assemble the modules by rows. First,
the six lower modules in a row were mounted and tested, and then the rows were completed by
mounting the six upper modules. Travel along the length of a row, and the actual insertion, wasachieved by mechanical stages driven by stepper motors. Different rows on the barrel were selectedby rotating the barrel by hand and locating it with an indexing wheel. Module insertion was always
done from the side of the barrel. A picture of the robot in front of an SCT barrel is shown infigure 19. Once the assembly procedures were well established, modules in several adjacent rows
were mounted in a group to make the switch between mounting and testing more efficient.

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Figure 18. One of the 48 assemblies of the barrel-flange FSI grid that monitor rotations and translations
between support cylinders of the SCT barrel.

In addition to the robotic procedures, additional assembly steps had to be performed by theoperators using dedicated tooling: 1) the application of thermal grease,19 on the cooling blockbefore the insertion of the module, and 2) the application of two clips to clamp the cooling blockto the module baseboard after the mounting. During the removal of a module, this grease joint
had to be split by hand using a wedge tool. The robotic assembly of the barrel SCT was highly
successful. Including replacements, over 2180 modules were mounted on the barrels. 40 modules
had to be removed and replaced for various reasons, most of which were not associated with theassembly process. Only four modules were damaged during assembly. The precision requirement
for the robot was moderate, as the location of the modules on the barrels was entirely determinedby the precision holes on the mounting brackets and module facings. As long as the module waspositioned with sub-millimetre precision, the shape of the bushes pulled the module into its correct
location. However, an estimate of the precision could be made from the survey data of the mounting
holes on the barrel. The r.m.s. of the distribution of measured versus expected separation between
adjacent module mounting holes was 62 µ m — this also includes the build tolerance of the barrel
structures. All modules on all cylinders were tested after mounting at a coolant temperature ofabout 17◦ C (“warm operation”). These tests comprised small groups of modules (typically about24) and were interleaved with the mounting. After the module mounting was completed, the fullbarrel was tested at different operation temperatures as described in section 5. During the mounting
and testing, the barrels were protected in an aluminium frame, which was also used as the innermost
layer in a double-walled container used for the shipment of the barrels to the module assembly site
and from there to their final destination at CERN.
19 Dow Corning DC340– 25 –

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Figure 19. Robot in its assembly position in front of barrel 3.

3.7 Transport of the barrels
3.7.1 Requirements

3.7.2 Designs of the transportation containers
At the last two assembly sites, the assembly frame was built up into a protective box using aluminium profiles and panels. This “inner shipping container” was then mounted on springs inside asecond box, which was steel framed with plywood panels. The completed transport box was 3.6 m(l) by 2.1 m (w) by 2.1 m (h), and weighed 2.4 tonnes. The arrangement is shown in figure 20.
The springs were wire rope isolators, which were selected to provide both vibration isolationand high damping, which is essential to prevent oscillations.
Both the inner shipping container and the outer transport box were instrumented with accelerometers to record acceleration in all three directions, as well as temperature.
3.7.3 Transport
Prior to shipping barrels, a dummy load with the same mass as the inner shipping containers was
instrumented and mounted in the outer transport box. This was loaded into a lorry using exactly
the same process as was planned for the real shipments. Short test runs were undertaken and theresults from the accelerometers indicated that the procedure would be safe. A final transport test
was performed prior to the transport of barrel 3 by sending a detector test sector, equipped withfunctional barrel modules, to CERN. This concluding test with shipping containers exercised thepre-defined transport, loading and unloading procedures and was followed by retests of modules
on the test sector after arrival at CERN.
For the transportation of the barrels, the wheels of the transportation frames were raised andthe boxes were held in position by load-lock bars fixed to the sides of the lorry. The barrels wereshipped one at a time, the route was along main roads, including the Channel tunnel, and wasscheduled for minimum traffic congestion.
The journeys were carried out at safe speeds, taking around two days. On arrival at CERN,
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The cylinders underwent three major stages of transportation. At the first assembly site brackets
were attached to cylinders and metrology performed. They were then shipped to the second assembly site where cooling, power and FSI were attached. From the second assembly site they weremoved to the third where modules were attached and the barrels tested. The final transport was to
CERN, the latter being a long journey with fully assembled detector barrels.
The barrels, when mounted in the assembly frames, weighed between 600 kg and 700 kg andwere of dimensions 3.1 m (l) by 1.6 m (h) by 0.9 m (w) for the smallest and 3.2 m by 1.7 m by
1.1 m for the largest. Once equipped with services and modules, the delicacy of the components
meant that it was crucial that the barrels were transported with extreme caution during the last twotransportation steps.
Transportation took place in an air-sprung, temperature-controlled, humidity-controlled lorry.
The acceleration experienced by the shipping container was required to be minimised to avoid
damage to the silicon modules and shaking loose connectors. The temperature was required to be
20±3◦ C to avoid thermal stresses and the humidity kept at around 40%.

visual inspection. During the journeys, no damage was sustained, however a small fraction (<10%)of the cooling pipe clips moved position and were repositioned.
Once the lorry was sealed up, the temperature of the barrels remained between 18◦ C and
22◦ C and the humidity between 36% and 56%. Figure 21 shows the accelerometer record for the
shipping of barrel 6. It shows the maximum acceleration in the vertical and horizontal directions
recorded in each half hour period of the journey. High g forces (tens of g) are commonplace instandard transportation and do not necessarily represent a significant concern. The duration of
the acceleration, any subsequent compensating accelerations, and the design of the object being
accelerated are also crucial in determining the impact of a shock event. The energy delivered to acomponent compared with its mass is crucial, which is why vibration damping is vital to remove
any resonant build up of energy absorption in suspended components.
Figure 21 shows two successive half hour periods at around 21 hours into the journey where
events exceeding 1.2 g were recorded. In the vertical direction these were close to 13 g, but wereof shorter duration than the resolution of the accelerometer (10 s) and of unknown origin.
3.8 Barrel thermal enclosure and grounding
3.8.1 Overview and requirements
The SCT barrel is contained in the thermal enclosure (TE). This consists of two outer thermal
enclosure (OTE) half-shells and two side panels and end panels which are concentrically mounted
to an inner thermal enclosure (ITE) cylinder. A photograph of the pre-assembled, empty thermal
enclosure at the manufacturer’s is shown in figure 22.

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Figure 20. The inner shipping container and the outer transport box with the panels removed. The inverted“L” brackets on the outside of the containers mount wheels.

The TE was manufactured by Lola Cars International.20 It has the following functions:• It provides mechanical protection for the SCT assembly.
• It provides thermal insulation between the SCT and TRT, which operate at different temperatures of −7◦ C and +25◦ C, respectively.
• It contains a dry-nitrogen atmosphere to avoid condensation at a nominal overpressure of
4 mbar and separates the SCT and TRT gas volumes (the volume between SCT and TRT is
flushed with CO2 , which is required for TRT operation).
• It acts as an electrical shield for the SCT barrel and provides a low-impedance path between
the patch panels at opposite ends of the SCT.
The above functions must be realised while satisfying the constraints of minimum mass and tightenvelope requirement — these made the design challenging. Heat transfer through the thermal
enclosure walls is estimated to be less than 200 W total and is dominated by conduction throughthe OTE, both due to its large area and because it is adjacent to the warm TRT.
3.8.2 Outer thermal enclosure
The outer radius of the thermal enclosure is constrained by the radial envelope of the SCT barrel of
549 mm and is allowed to have a maximum thickness of 5 mm to clear the SCT modules on barrel 6.
Along the horizontal axis, the OTE has to clear the SCT support rails inside the inner TRT cylinder.
This requirement and its assembly led to a design which consists of two identical half-shells withflat side panels mounted on the horizontal axis (see figure 22). Using flat side panels along thehorizontal axis allows to reduce the radial envelope of the thermal enclosure locally where moreclearance between SCT and TRT is needed to accommodate the SCT support rails and support
mechanics between SCT and TRT barrel. The disadvantage of side-panels is that the number andoverall length of seams increases.
20 Lola Cars International Ltd., Glebe Road, St. Peter’s Road, Huntingdon, PE29 7DS, UK.

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Figure 21. Maximum accelerations in each half hour interval in the three directions experienced by barrel 6
during transportation from Oxford to CERN. (rms values are much smaller)

The OTE is built as a sandwich structure of 0.6 mm thick CFRP, a foam core providing thermal
insulation and stiffness, and an outer 0.5 mm thick aluminium sheet.
The CFRP is based on the CYCOM 919 prepreg by Cytek,21 an epoxy resin system with a
T300 fibre, frequently used in the racing-car industry. (Other parameters are 45% resin content,
2×2 twill woven cloth, 0.2 mm cured ply thickness; the elastic modulus is 57 GPa at 0◦ and 48 GPa
at 90◦ .) Aluminium rather than CFRP was chosen for the outer layer to reduce costs by avoiding theconstruction of an additional precision mold tool. The use of dissimilar materials does not causesignificant stresses due to thermal contraction, since during operating conditions, the aluminium
skin is at room temperature (TRT temperature) and the thermal expansion of the CFRP (at −7◦ C)
is small.
For thermal insulation, the cavity is filled with Pyrogel AR5401,22 an aerogel with a thermal
conductivity of 0.012 W/mK. The Pyrogel was wrapped in aluminised polyimide foil to minimise
heat transfer by radiation. The Pyrogel panels are separated by strips of Airex foam as shown infigure 23 to provide dimensional rigidity to the sandwich. The overall thickness of the assemblyis 5 mm.
21 Cytek Engineered Materials, Anaheim, CA, USA.
22 Aspen Aerogels, Inc., 30 Forbes Road, Building B, Northborough, MA 01532, USA.

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Figure 22. The thermal enclosure as pre-assembled at the manufacturer’s.

The OTE CFRP inner skins were cured on a carbon-fibre rather than a steel mould tool tonegate any differential expansion problems during curing. The foam spacers and Pyrogel insulation panels were laid on, then the aluminium outer skin was bonded in place using Araldite 2011
epoxy adhesive under vacuum. This approach was followed also for the end and side panels andworked well. The inner surface of the OTE half-shells was laminated with 25 µ m thick aluminised
polyimide foil, again to minimise radiative heat transfer.
The OTE half-shells are not true half cylinders, but cover an angular spread of around 165◦ toallow for the mounting of the side panels. The CFRP inner skin of the half-shells incorporates amoulded end flange for mounting the services bulkhead.
The material represented by the OTE corresponds to 1.1% of a radiation length for particles atnormal incidence.
3.8.3 Side panels
The side panels join the upper and lower half-shells to make up the OTE and allow the barrel
mountings and their penetrations to be decoupled from the main TE assembly, which was an essential schedule requirement. The joint between these parts needed to be quite strong structurally,
provide a seal to maintain the positive-pressure dry-nitrogen atmosphere within the thermal enclosure and ensure the electrical continuity of the outer skin be maintained around the circumference.
In terms of manufacture, the side panels are very similar to the OTE half-shells, but slightly lesschallenging due to their smaller size and flat profile. The inner skin was again a CFRP moulding andincludes end flanges. The end flanges, which provide mounting points for the services bulkheads,
were machined from aluminium and bonded to the inner skin.
3.8.4 Inner thermal enclosure cylinder
The inner thermal enclosure (ITE) has a diameter of 0.6 m. Unlike the OTE, it is made of a single
wall of CFRP. This is possible since the SCT and the pixel detector both operate in a nitrogen atmosphere and their temperatures differ by only 7◦ C. The wall thickness of 0.5 mm is a compromise
between optimum mechanical properties (pressure and mechanical envelope) and the requirementof minimum mass. A 100 µ m thick aluminium skin on the inside of the ITE serves as the electricalscreen. The ITE has rigid CFRP and aluminium end rings that provide rigidity, a mechanical seal

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Figure 23. OTE half-shell with Pyrogel insulation and Airex stiffeners during manufacture (left) and ITE
cylinder (right).

3.8.5 Bulkhead
An aluminium bulkhead is bolted between the end panel and outer cylinder to feed through the
LMT’s, cooling pipes, optical fibres and DCS cables. A picture of the bulkhead is shown in figure 24.
The bulkhead is a three-layer assembly that allows sequential insertion of the SCT barrel
services passing through to their respective patch panels. The services are sealed in using a combination of Techsil24 silicone rubber elastomer and Loctite 5145.25 With the exception of the coolingexhausts, the services do not have a connector break at this point to save space and reduce detector
mass. The services bulkhead was CNC machined, while the many different types of PEEK cable
feed-throughs were machined on a conventional mill.
3.8.6 End panels and heaters
Annular end panels incorporate channels on their outer face that increase rigidity and accommodate
heater cables from the ATLAS pixel detectors. The build-up and thickness of the end panels is again
similar to that of the OTE and side panels.
Due to limited space throughout the inner detector, the thermal insulation was chosen to be
partly active. The outer surface of the TE is thus kept at a constant temperature (typically TRT temperature) above the prevailing dew point to avoid condensation. This is done using large laminatedcopper-polyimide heaters, the surfaces of which are etched, before gluing to the TRT inner surface
with Araldite 2011. Similar heaters are fitted to the end panels.
The heater copper tracks are typically several metres long to create sufficient resistance forheating. The copper-polyimide heater panels surrounding the SCT barrel thermal enclosure are
glued onto the curved inner surface of the TRT enclosure and to the end plates of the barrel. The
former include 5 micrometers thick copper and are 6.4 mm wide. The latter are 8 micrometers
23 EP79 from Master Bond Inc., 154 Hobart Street, Hackensack, New Jersey, USA.
24 Techsil Ltd., 30 Bidavon Ind. Estate, Waterloo Road, Bidford on Avon, B50 4JN, UK.
25 Henkel Loctite Adhesives Ltd., Technologies House, Wood Lane End, Hemel Hempstead, HP2 4RQ, UK.

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face and a continuous electrical connection to the bulkhead. A photograph of the ITE is shown infigure 23.
A CFRP mould tool was used for the ITE production and the aluminium skin of the ITE was
co-cured with the CFRP outer skin. This entailed taping the aluminium to the mould before laying
the prepreg. A large enough single sheet of aluminium was not available, so three pieces were used,joined with conductive epoxy EP79.23
Tight envelope limits dictated challenging manufacturing tolerances. For most general features, an acceptable tolerance of ±0.25 mm in the completely assembled form was achieved by
the manufacturer. Because the ITE has little natural stiffness and could not support its own weight
before the rigid end rings were applied, the mould tool was accurately surveyed before curing.
A geometric parallelism tolerance of 0.4 mm was maintained over the end-ring faces and holepositions were located within a positional tolerance of 0.2 mm.
The material represented by the ITE corresponds to 0.4 % of a radiation length for particles atnormal incidence.

thick and have a track width of 0.75 mm. Total pad thicknesses (including polyimide) are about
0.5 mm. Each heater has two independent and parallel heating tracks for redundancy and two
negative thermal coefficient (NTC) thermistors for monitoring the temperature.
All low voltage heater pads are supplied by 48 volt DC power supplies, located in the ATLAS
US15 and USA15 service caverns, that uses pulse-width-modulation to provide the time-averaged
power necessary to maintain the desired temperature. Typical heater currents are 1 A.
3.8.7 Grounding and shielding
The thermal enclosure is essential to achieve good electrical performance of the SCT barrel detector. It must act as a shield (Faraday cage) to suppress external electromagnetic radiation and provide
a low-impedance path between the cable patch panels at each end of the SCT barrel. This required
implementing high-quality corrosion-proof electrical joints that will not deteriorate throughout the
lifetime of the SCT. The electrical shield has to be as continuous as possible; gaps and seams haveto be minimised.
The electrical design of the thermal enclosure and its implementation were not straightforward.
This was in part due to the difficulty of joining aluminium. Other difficulties arose when sheets
or foils could not be bought or handled in the desired sizes and several pieces had to be joined.
To prevent corrosion and increased resistance across aluminium joints, all aluminium parts weretreated with Alochrome 1200 to resist oxidation, thereby providing a more reliable, long-term
electrical connection. A novel spot-welding technique was developed at RAL to provide efficientelectrical coupling in as many places as possible. This was used to weld aluminium tags to the OTE
aluminium skin (see figure 22). During assembly, the tags were folded over the rim of the OTE and– 32 –

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Figure 24. A photograph of the bulkhead for service feedthrough. Only the upper half is shown.

3.8.8 Production techniques and experience
The TE is a technically complex project which was started relatively late and went through several
design iterations due to changing requirements. The very limited envelope of 5 mm thickness
allocated to the TE was a significant constraint. Given a more generous space allocation for thethermal enclosure envelope, the design of the TE could have been significantly simplified, possibly
allowing for a fully cylindrical shape, relaxed tolerances and choosing a CFRP/insulator/CFRP
sandwich for the OTE.
Sealing the thermal enclosure was cumbersome. The sealing of the long seams would havebeen simplified by giving the joint adequate stiffness.
Grounding and shielding specifications were considered late in the design and required a number of inventive solutions. Using copper rather than aluminium at the larger radii would have
simplified the design greatly. Once the design was finalised, production of the thermal enclosure
proceeded smoothly. Manufacturing of large precision CFRP objects and the various laminating
steps did not pose a particular challenge to the manufacturer.

4. Assembly of the four barrels and integration to TRT
After the single barrels were assembled they were transported to CERN to complete the integrationof the SCT barrel detector. Tests (described in detail in section 5) were carried out on the single barrels to assure their proper functioning before all four barrels were mechanically assembled
together and inserted into the thermal enclosure.
4.1 Four-barrel assembly
4.1.1 Description of barrel assembly
The four barrels were individually assembled at RAL and Oxford University, before being shippedto CERN for their integration with each other and into the TRT. This phase took place in the CERNSR1 building, where the room temperature was controlled to ±2◦ C and the relative humidity toabout 40±10%. The goal of this work was to install the four nested barrels inside the thermal
enclosure. The thermal enclosure comprises two half-cylinders (half-shells) and two side panels,
forming the Outer Thermal Enclosure (OTE), the Inner Thermal Enclosure (ITE) and two end
panels. Details of this structure are contained in section 3.8. The heater pads that define the exterior
26 Laird Technologies, http://www.lairdtech.com, part number 0C98-0550-17.

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were bolted to the bulkhead. A similar technique was used for the ITE aluminium foil, where thetags are part of the foil.
A silver-coated nickel epoxy, EP79, was used for some electrically conductive joints. Many
joints included commercial tin-plated Be-Cu radio-frequency gaskets.26
The DC resistance of the important electrical joints was monitored repeatedly during assemblyand four-barrel integration, and showed no deterioration with time. The resistance across the lap
joint of the OTE aluminium skin is ∼0.16 mΩ; the resistance between the adjacent aluminium foils
lining the ITE ranges from 1 to 3 mΩ; the resistance between the OTE top and bottom shells acrossthe side panel is ∼0.5 mΩ; the resistance of the ITE to the bulkhead is ∼10 mΩ.

operating temperature of the SCT barrel are mounted on the inside of the TRT barrel and on the
TE end panels. The interface temperature between the SCT barrel and pixel barrel is controlled byheater pads on the pixel support tube (PST).
4.1.2 ESD precautions

4.1.3 Integration with the thermal enclosure and sealing
The ITE, OTE and TE side panels were delivered to CERN after a trial assembly and sign-off at the
manufacturer’s site. The OTE half-shells were attached to the upper and lower supports of the fourbarrel assembly cradle (“SCT cradle”). They were moved vertically outwards to give the maximum
clearance for the insertion of barrel 6 (see figure 25). The next stage of the OTE assembly involved
the fitting of the side panels to barrel 6 (see figure 26). The clearance between the modules andthe inner surface of the OTE was of the order of 2 mm, and to check that this was maintained atall φ locations, the barrel was rotated before insertion and the concentricity of the barrel moduleconnectors and the spindle adjusted. Each half-shell of the OTE was lined with aluminium foil
which would be electrically bonded to the outer aluminium skin of the OTE side panels. Although
this would ensure that the SCT grounding was complete and also ensure that all metallic surfaces
were referenced, it did present a potential hazard to the high-voltage supply of the silicon modules.
To overcome this problem, 12 bands of 25 µ m polyimide foil were glued around the inside of eachhalf-shell at the correct z positions to ensure insulation from the module connectors carrying the
HV connection.
4.1.4 Description of the procedure
The barrel 6 inner transport-container was dismantled, leaving the barrel, service cages and spindle
supported on the pedestals and baseplate. The baseplate was lowered onto the rolling platformof the four-barrel assembly tooling and roughly aligned with the axis of the tooling. Extension
supports were fitted to the ends of the spindles and these were attached to the assembly tooling.
The spindle was disconnected from the pedestals and the baseplate lowered, thus transferring theweight of the barrel and services cages to the assembly tooling. After removing the baseplate
and pedestals, the barrel was carefully aligned to the tooling axes. The weight of the barrel wastransferred to the main cradle by attachment to the inserts that pass through the OTE side panels.
The upper and lower half-shells of the OTE were lowered into position and attached to the side

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Problems with electrostatic discharge (ESD) causing damage to components had been observed
during earlier assembly stages at both Oxford and RAL. However, by the time the barrels arrived at
CERN for the four-barrel assembly, these problems were well understood. A room was built inside
the CERN-SR1 cleanroom which was used for the assembly of the barrels. Strict controls were
put in place to diminish the risk of ESD damage to the barrel components. Access was controlled
and all fixtures and personnel were electrically connected to a central ground point via 1 MΩ
resistance. Personnel wore ESD-safe coats, overshoes and ankle or wrist grounding straps whenworking on the barrel. The walls of the room were of metal construction and were coated with an
ESD-dissipative paint. The floor was also dissipative and the resistance measured to the underlying
copper-mesh ground plane was around 10i Ω. The surface resistivity was checked monthly andwas always found to be approximately 1010 Ω/square.

panels of the OTE, thereby completing the cylindrical part of the gas-tight Faraday cage of the
OTE. The side-panel joints were bolted and sealed. At this stage, it was necessary to unfold all theservices from the barrel service cage and mount them on the radial support structure (RSS). The
rear TE bulkhead was bolted in place, the rear feedthroughs were added and the LMT services were
folded out, their outer ends being supported in a separate frame. Each LMT was aligned with thecorrect slot position in the inner bulkhead of the thermal enclosure. For most LMTs, the temporary
patch panel (TPP) was removed and the PPB1 final patch panel was protected with ESD-safe foam
packaging. For those LMTs that were to be used in the cosmic-ray test with the TRT, the TPPs
were left untouched.
Readout fibres were wrapped in soft, anti-static foam over the MT connector and at each endof the spliced section and they were fixed to the RSS in the gaps in between the LMTs. The FSI
fibre spools were mounted on specially constructed stands, which screwed onto the RSS. One FSI
fibre was damaged during this operation and was repaired using a fusion splicer. Detector control
system (DCS) sensor wires were grouped according to their final feedthrough allocation and theirlengths were adjusted to fit with the final patch panels inside the ATLAS solenoid magnet. The
Cu-Ni capillaries (with pre-installed sleeves) were fitted to the individual SCT barrels before theirintegration into the four-barrel assembly. The capillaries were leak tested on the barrels with an– 35 –

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Figure 25. View of the Outer Thermal Enclosure (OTE) on its tooling with the Radial Services Support(RSS) in place.

acceptance criterion of 4×10−9 mbar·l/s. The Cu-Ni capillary connectors were sealed using 0.6 mm
diameter indium-wire seals. The wire was degreased in acetone for five minutes, then washed inde-ionised water, followed by another five minutes in 10% hydrochloric acid. They were washedagain in de-ionised water and allowed to dry. A matched pair of capillaries was chosen for eachcooling loop to ensure identical mass-flow rates in each half of a loop. The φ location of eachcapillary (each has a unique serial number) was noted. During the installation of barrel 5, one of
the barrel 6 capillaries was damaged. It was successfully replaced with a spare with a very similar
mass flow at the standard pressure of 13 bar(a).
Once all the services had been removed from a barrel, the service cage was removed and thespindle withdrawn ready for the insertion of the next barrel. The unfolding process was repeated
for all barrels, with services from subsequent barrels being placed on top of the earlier ones. Extra
shelves running the length of the cradle were installed for the top and bottom test sectors in orderto support the extra weight of the TPPs. Each barrel was supported from the one outside it bytemporary interlinks. These were extended as each barrel was added. Eventually, these were
replaced with the final CFRP interlinks.
With all services for the four barrels mounted on the RSS, the sealing of services into bulkheads could take place. The front covers of the feedthroughs were then fitted and sealed with either
Techsil27 or Loctite 5145 (Tempflex) and left to cure. Service drums consisting of mesh cylindersof the same radius as barrel 3 were mounted at each end of the four-barrel structure. The readout
fibres, FSI fibres and DCS wires and patch panels were removed from the RSS and secured to the
27 A two-part adhesive to which talcum power was added as a viscosity modifier. Techsil Ltd., 30 Bidavon Ind. Estate,

Waterloo Road, Bidford on Avon, B50 4JN, UK.

– 36 –

2008 JINST 3 P10006

Figure 26. Barrel 6, with thermal enclosure side panels ready for insertion into the OTE.

4.1.5 Barrel interlinks
The Interlinks are precisely machined radial CFRP spokes which link the four SCT barrels together.

– 37 –

2008 JINST 3 P10006

drums. PEEK grommets were fitted into slots on the inner bulkhead and glued into place using
Techsil. Services were sealed into the bulkheads, again using Techsil. It was found that although
Techsil forms a very good seal around fixed objects, it does not work so well if the services arelikely to be manipulated at later stages of the installation. This was a particular problem for the
LMTs, where a gap could open at the back of the grommet when the tapes were folded forwards.
Refilling the gaps with Techsil was found not to be viable, so a thin bead of Loctite 5145 wasapplied to the back of the tapes in an attempt to improve the sealing. Once a grommet was filledwith its 6 or 12 LMTs, the grommet lid was secured with two M1×4 stainless-steel screws.
The final interlinks between the barrels were installed at this stage. A few grommets were also
secured with Loctite 5145, as the weight of the LMT bundle was found to cause the grommet tobe pulled away from the bulkhead. There were also two cases at each end where the castellations
between grommets in the inner bulkhead had to be removed to allow a special double grommet to beinserted to correct an error in the bulkhead services routing. Before the adhesive had cured, pressure
plates were fitted under each lid. These were clamped into place by a nylon screw threaded throughthe cover plate. After the glue had set, the portion of screw exposed above the lid was removed.
The middle bulkhead was then installed. This ensured the correct location of the capillaries and“soft” services. The capillary sleeves were fitted into their grommets in the middle bulkhead andglued into place using Araldite 2011. The remaining length of the capillary was coiled and stored
on the support drums. DCS wires and opto-fibres were sealed into grommets using similar methods
as for the LMTs. The fibre grommets were sealed first with un-filled Techsil (without talcum filler).
The low viscosity of this mix allowed the glue to flow around the individual fibres. Once this had
set, Techsil was then injected into the grommet to fill up any larger gaps. The two halves of theouter bulkhead were screwed to the middle bulkhead, using Techsil as a sealant.
At this stage, the pipes (“spiders”; see section 3.4.3) that would connect the ends of the cooling loops to the bulkhead connectors were installed. The copper spiders were annealed and in thisstate, fine adjustments could be made to ensure they fitted without applying significant force on
the exhaust connection or other services. Indium wire of 0.8 mm diameter was used for the seal.
The other end of the spider was then clamped into the TE bulkhead and then leak tested by pressurising the loop with helium, with both capillaries closed, and searching for leaks greater than
∼2×10−9 mbar·l/s. Figure 27 shows one barrel end with all services routed through the bulkhead
and the spiders installed.
The cooling reference disk (CRD) was cut to shape using a template. The Litz wires fromthe cooling loops were pulled through holes in the CRD and were soldered to the disk. The CRD
was grounded by trapping strips from its outer edge between the middle and outer bulkheads. The
ITE was then inserted using an adaption of the four-barrel assembly tool. The TE end panels
were bolted in place, making sure to connect all foil connections, while the continuity between
the side panels and the OTE was made using screws and Alochromed aluminium foil bonded with
conductive epoxy EP79. Once the end panels were in place, the foils from the ITE were folded out,
trimmed to length and sandwiched in between the end panel and the retaining ring. The final task
was to install the manifolds and connections of the environmental nitrogen pipes.

There are eight equispaced spokes at each barrel end. The two horizontal ones on each barrel end
are linked to the SCT barrel support on their outer end and provide support points for the pixel
detector on their inside end.
The assembly method chosen dictated the use of temporary interlinks for all the phases ofthe four barrel assembly. These temporary interlinks were heavier and made of successive shortaluminium alloy parts, allowing each barrel to be fastened into its external neighbouring barrel
without obstructing the way for the next barrel to be installed.
When all the four barrels were assembled and precisely attached together, the temporary interlinks were removed sequentially one by one and replaced by the final CFRP parts which are
dowel-pinned and bolted to the cylinders. A photograph showing one of the final interlinks in the
installed 4 barrel assembly is shown in figure 28.

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2008 JINST 3 P10006

Figure 27. Barrel end with all services routed through the bulkhead.

Figure 29. The SCT barrel and support cradle being set up for its transfer on the cantilever tool.

4.1.6 The Intermediate Service Support Structure (ISSS)The ISSS, shown in figure 29 on both ends of the SCT barrel and in figure 30 from one end of
the SCT barrel before the SCT was inserted into the TRT, consisted of two lightweight, aluminium
structures mounted at each end of the SCT barrel. These structures were supported on steel framesthat were fixed to either the barrel cradle or to the ID rails in either the ID trolley or inside the
ATLAS solenoid respectively. The ISSS was designed to contain all of the SCT services in eightsegments at each end of the barrel. The ISSS was designed to provide the safe storage of allservices during the SCT insertion into the TRT and the subsequent connection of services inside– 39 –

2008 JINST 3 P10006

Figure 28. Photograph showing one of the 8 interlinks at one end, after the four barrel assembly.

the cryostat. For all services, the respective minimum bend radii were observed. After securing all
cables, fibres and pipes inside the ISSS, it was decided not to fit the sector lids during insertion into
the TRT as this would have violated the envelope — they were fitted immediately after insertion.
On completion of the final envelope checks, the SCT barrel assembly was declared ready to moveonto its cantilever stand, prior to insertion into the TRT.
4.1.7 Tests done during four-barrel assembly

28 Henkel Corporation Aerospace Group, 2850 Willow Pass Road, P.O. Box 312, Bay Point, CA 94565, USA.
29 Varian, Inc. Corporate Headquarters, 3120 Hansen Way, Palo Alto, CA 94304-1030, USA.
30 N

2 losses to the ID volume were expected to be .100 l/hr for the full SCT– 40 –

2008 JINST 3 P10006

Prior to the four-barrel assembly, all cooling loops had been tested and no leaks had been found.
During the four-barrel assembly, each cooling loop was evacuated from the exhaust connector, withthe two capillaries blanked off. A helium leak-detector with a sensitivity of around 5 ×10−10 mbar·l/s.
was connected to the loop while helium gas was directed at the visible joints. During these tests,
three of the exhaust flanges were found to have developed leaks (of the order of 10−4 mbar·l/s).
The leaks were on barrel 3 (2 leaks) and barrel 6 (1 leaks). In each case, the leak was located at asolder joint near the outside of the exhaust manifold. All three exhaust connectors were from anearly production and the problem was traced to the Ni/Cu/Au plating on the manifold. It seemedthat the copper was delaminating from the underlying nickel layer. To overcome this problem,
PEEK inserts were machined and glued into 42 of the 44 manifolds using Hysol 939628 and secured with a bead of Araldite 2011 along the visible edge. In two manifolds, it was not possible tofit these inserts so the inner surface was sealed with Araldite 2011. On all manifolds, TorrSeal29
was applied externally around the solder joint to provide a second seal. An analysis was carriedout to estimate the maximum force that could be transmitted to these exhaust manifolds by thermal
expansion and contraction of the spiders — see section 3.4.3 for further details. The effect was
found to be most pronounced for the long (barrel 3 and barrel 6) exhausts. The hinged support was
implemented by loosening the pipe from its rigid bulkhead mounting to give it an 80 µ m float, andthen the couplings were sealed from the inside using Loctite 5145.
The barrel assembly involved the removal of most of the TPPs. For those to which accesscould be arranged, the integrity of the LMTs was checked by measuring the loop impedances ofthe lines and by checking the hybrid thermistor values. The few breaks that were discovered were
either repaired or overcome by modification to the final patch panel.
To leak test the thermal enclosure, a dry-air supply was mixed with argon and connected toa digital flow meter (0-40 l/min). The input was split to feed the four internal nitrogen pipes. A
0-to-4 mbar pressure switch, set to operate at 0.9 mbar, was connected to the middle bulkhead. A
gas bubbler, set to 0.6 mbar, was attached to the exhaust line to create a back pressure. Initially,
many leaks were found and the leak rate was so large that an overpressure of 0.15 mbar could only
be achieved with a flow rate of 2500 l/hour. The most significant leaks were at the exit of the DCS
wires, around one capillary and several fibre grommets, behind most LMTs, some exhaust spiders
and the joints between the TE side panels and the OTE half-shells. After sealing these, a 0.6 mbar
overpressure was achieved using 2.5 times the nominal flow rate (approximately 2.5 m3 /hour). At
nominal flow, there was an overpressure of 0.2 mbar. This will lead to larger than expected30 losses

of nitrogen to the Inner Detector environmental gas (CO2 ). The consequences of this leakage are
currently being studied.
4.1.8 Photogrammetry survey
Three consecutive photogrammetry surveys were performed on the SCT barrels shortly after fourbarrel assembly:• Survey 2 [28]: This survey recorded a change of the external reference targets, with a precision of 100µ m, before the SCT was inserted into the TRT.
• Survey 3 [29]: This was recorded just after the SCT had been inserted into the TRT, with anestimated precision of 150µ m to the concentricity of the detectors.
The first set of the three photogrammetry measurements was used to investigate barrel deformations. An analysis of the data at each end of the barrel, in comparison with circle hypothesis
revealed radial deviations of the order of 100µ m. The data could be well fitted by ellipses. There
was a rotation of about 0.5 radians in the directions of the major axes of the ellipses found at thetwo ends of the barrel. These features can be explained by a finite-element analysis (FEA) model
assuming a three point mount scenario. An interpolation of the measured targets to the module
positions was performed. Details on survey 3 and its comparison to track based alignment measurements from cosmic ray data are given in section 4.2.3.
4.2 Integration into the TRTThis section describes the final integration of the completed SCT barrel inside the TRT barrel.
4.2.1 Support requirements of the SCT inside the TRT
In accordance with the general support scheme of the inner detector [26], the SCT is supported
from rails integrated into the TRT inner shell. The rails are made of CFRP except for the bearing
areas, which are made of aluminium alloy. These aluminium parts also offer features to be connected to the tooling for blocking things safely during transport. The four SCT supports are rigidly
attached to the outermost barrel and to the interlink system linking the four barrels together andthey penetrate through the thermal enclosure. They have a short vertical adjustment capability andcan be locked for transport.
4.2.2 Integration procedure
The tools used for the integration of the SCT into the TRT are:
• The ID barrel trolley, designed to support the ID barrel for tests, transport and installation
into the pit.

– 41 –

2008 JINST 3 P10006• Survey 1 [27]: This is the most detailed survey of the three. It recorded the relative positionof the eight radial interlinks that support the four cylinders at either end of the barrel, as wellas the FSI targets and pixel supports (with a horizontal and vertical x− y precision of 20µ m.).
Four external reference targets were surveyed.

• The SCT cradle, designed to support the SCT barrel during its assembly and tests. The cradleis also used to transport the SCT barrel inside CERN-SR1 up to the insertion area.
• The ISSS, attached at both ends of the SCT barrel to support the service extensions during
insertion into the TRT, transport and installation.
• The SCT barrel cantilever tool, designed to support the SCT barrel from one end during theinsertion operation.
As described in section 4.1, at the end of the four-barrel assembly, the SCT barrel detector
was supported in the SCT cradle using temporary non-adjustable supports fastened to the four
final support positions. All temporary aluminium interlinks were replaced one-by-one by the final
CFRP ones; these were bolted and pinned in place with dowels. These interlinks also provide
precise survey-target locations, which were used to do a photogrammetric survey before closing
the thermal enclosure and arranging the services on the ISSS’s.
After completion of the four-barrel assembly, the cradle and SCT barrel were transferred fromthe barrel assembly area to the insertion area and raised by hydraulic jacks on a rail system to allow
for its introduction onto the SCT barrel cantilever tool. Figure 29 shows the SCT barrel on thecradle being set up for its transfer on the cantilever tool. The barrel was raised until its axis wasabove the axis of the cantilever tool. The SCT barrel was moved over the fixed cantilever standwith the cradle being guided by the floor rail system (figure 30). Then, by lowering the cradle,
the SCT was simply docked to the saddle system attached to the cantilever arm passing throughthe bore of the SCT barrel. The cradle was removed and the cantilever/SCT barrel adjusted withrespect to the rail system on which the TRT barrel trolley rolled during the insertion manoeuvre. A
geometry check was performed by moving a template along the rails.
The TRT barrel was transferred from its assembly cradle into the ID barrel trolley. The four
TRT supports were adjusted vertically in such a way as to have the same compression load on thepressure-sensitive foils integrated in its feet. The ID trolley was then transferred from its wheels to
the insertion rail system and its axis adjusted to be properly positioned and parallel with the axis

– 42 –

2008 JINST 3 P10006

Figure 30. Transfer of the barrel onto the cantilever tool.

Figure 31. SCT support installed on thebarrel. The end of the support rail inside
the TRT barrel can be seen to its left.

of the rail system. Fine tuning of the relative adjustment of the TRT and SCT was done and theinsertion was achieved by translating the ID trolley on the rails around the SCT, which was only
supported from its far end on its cantilever tool (figure 32). Since the clearances in the region of
the SCT supports (figure 31) were too small, the SCT supports were removed. Consequently theinsertion was stopped approximately 100 mm before the SCT was in its final position, and the final
SCT supports were fastened to the near end of the SCT barrel — see figure 31. Then the insertion
continued up to a point where the SCT protruded from the TRT on the far end to allow for themounting of the two remaining SCT supports. The trolley was then moved backwards to reach thefinal relative position in z of the two subsystems. Some final adjustments were made and the SCT
feet were lowered to make contact with the rails of the TRT. A front strut was installed to blockthe cantilever beam in a way so as to avoid spring-back problems. Then the cantilever support was
lowered to transfer the weight of the SCT from it to the TRT.
4.2.3 Relative alignment between SCT and TRT
After the successful insertion of the SCT barrel into the TRT and the transfer of the SCT weight
onto the support rails mounted inside the TRT inner cylinder, a theodolite survey [29] was carriedout to measure the position of the SCT barrel with respect to the TRT barrel. Table 3 summarises
the observed misalignments of the SCT with respect to the TRT central axis. The detectors are
positioned at their nominal relative z positions to better than the survey measurement precision
along the z-axis of 0.2mm.

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2008 JINST 3 P10006

Figure 32. Translation of the TRT barrel inside the trolley over the SCT barrel.

Table 3. Relative translation and rotation between the SCT barrel and the TRT barrel at the origin, extracted
from the survey measurements.

∆x [mm]−0.42∆y [mm]+0.33∆rotX [mrad]+0.154∆rotY [mrad]+0.165

The relative alignment of the SCT and TRT barrels was verified by using the reconstruction ofcosmic-ray tracks in the SCT and TRT during combined tests in CERN-SR1 (see section 6.3.2). A
special procedure was implemented to determine the relative SCT-TRT displacement:• A 2D residual was formed from the difference in the track coordinates in the plane perpendicular to the straw-layer surface (the TRT has no sensitivity along the wire direction).
• The track χ 2 was calculated and minimised by shifting the location of the SCT and TRT
in the detector geometry description used for track reconstruction. Because of the setup
used during these tests (tracks almost vertical and no TRT sensitivity along z), only thedisplacement along x (∆x) and the rotations (∆rotY and ∆rotZ) around the y- and z-axes,
respectively, could be determined.
This alignment procedure was applied on tracks collected from two different runs: one at the
beginning and one at the end of a stable period of data taking. Further details on track reconstruction and the alignment methods used are given in reference [23]. The two resulting estimates of
the SCT-TRT misalignment were consistent within their estimated uncertainties. The track-based
measurements were found to be consistent with the survey measurements summarised in table 3.

5. Tests of single barrels
The performance of SCT barrel modules was monitored through the assembly and integration tocheck for damage of components or deterioration of their performance (e.g. noise performance ofmodules). Detailed tests were carried out on every module before mounting to barrels, followed
by a retest of each module after it was mounted to its barrel. Once a barrel was completed, thefunction of all modules was tested in simultaneous operation and performance characterisation,
the so-called “single-barrel tests”. Single-barrel tests were carried out in Oxford before transport,
and at CERN-SR1 after transport for barrel 3, the first barrel to be shipped to CERN. This showedno sign of deterioration. Single-barrel tests on barrels 4 and 6 were carried out before transport
only; and on barrel 5, after transport only. Visual inspection and a minimal set of functional checks(e.g. continuity tests of LMT’s, verification of DCS sensors) were carried out on all barrels after
transport.
5.1 Test setup for single barrels
For the operation and tests of the SCT barrels during macro-assembly, the first implementation of
the final SCT off-detector system was used. The test systems in Oxford and CERN-SR1 used the– 44 –

2008 JINST 3 P10006• TRT and SCT tracks were fitted separately and the SCT tracks were extrapolated to the first
TRT straw layer.

same off-detector supply and readout as well as test procedures, with the exception of the detector
cooling plants, which were specifically built for each site. The test system required for the operation
and readout of the SCT barrels has the following main components:• Power supply and environment sensor system, which are controlled by the detector control
system (DCS). The SCT interlock system provides temperature-based safety interlocks tothe power supply system.
• The readout system of SCT modules based on the final SCT readout driver module (ROD)
system.

These tests used the final DAQ [21] and DCS [20] supply hardware. In addition to enabling themodules to be re-characterised electrically [21], the tests provided the opportunity to monitor the
longer-term stability and uniformity of other operation-related parameters such as cooling, moduletemperatures and the optical communication.
The visual inspection after transport focused on abnormalities on or near modules (displacement of module/cooling block clips or shunt-shield connections, displacement of services at barrel
ends, damage to wire bonds). High-resolution photographs of the full barrel surface and ends wererecorded for future reference. The visual inspection was followed by electrical continuity tests of all
low-mass tapes to detect any broken tracks. Figure 33 shows barrel 5 during the visual inspection.
5.1.1 Detector control system
The SCT DCS system is divided into two main parts: the power supply (PS) [10] control and theenvironmental monitoring. The PS system controls and monitors the powering of the SCT detector
modules. Each module is powered by two independent, floating power supplies. The high voltage(HV) card provides the bias voltage to the silicon sensor and the low voltage (LV) card powers theanalogue and digital voltage needed for the readout chips and the opto components on the harness.
The LV card also reads out thermistors located on the hybrid (1 NTC sensor per module side). The
cards are connected to a crate controller board (based upon the ELMB31 ), which is then read out
through a CAN32 bus and the data are sent to a PVSS33 project.
The environmental project monitors the temperature and humidity inside the detector volume.
Several temperature sensors are placed in critical locations: at the exhaust of the cooling pipes, next
to the FSI nodes to measure gas temperature, and attached to the CFRP mechanical structure. In
addition, radiation-hard humidity sensors are placed on each barrel. Their measurements, coupledwith temperature readings, are used to calculate the dew point, which is continuously monitored andcompared to ambient temperatures to avoid condensation. The environmental parameters are readout using ELMB’s connected to the DCS project with a CAN bus, similar to the PS system. The
temperature sensors on the cooling pipe are also used in the interlock system. This fully hardwareimplemented system turns off the power to the modules in about one second if the temperature goes
31 ATLAS Embedded Local Monitor Board.
32 CAN in Automation(CiA), http://www.can-cia.de.
33 PVSS-II, ETM, Austria,http://www.pvss.com/.

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2008 JINST 3 P10006• Detector cooling is provided by an evaporative cooling system based on C3 F8 (CERN-SR1
and Oxford) and C4 F10 coolants (Oxford only).

above a predefined threshold, hence protecting the modules from overheating if the cooling stops.
The interlock acts on all modules of one cooling loop.
5.1.2 Detector readout system
Control, readout and online calibration of the SCT were performed using the SctRodDaq [21]. The
SctRodDaq was developed within the ATLAS TDAQ software framework [22], whereby multiple
C++ and Java applications can communicate across multiple processors using CORBA.34 The version of SctRodDaq used for tests in Oxford and CERN-SR1 comprised several PC’s, a 6U VME
crate for trigger timing control (TTC) specific modules, and a 9U VME64x crate housing up to
14 readout driver modules (ROD’s), 14 back of crate cards (BOC’s) and a timing interface module(TIM). The ROD’s are used for the main control and data handling, whereas the BOC’s provide
the ROD interface to the front-end modules and to the rest of the ATLAS DAQ chain. The TIM
provides the interface to the ATLAS TTC system. Each ROD/BOC pair is connected optically to
up to 48 SCT modules. Details of this readout system, its hardware and software components aredescribed in [21].
5.1.3 Test procedure
The SCT uses a binary readout architecture whereby a hit is registered if the pulse height fromthe strip exceeds a preset threshold. For calibration of the SCT modules, triggers are generated
34 Common Object Request Broker Architecture defined by the Object Management Group.

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2008 JINST 3 P10006

Figure 33. Complete barrel 5, with all services and modules installed, during visual inspection on arrival at
CERN.

5.2 Results of electrical tests on single barrels
Results presented here were obtained during single-barrel tests after a cylinder was fully populatedwith modules. Module ENC noise (average ENC noise per chip derived from the 3-point-gain test)
for the four barrels immediately after assembly is shown in figure 34. The measured ENC noise
varies between the barrels due to their different operating temperatures. The barrels were operated
at hybrid temperatures of 11.3◦ C, 12.4◦ C, 16.4◦ C and −0.4◦ C for barrels 3, 4, 5 and 6 respectively.
ENC noise results for warm operation, with hybrid temperature typically ∼28◦ C, are given in [21]for single barrels and in section 6 for the completed SCT barrel inside the TRT.

– 47 –

2008 JINST 3 P10006

internally by the ROD’s or by the TIM and sent to the modules. Known test-charges are injectedby the front-end chips and the occupancy (the per-channel fraction of triggered events for which
the pulse height exceeds the threshold) is determined as a function of threshold. Histogrammingof the threshold scan is done by the ROD and analysed offline on a channel-by-channel basis. The
test procedure is described in further detail in reference [25]. The following tests are carried out oneach module.
In the setup phase of the module readout, each module is configured, a trigger is sent to it andthe returned event of this module is checked to confirm that the communication works (“module
probe”). A “hard reset” is issued by the LV power supply to the module which reverts back to its
default “clock/2” state, whereby the input clock is divided by two in frequency and passed onto
the output data-link, if the reset functions correctly. After the modules are configured, the optoreceiver threshold on the BOC is optimised for reliable readout by an “RX threshold scan”. The
correct communication is further verified with a so-called “NMask scan” in which a known test
pattern of varying length is written to the ABCD3TA mask registers, read back and compared.
After module setup, the digital function of the module is verified with a test of the ABCD3TA
pipeline cells and a check of the redundancy readout, which allows to by-pass a chip if it isfaulty [8]. Following the successful completion of digital tests, the analogue performance is tested
for each channel. In the “3-point-gain” test, three test charges (1.5 fC, 2.0 fC and 2.5 fC) are injected in each channel and its gain is measured by a threshold scan for each test charge. From theS-curve of the threshold-scan data, the gain is extracted as the threshold at 50% efficiency. A fit of
a complementary error function to the threshold scan yields the output noise at 2.0 fC. This is used
to extract the equivalent noise charge (ENC), defined as the output noise divided by the channel
gain. With this test, it is possible to identify defective channels (noisy or dead) as well as bonding
defects (noise too high or low).
The noise occupancy is defined as the single-channel noise occupancy per bunch-crossing. It
is usually averaged over an ABCD3TA chip (128 channels) or a complete module (1536 channels).
The noise occupancy is measured with a threshold scan without charge injection. The noise occupancy is given as the number of noise hits per channel and Level-1 (L1A) trigger, at a threshold of
1 fC (the operational threshold in the experiment). The special test, using two consecutive triggers
of varying delay [25], was used to check for light-leaks in the opto readout system. This can cause
additional noise when the 850 nm laser light leaks out of its enclosures and is absorbed in the active
silicon sensors. Furthermore, this test is sensitive to electrical pick-up of the digital signals present
during the readout cycle.

Table 4. Number of dead channels in SCT barrels.

Barrel 3
1190

Barrel 4
483

Barrel 5
1342

Barrel 6
3667

Total
6683
0.206%

The characterisations of modules on the barrels were carried out at a range of temperatures.
Figure 35 shows the dependence of ENC noise on the hybrid temperature. This shows a slope of
5.9 e− /◦ C and an ENC noise of 1480 e− at 0◦ C.
The number of defective channels per module is shown in figure 36. The tail in the distribution, corresponding to modules with ≥10 defective channels, is due to the use of ASIC’s with
1 faulty channel [4]. The specification for the number of defective channels is <1%, i.e. <15 channels per module. The total number of defective channels per barrel is summarised in table 4. The
overall total number of defective channels is 0.2% of channels in the SCT barrel. No deterioration
in ENC noise or numbers of defective channels was observed as a result of assembling the modules
to the cylinders.
The channel noise occupancy at 1 fC threshold, averaged over the module, is shown in figure 37. The average channel noise occupancy of 4.45×10−5 , measured on the completed barrels,
matches the noise occupancy measured during module production [4]. The specification for the
SCT single-channel noise occupancy is 5×10−4 , hence all modules are well within specification.

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2008 JINST 3 P10006

Figure 34. Average chip ENC noise for the four barrels as determined in a 3-point gain measurement during
single-barrel tests. The barrels were operated at hybrid temperatures of 11.3◦C, 12.4◦C, 16.4◦C and −0.4◦Cfor barrels 3, 4, 5 and 6 respectively.

chip_noise [e-]from single barrel tests

1750
1700
1650
1600

slope = 5.89 e-/C

1500

const @ 0C = 1480 e1450
0

5

10

15

20

25
30
35
hybrid temperature [C]number of modules

Figure 35. ENC noise versus hybrid temperature measured during single-barrel tests.

900

Mean 2.275

800

RMS

3.416

700
600
500
400
300
200
100
0 0

2

4

6

8

10
12
14
16
18
number of defect channels/module

Figure 36. Number of defective channels per module as determined in a 3-point gain measurement aftermodule assembly to the barrels.

5.3 Results of thermal tests on single barrels
A crucial aspect of the assembly of the SCT is the proper performance of the module cooling. Up
to 10 W of power per module has to be removed at a temperature which safeguards against thermal
runaway. To limit the effects of radiation damage, the temperature on the silicon sensors should
be around −7◦ C. The design of the module and the cooling system achieves this goal at a coolant
temperature in the pipe of around −25◦ C. Any serious degradation of the thermal impedance between the heat sources (ASIC’s) and the coolant will compromise the long-term operability of theaffected module in the high-rate environment of the LHC.

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2008 JINST 3 P10006

1550

number of modules

140

Mean 4.449e-05

120

RMS

3.498e-05

100
80
60
40

-3

00

0.05

0.1

0.15

×10
0.2
0.25
0.3
module noise occupancy

Figure 37. Average module noise occupancy measured on the completed barrels.

Of particular concern is the quality of the grease joint between the cooling block surrounding
the cooling pipe and the module baseboard: firstly, because of the delicacy of the grease applicationand secondly, as the expected temperature gradient across this joint is already about 2◦ C.
In the evaporative cooling system, the coolant temperature in the evaporator can be adjusted
by controlling the back-pressure (see section 3.2). In the ATLAS ID evaporative-cooling system,
each cooling loop on the detector is connected to one cooling circuit with its own back-pressure
regulation as shown in figure 14. During the assembly, the uniformity of module temperatures wasstudied in “warm” (coolant temperature about 17◦ C) and “cold” (about −7◦ C) operation. Cold
operation was achieved by using C3 F8 as a coolant at an evaporation pressure of about 3.3 bar(a),whereas warm operation was achieved either by operating the system with perfluorobutane (C4 F10 ),for which the saturation curve is shifted towards higher temperatures (for tests at Oxford), or byevaporating the original coolant, C3 F8 , at higher pressure (6.5 bar(a), for tests at CERN). The
choice of these temperatures was driven by the dew point achieved during the tests — this had tostay safely below the coolant temperature.
The temperature of the hybrids on the SCT modules is measured by two thermistors,35 oneon the top and one on the bottom side of the module [4]. The data from the thermistors are readout through the low voltage supply cards, from where they are transferred via CANbus to the
PVSS-based DCS software projects. While this system monitors temperatures continuously, forthe analysis discussed, the focus is on the spatial uniformity of the thermal behaviour and not the
dynamic behaviour of the system. Therefore module temperatures discussed here are single values
obtained in stable conditions.
For this analysis, the temperature of a given module was obtained as the mean of the twotemperature sensors on the two hybrids. On average, modules were slightly hotter by 0.33◦ C on theside facing the CFRP barrel (bottom) than on the outside (top). The uncertainty on the temperature
measurement of one module is 0.66◦ C. Before the temperature uniformity of the assembled barrels
could be assessed, two systematic effects had to be removed:
35 Semitec 103KT1608-1P.

– 50 –

2008 JINST 3 P10006

20

13.5
13.0
12.5
12.0
11.5
11.0
10.5
10.0
9.5
1

5

9

13

17

21

Module #Figure 38. Average module hybrid temperature along a stave for barrel 3 during cold tests at Oxford. The
coolant enters the pipe near Module 1 and leaves the pipe near Module 24.

1. Fluctuations between cooling loops, which were likely caused by small differences in the
back-pressure regulators and/or gravitational effects.
2. Variations of the temperature along the loop due to the frictional and momentum pressure
drop in the pipe (as an example, see figure 38).
After removal of the systematic effects, the r.m.s. of the distribution of the module temperatures for the complete barrels (for warm and cold test runs) was between 0.6 and 1.1◦ C, with thelarger values typically obtained for the outer barrels. On barrels 3, 4, 5 and 6, the numbers ofmodules whose hybrid temperature exceeds a threshold 2.3◦ C above the average temperature were0, 4, 5 and 14 modules respectively (0%, 0.8%, 0.9% and 2.1% of the total number of modules
on each barrel). This threshold was taken as three times the measurement error plus variations
expected from powering. These 23 modules (1.1 % of the complete barrel) with larger than the
typical temperature variation fall into two categories:• Modules where the temperature of one thermistor is significantly different from the temperature recorded in the other thermistor on the same module. Modules with this characteristic
have only been used on the outer barrels, where radiation damage to the silicon strip detectors
will be lower.

– 51 –

2008 JINST 3 P10006

Average T [°C]

B3 cold

[OC]2.5

Module position

Temperature variation map for Barrel 3
12

2
1.5

10
1
0.5

8

6

-0.5
-1

4
-1.5
-2

2
5

10

15

20

25

30
LMT #

-2.5

Figure 39. 2-dimensional distribution of the relative hybrid temperatures after corrections for systematiceffects for the barrel 3 reception tests at CERN.

• Modules which are close to the input and output of the cooling pipe. The cooling pipe
was clamped to the structure close to these modules and this additional constraint prevented
the pipe from conforming well to the module baseboard surface. In a few cases, thermal
performance could be recovered for this class of modules by loosening and re-tightening of
the cooling-loop support bracket at the barrel end.
These studies show that a high degree of uniformity of the thermal impedance throughout the barrel
SCT has been achieved. This is essential for the safe operation of the modules over the lifetime ofthe experiment (see figure 39).

6. Final combined tests before installation
The tests described in this section were carried out in the CERN-SR1 test area with the alreadyassembled SCT and TRT barrels, following the insertion of SCT into TRT barrel. For the final testof the completed barrels, two opposite detector sections were connected to the supply and readout
systems of the SCT and TRT. The schematics in figure 40 (right) illustrate the chosen test sectors
for the TRT and SCT, as well as the scintillator arrangement. The photograph shows the barrel
during the cabling of the top sector, before the bottom sector was cabled. Further details on thistest setup and combined tests between SCT and TRT are given in references [23, 24]. The goalsof the final tests were to gain operational experience with the combined barrels, to measure thenoise performance in the final arrangement prior to installation in the ATLAS cavern and to collect– 52 –

2008 JINST 3 P10006

0a cosmic-ray data set. This was subsequently used to test the tracking performance of the SCT and
the SCT combined with the TRT [23].
6.1 Operation of the SCT inside the TRT
Two SCT sectors, comprising a total of 468 barrel modules on barrels 3, 4, 5 and 6, were connectedto the power supply, cooling and readout systems. The power supply, interlock, environmental
monitoring and readout systems were identical to those described for the CERN-SR1 system in
section 5.1.1, with the addition of the SCT timing and trigger control (TTC) system for synchronisation to the TRT and cosmic-ray trigger. The detector services were arranged in a configuration
as close as possible to the final configuration for ATLAS. Two sections of the inner and outer heatspreader plate (HSP) were mounted around the SCT services. (The HSP is required for electromagnetic shielding and for cooling of services. Once the SCT is installed together with the TRT
in the cryostat, the heat spreader plate encloses the SCT barrel services along their routing above
the TRT barrel electronics.) The HSP ground was connected to the thermal enclosure bulkhead
and to the SCT detector ground. The SCT was electrically isolated from the TRT, tooling andsupport structure (except for special tests) to match the final ground installation of ATLAS and it
was connected to a single grounding star-point. The SCT barrel volume was flushed with dry air
at a rate of 36 l/min to maintain a dew point of −40◦ C during the tests. Additional dry-air flushing was provided in the service volume between the TRT electronics and the SCT HSP to prevent
condensation on services or the bulkhead.
During the final tests, a total of 12 barrel cooling loops were operated with the CERN-SR1
C3 F8 evaporative cooling system at a typical back-pressure setting of 5.6 to 5.8 bar. For thissetting, with modules powered, configured and read out, a cooling-pipe temperature of 14±1◦ C– 53 –

2008 JINST 3 P10006

Figure 40. ID barrel setup for the cosmic-ray test (left) and the configuration of module groups chosen forthis test (right).

(a)Mean 28.28
RMS 1.513

30
25

number of modules

number of modules

35

80

(b)
70

Mean 5.535
RMS 0.1186

60
50

20
40
30

10

20

5

10

0
20

22

24

26

28

0
4

30 32 34 36 38 40
hybrid temperature [C]

4.5
5
5.5
6
6.5
module analog+digital power [W]7

Figure 41. (a) Hybrid temperature for warm operation and (b) module power consumption at the start of atypical noise occupancy scan.

was measured. Figure 41a shows the distribution of hybrid temperatures measured at the start ofa noise-occupancy calibration scan. For this back-pressure setting, the mean was 28.3◦ C and ther.m.s spread was 1.5◦ C. No correction for loop-to-loop variation or variation along the cooling loop
was carried out for this study. The recorded temperatures indicate that the thermal performance ofbarrels was maintained during four-barrel assembly and integration to the TRT. The module power,
as the sum of analogue and digital power consumption of the ASIC’s, is shown in figure 41b forthe same run.
6.2 Results of barrel calibration tests inside the TRT
The calibration tests after integration with the TRT were aimed at determining the number of defective channels in the tested sectors and the analogue performance (noise, gain) in comparison to
the tests on single barrels. While this was not a complete test of the detector, the results can beconsidered as representative for the SCT barrel performance after integration with the TRT. The
following tests have been carried out (see section 5.1.3 and [25] for reference):• Check of module supply: Vdd and Vcc supply currents and voltages, detector bias voltage
and current, temperature readout of hybrid thermistor.
• Digital tests: opto scan (RX threshold), NMask, bypass tests [21].
• Analogue tests: three-point gain, response curve, noise occupancy, double-trigger noise,
synchronous-trigger noise-occupancy.
• Sensor leakage current for all modules.

– 54 –

2008 JINST 3 P10006

15

number of chips

number of chips

180
Mean 4.374e-05

160
RMS

Mean 4.323e-05

1202.631e-05

140

RMS

2.465e-05

100

120

Bottom sector

Top sector
80

100

60

60
40
40
20

20
×10
0
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
chip noise occupancy

-3

×10
0
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
chip noise occupancy-3

Figure 42. Noise occupancy for front-end chips in bottom and top test sectors after the SCT integration with
the TRT.

• Detector timing: check of hit coincidences between module sides during a scan of the level-1
trigger accept (L1A) delay with respect to cosmic-ray trigger.
Checks of the module supply and digital tests were carried out during the setup phase of thedetector in order to achieve a reliable readout of all modules.
During the setup phase for these calibration tests, the total number of defective links wasdetermined. For the total barrel, not only for the sections used in the final test, six and four data and
TTC links respectively were found to be dead after the final integration of the four single barrels.
These are thought to be due to fibre breakages at the services thermal feedthroughs. It should be
noted that the data for defective data links are not lost but can still be accessed through the readout
redundancy [11].
The initial tests were followed by the commissioning of the top sector and the bottom sector
with analogue calibration tests. Figure 42 shows the noise occupancy for all chips on the top and thebottom sector. The average noise occupancy was 4.4×10−5 — this matches the noise occupancy
found in the single-barrel tests. The ENC noise, as measured in a three-point gain measurement, isshown for the four barrels individually in figure 43. For a hybrid operation temperature of 28◦ C,
the mean ENC noise was 1681 e− , consistent for the four barrels.
To allow a direct comparison of ENC noise between (a) tests after the four-barrel assembly andintegration with the TRT with (b) tests carried out on single barrels, it was important to correct for
the known dependence of noise on the operating temperature (as shown in figure 35 with 6 e− /◦ C).
Figure 44 shows the difference between the ENC noise as measured on the SCT inside the TRT
and the temperature-corrected ENC noise measured on single barrels. A slight increase in noise of
32 e− (2%) is observed after the four-barrel assembly and integration with the TRT. The gain, as– 55 –

2008 JINST 3 P10006

80

n3

Noise Barrel 3
9080

Noise Barrel 4

Entries

1008

Mean

1684

RMS

75.62

120

n4
Entries 1272
Mean
1680
RMS
63.6

100

70
60

80

50
60

40

40

20
20
10
0
1000
1200 1400 1600 1800 2000 2200 2400
chip noise [e-]Noise Barrel 5
160
140

0
1000
1200 1400 1600 1800 2000 2200 2400
chip noise [e-]n5Noise Barrel 6

Entries 1728
Mean
1681
RMS
76.66

160

n6
Entries 1572
Mean
1679
RMS
79.95

140

120

120
100
100
80
80
60

60

40

40

20

20

0
1000
1200 1400 1600 1800 2000 2200 2400
chip noise [e-]010001200 1400 1600 1800 2000 2200 2400
chip noise [e-]Figure 43. Chip ENC noise for individual barrels after the SCT integration with the TRT.

measured after integration, is shown in figure 44 and shows no change.
As preparation for cosmic-ray data taking, the SCT L1A trigger was timed in with respect tothe scintillator trigger. This was done using the ROD histogramming of hit coincidence between thetwo sides of modules while the offset between SCT L1A and the cosmic-ray trigger was varied [30].
A coincidence is defined as a hit on each sensor side of a module, with the additional requirementthat the physical location of the hits overlap within the readout width of a chip (approximately
1 cm). With the typical noise occupancy, the rate of accidental noise coincidences is minimal.
Figure 45 shows the number of coincidences as a function of the L1A delay in units of bunchcrossings. The plot shows the enhancement of coincidences when the correct trigger delay (set to 0
in this measurement) was set. Since the cosmic-ray trigger is uniformly distributed over the 25 ns
wide clock-cycle, coincidences are registered in two adjacent time-bins.

– 56 –

2008 JINST 3 P10006

30

htempEntries 5580
Mean 53.29
RMS
2.328

number of chips

700
300

Mean 32.66

600

RMS 43.22

250

500
200

400

150

300
200

50

100

0
-300

-200

-100

0

040

100
200
300
Noise difference [e-]

45

50

55

60
65
70
chip gain [mV/fC]

Figure 44. Difference in ENC noise between measurements with the SCT in the TRT and the single-barrel
noise measurements (left), and the mean analogue gain of front-end chips as measured after the SCT with
the TRT integration (right).

Figure 45. Number coincidences between hits in both sides of a module as function of the delay between
L1A and the cosmic-ray trigger. This information was collected during SCT timing-in for cosmic-ray runs.

6.3 Noise and cosmic-ray tests with the TRT
In addition to the calibration tests, the SCT was run in a physics mode, analogous to the final
ATLAS configuration. Data were passed through the ATLAS ATHENA [31] framework. This
converts the raw-hit information in the SCT into space-points and tracks for further analysis. This
output was then used to examine the noise occupancy, efficiency and tracking performance of thedetector, as well as providing a basis for detailed studies of detector alignment. A brief summaryof the tests performed in CERN-SR1 is given here; full details may be obtained from [23].

– 57 –

2008 JINST 3 P10006

100ln(N)

-6
-8

-10

-14
-16
-18

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

2

2

t /(fC)

Figure 46. A plot of ln N vs t 2 for one side of a barrel SCT module. A straight line has been fitted, indicatingthat the distribution follows the expected shape.

6.3.1 Noise tests
Several noise tests were performed with random triggers during the CERN-SR1 commissioning
period in an attempt to characterise thoroughly the behaviour of the detector in the presence of the
TRT. Data were taken at trigger rates of 500 Hz, 5 kHz and 50 kHz, from which it was concluded
that there is no significant variation of the SCT barrel noise occupancy with trigger rate. Data
were collected with different module-threshold settings in order to obtain a curve of ln(N) vs t 2 ,where N is the single-strip noise occupancy per bunch-crossing and t is the threshold. Figure 46
shows the results of the threshold scan as ln(N) versus threshold squared. The straight line fit to
the data confirms the expected gaussian noise distribution. The results revealed that all modules
were performing comfortably within specification. Grounding issues were investigated by runningthe barrel under two different grounding schemes, without effect on the noise occupancy.
One significant test with regards to the SCT/TRT integration involved the examination ofcross-talk between the two subsystems. Runs were taken in which the trigger offset between the
SCT and TRT was slowly varied around the correct value, thus providing a probe of possible pickup
induced in the SCT by the TRT readout cycle. Runs were taken with TRT off and on. No evidence
for cross-talk was found in any of the tests, and the final noise occupancy values for the SCT
running in conjunction with the TRT indicate a noise performance within the design specification.
6.3.2 Cosmic-ray tests
Observation of cosmic muons passing through the SCT barrel provides valuable input into tracking
and alignment studies. In addition, the large samples of tracks collected during the CERN-SR1– 58 –

2008 JINST 3 P10006-12

Event: JiveXML_2850_00023

Run: 2850

Event: 23-100

0

X (cm)

100

Figure 47. A reconstructed cosmic-ray track with SCT and TRT hits.

commissioning period allowed a first preliminary efficiency measurement to be made of the SCT
modules in the final detector configuration.
Figure 47 shows a reconstructed cosmic-ray track with both SCT and TRT hits. From this, it
can be deduced that both that the TRT and SCT are well timed-in and that the pattern recognition
and tracking algorithms are performing well. The selection of high-quality tracks (obtained by
cutting on such quantities as the number of hits in the SCT, the track χ 2 and the incident angle of the
track on a module) permits detailed studies of module efficiency and alignment. Figure 48 shows
the hit efficiency per layer, with nominal module locations and after alignment. After alignment,
the unbiased hit efficiency in all barrel layers is measured to be within specifications, i.e. greaterthan 99%. Furthermore, since the cosmic-ray trigger was not synchronised with the readout clock,the collected data could also be used to study the dependence of the SCT efficiency on the arrival
time of the particle. The hit efficiency was found to remain high for a range of 4 ns around thenominal trigger time.
With respect to the tracking performance, the SCT detector resolution can be extracted fromthe r.m.s. of the residual distribution for the SCT hits after allowing for track extrapolation errors
and correcting for any misalignments. This procedure is most reliable for high-momentum tracks
for which an r.m.s of around 28 µ m is obtained. With a track extrapolation uncertainty of around
16 µ m, this corresponds to a detector resolution of 23 µ m, in agreement with expectations. Due tothe absence of a magnetic field, it was not possible to determine the particle momentum. Therefore,
the influence of low-momentum tracks on the measured residual distribution had to be minimisedby cutting on the track χ 2 , as described in [23].

– 59 –

2008 JINST 3 P10006

-100

0

Y (cm)

100

ATLAS Atlantis 2006-06-02 17:45:23 MET DST

Unbiased Hit Efficiency

1
0.998
Cosmic Data
0.996
0.994
0.992
0.99

0.986

No alignment

0.984

Global Chi2 alignment

0.982
0.98
0

0.5

1

1.5

2

2.5

3
3.5
4
layer and side

Figure 48. Hit efficiency measured for the different barrel layers from cosmic-ray data. The efficiencies
are shown separately for each side within a layer. Use has been made of the different available alignment
corrections obtained by the SCT alignment methods described in reference [23].

7. Summary
This paper describes the engineering design philosophy, fabrication, construction and integrationof a low-mass charged-particle tracking system that has been installed in the ATLAS detector. The
tracker is required to operate inside a 2 T solenoidal magnetic field, to provide hermetic coverage
up to |η | =2.5 and to generate up to four space-points for each charged track. This performancemust be delivered without placing excessive material in front of the calorimeters. Therefore, thedesign has been driven by the need to select materials with the longest radiation lengths and optimal
strength-to-weight ratios. The complete SCT barrel supports 2112 double-sided silicon modules
and their associated services. Early results from the cosmic-ray runs in the CERN-SR1 assemblybuilding indicate that the modules are positioned with the required accuracy. For high momentumtracks, an r.m.s of around 28µ m is obtained for the hit residuals. Allowing for the track extrapolation errors of around 16 µ m, this is compatible with the expected detector resolution of 23 µ m.
For the complete SCT barrel, the average channel noise occupancy per bunch-crossing is
4.4×10−5 — this matches the noise occupancy found in the single barrel tests. For a hybrid operation temperature of 28◦ C, the mean ENC noise is 1681 e− , consistent for the four single barrels.
Furthermore, the unbiased hit efficiency in all barrel layers was measured to be within specifications, i.e. greater than 99%.
Clearly, some compromises have been made to achieve these goals and the resultant decision to– 60 –

2008 JINST 3 P10006

0.988

reduce the number of connectors within the solenoid volume resulted in many variants of services
assemblies and increased the lengths of services that needed to be stored and manipulated during
the integration process. As a corollary to this, it is hoped to achieve benefits in the long-term
reliability of the tracker.
The engineering challenges offered by the construction of the SCT barrel have brought together a large, multi-national team of technicians, engineers and scientists who have worked in true
collaboration to meet the deadlines and targets of the project. Not all scheduled dates were achieved
but perhaps this was to be anticipated, given the complexity and technical demands imposed on theteam by the innovative design.

We are greatly indebted to all the technical staff who worked on the barrel engineering and integration project from the ATLAS SCT Institutes. We acknowledge the support of the funding
authorities of the collaborating institutes including the Spanish National Programme for Particle
Physics; the Research Council of Norway; the Science and Technology Facilities Council of theUnited Kingdom; the Polish Ministry of Higher Education and Science; the German Ministry of
Science; the Swiss National Science Foundation; the State Secretariat for Education and Researchand the Canton of Geneva; the Slovenian Research Agency and the Ministry of Higher Education,
Science and Technology of the Republic of Slovenia; the Ministry of Education, Culture, Sports,
Science and Technology of Japan; the Japan Society for the Promotion of Science; the Office ofHigh Energy Physics of the United States Department of Energy; the United States National Science Foundation; the Australian Research Council (ARC) and Department of Education, Scienceand Training (DEST); the Dutch Foundation for Fundamental Research on Matter (FOM); the Ministry of Education, Youth and Sports of the Czech Republic, Ministry of Industry and Trade of the
Czech Republic, and Committee for Collaboration of the Czech Republic with CERN; the National
Science Council, Taiwan; the Swedish Research Council.

A. Acronyms
CF
CFRP
CTE
CRD
DCS
ENC
ESD
FEA
FSI
HEX
HSP
ISSS
L1A

Carbon Fibre
Carbon Fibre Reinforced Plastic
Coefficient of Thermal Expansion
Cooling Reference Disk
Detector Control System
Equivalent Noise Charge
Electrostatic discharge
Finite Element Analysis
Frequency Scanning Interferomtry
Heat Exhanger
Heat Spreader Plate
Intermediate Service Support Structure
Level 1 Accept trigger– 61 –

2008 JINST 3 P10006

Acknowledgments

LMT
PPB n
ROD
RSS
TDAQ
TE
TIM
TPP

Low Mass TapePatch Panel Barrel n
Readout Driver Module
Radial Service Support
ATLAS data DAQ framework
Thermal Enclosure
Timing Interface Module
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Hadron Collider at CERN, CERN-LHCC-94-43.
[2] ATLAS collaboration, G. Aad et al., The ATLAS Experiment at the CERN Large Hadron Collider,2008 JINST 3 S08003.
[3] ATLAS collaboration, ATLAS Inner Detector Technical Design Report, CERN, April 1997.
[4] A. Abdesselam et al., The barrel modules of the ATLAS semiconductor tracker, Nucl. Instrum. Meth.
A 568 (2006) 642.
[5] A. Abdesselam et al., The ATLAS semiconductor tracker end-cap module, Nucl. Instrum. Meth. A 575(2007) 353.
[6] A. Ahmad et al., The silicon microstrip sensors of the ATLAS semiconductor tracker, Nucl. Instrum.
Meth. A 578 (2007) 98.
[7] R. J. Apsimon et al., Application of advanced thermal management technologies to the ATLAS SCT
barrel module baseboards, Nucl. Instrum. Meth. A 565 (2006) 561.
[8] F. Campabadal et al., Design and performance of the ABCD3TA ASIC for readout of silicon strip
detectors in the ATLAS semiconductor tracker, Nucl. Instrum. Meth. A 552 (2005) 292.
[9] Y. Unno et al., Application of Cu-polyimide flex circuit and Al-on-glass pitch adapter for the ATLASSCT barrel hybrid, Nucl. Instrum. Meth. A 541 (2005) 286.
[10] P.W. Phillips, The ATLAS SCT Power SupplySystem, Proceedings of Topical Workshop on Electronics
for Particle Physics (TWEPP 2007), Prague, Czech Republic, 3-7 September 2007.
[11] A. Abdesselam, The optical links of the ATLAS Semiconductor Tracker, 2007 JINST 2 P09003.
[12] V. Vacek, G. Hallewell, S. Lindsay and S. Illie, Perfluorocarbons and their use in cooling systems forsemiconductor particle detectors, Fluid Phase Equlibria 174 (2000) 191.
[13] V. Vacek, G. Hallewell and S. Lindsay, Velocity of sound measurements in gaseous per fluorocarbons
and their mixtures, Fluid Phase Equilibria 185 (2001) 305.
[14] M. Olcese et al., Evaporative Cooling System for the ATLAS Inner Detector, 2008 JINST 3 P07003.
[15] P. Bonneau, Evaporative cooling system for Pixel/SCT, ATC-TL-EP-0001, EDMS Id 391168 (2006).
[16] P.A. Coe, D.F. Howell and R.B. Nickerson, Frequency scanning interferometry in ATLAS: remote,multiple, simultaneous and precise distance measurements in a hostile environment, Meas. Sci.
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[18] S.M. Gibson, The ATLAS SCT alignment system and a comparative study of misalignment at CDF
and ATLAS, D.Phil. Thesis, Oxford UK (2004).
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