Ultra-light and stable composite structure to support and cool the ATLAS pixel detector barrel electronics modules

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Abstract

The design of an ultra light structure, the so-called “stave”, to support and cool the sensitive elements of the Barrel Pixel detector, the innermost part of the ATLAS detector to be installed on the new Large Hadron Collider at CERN (Geneva), is presented.

Very high-dimensional stability, minimization of the material and ability of operating 10 years in a high radiation environment are the key design requirements.

The proposed solution consists of a combination of different carbon-based materials (impregnated carbon–carbon, ultra high modulus carbon fibre composites) coupled to a thin aluminum tube to form a very light support with an integrated cooling channel.

Our design has proven to successfully fulfil the requirements. The extensive prototyping and testing program to fully qualify the design and release the production are discussed.

Introduction

The Pixel detector [1] is the innermost part of the ATLAS detector to be installed in the new Large Hadron Collider under construction at the CERN Particle Physics Laboratory in Geneva. Two proton beams will collide in the central point of ATLAS (interaction point).

The ATLAS Pixel Detector (Fig. 1) has to provide critical tracking information for pattern recognition of charged particles generated at the interaction point, with very high efficiency and resolution. The support structure of the pixel electronic sensors has to fulfil tight precision and stability requirements and also has to integrate an efficient cooling system. In fact in the pixel volume a high electronics density produces a relevant amount of heat, which has to be efficiently dissipated in order to keep the pixel sensors at a temperature below 0°C to minimize the radiation damage.

Section snippets

The ATLAS Pixel Detector

The ATLAS Pixel Detector consists of over 100 million readout channels organized in an array of 1744 sensitive elements (modules). The module layout has been studied to achieve the required coverage of the solid angle around the interaction point.

The basic structure of the detector consists of a barrel section with three coaxial layers of staves and two disk sections each containing three disks (Fig. 1(a)).

Each barrel layer consists of a cylindrical turbine-like sequence (Fig. 1(b)) of

Requirements and constraints

The pixel detector support structure is subject to tight physics requirements:

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    achieve a full coverage of the sensitive area around the interaction point;

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    excellent transparency to particles (a “material budget” equivalent in terms of thickness to less than 1% of the radiation length of the material is a common requirement);

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    extremely high stability within few microns.

There are also additional constraints due to the installation and operating conditions:

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    very limited access to the detector for

Pixel sensitive elements

The pixel module is the core unit of the pixel detector (Fig. 2). It is a highly integrated electromechanical unit including:

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    a silicon pixel sensor;

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    16 readout chips bump-bonded on the pixel sensor;

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    a hybrid circuit wire-bonded to the readout chips, housing the Module Control Chip (MCC) and providing the interfaces to the optical, signal and power cables. Each module dissipates about 8W over a surface of about 2×6cm2.

The “stave”: concept and requirements

The pixel stave is the basic module local support unit of the barrel section of the pixel detector. The stave provides independent support and cooling to 13 modules allowing for a highly modular barrel assembly, which is critical for easy maintenance.

The 13 modules are arranged one after the other along the stave axis and they are partially overlapping in order to achieve the required coverage in the axial direction. Thus the surface of the stave in contact with the modules is stepped and the

Design and choice of the materials

The extremely demanding requirements restrict the choice of the materials to just a few options; this has implied a very long development and optimization process to converge to the best design.

The most difficult design issues to be addressed are cooling and stability. The cooling is critical for the good operation of the modules. The heat is produced over a relatively large area and it has to be transferred with high efficiency to a small cooling tube (Fig. 4).

To minimize the temperature

Analysis

A finite element model of the stave has been made using the ANSYS code,1 with a very fine mapped mesh to reproduce the small thickness of the different materials. The geometry has been modelled with all its major details.

Some conservative assumptions have been made:

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    the aluminum tube has been considered only as an added mass, as shear coupling through the grease layer is difficult to

Qualification program

Several real scale prototypes of staves have been assembled, and a very comprehensive test program has been carried out to validate the design.

Simulations and measurements were always found in good agreement.

Production and quality control

Given the tight requirements, the complexity of the manufacturing process and the relatively high number of items, technical specifications have been developed. This has also given the opportunity to review, document and optimize the pre-production experience. The manufacturing process has been analyzed, identifying the various phases, and procedures have been written down to describe in detail every single step, from raw material purchase to final assembling; critical phases have been

Conclusions

A support structure (stave) for the pixel electronic sensors of the barrel ATLAS pixel detector has been developed. Several severe constraints are imposed to this support structure by both the physics to be performed and by the foreseen operating conditions. Our project has been especially designed to cope with all of these constraints.

A stave is also intended to include a cooling system to dissipate the heat due to the electronics. The project has then used a combination of carbon-based

Acknowledgments

The authors want to thank their colleagues of the ATLAS pixel team for the many interesting discussions. Thank are also due to F. Gastaldo for his contribution in the early phase of this project.

References (7)

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  • E. Anderssen, et al., Fluorocarbon evaporative cooling developments for the ATLAS pixel and semiconductor tracking...
  • M. Olcese, et al., Impregnation process for thin gas-tight carbon–carbon composite structures, in: Proceedings of the...
There are more references available in the full text version of this article.

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