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  ![](media/image1.wmf){width="0.9326388888888889in"   Pixel Detector Active                   
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  ATLAS Project Document No:                           Institute Document No.  Created :       Page: **1
                                                                               **22/10/99**    of** **7**

  ATL-IP-EP-0004                                                               Modified :      Rev. No .:
                                                                                               **draft**
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| Pixel Detector Active |                       |                       |
| Area Layout           |                       |                       |
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| Abstract              |                       |                       |
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| This document         |                       |                       |
| describes the layout  |                       |                       |
| of the active area of |                       |                       |
| detector elements of  |                       |                       |
| ATLAS Pixel Detector. |                       |                       |
|                       |                       |                       |
| All relevant          |                       |                       |
| assumptions are       |                       |                       |
| presented and the     |                       |                       |
| p                     |                       |                       |
| hysical/technological |                       |                       |
| constraints are       |                       |                       |
| discussed.            |                       |                       |
|                       |                       |                       |
| The document is meant |                       |                       |
| to basically (The     |                       |                       |
| basic purpose of this |                       |                       |
| document is to...)    |                       |                       |
| provide the required  |                       |                       |
| input to those who    |                       |                       |
| are in charge of the  |                       |                       |
| calculation of the    |                       |                       |
| detector performance  |                       |                       |
| and to people in      |                       |                       |
| charge of finalising  |                       |                       |
| the design of the     |                       |                       |
| Pixel Detector        |                       |                       |
| support structure and |                       |                       |
| services.             |                       |                       |
|                       |                       |                       |
|                       |                       |                       |
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| Prepared by:          | Checked by:           | Approved by:          |
|                       |                       |                       |
| David Bintinger       |                       |                       |
|                       |                       |                       |
| Marco Olcese          |                       |                       |
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| Distribution List     |                       |                       |
|                       |                       |                       |
| Atlas Collaboration   |                       |                       |
| for comments          |                       |                       |
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  History of                            
  Changes                               

  Rev. No.   Date            Pages      Description of changes

  Draft      25/10/99        All        Draft release for comments     
                                        
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Table of Contents

1 INTRODUCTION [4](#introduction)

2 PHYSICS REQUIREMENTS [5](#physics-requirements)

3 LAYOUT CONSTRAINTS [5](#layout-constraints)

4 MODULE ENVELOPES [5](#module-envelopes)

5 BARREL LAYOUT [5](#barrel-layout)

5.1 Basic assumptions [5](#basic-assumptions)

5.2 Acceptance losses calculation method
[5](#acceptance-losses-calculation-method)

5.3 Module layout on stave [5](#module-layout-on-stave)

5.4 Stave layout on phi (Stave azimuthal layout)
[5](#stave-layout-on-phi-stave-azimuthal-layout)

6 DISK LAYOUT [5](#disk-layout)

6.1 Basic assumptions [5](#basic-assumptions-1)

6.2 Acceptance losses calculation method
[5](#acceptance-losses-calculation-method-1)

6.3 Module layout on disk [5](#module-layout-on-disk)

6.4 Disk layout in z [5](#disk-layout-in-z)

7 GLOBAL LAYOUT OF PIXEL ACTIVE REGION
[6](#global-layout-of-pixel-active-region)

# INTRODUCTION

The pixel detector is made of an array of sensitive elements organised
in two main structures: barrel and disks.

The barrel region is made of three coaxial cylinders (B-layer, layer 1
and layer 2) each containing a different number of staves.

One stave is a basic element with 13 detector elements.(The basic
element of a barrel layer is a stave with 13 detector elements.)

The two disk regions, one at each end of the barrel, contain five disks
each.

Each disk is made of sectors, one sector supports 6 detector elements
three on each face.

The layout is driven by many constraints and its final design is a
result of a very sophisticated and refined optimisation involving almost
all fields of the Pixel Detector project (mechanics, physics,
electronics, ..).

# PHYSICS REQUIREMENTS

I would illustrate the basic criteria of the layout of disks and barrel
layers, explaining the physics requirements (acceptance, ...) saying how
the barrel layout is linked to the disk one.

# LAYOUT CONSTRAINTS

A few words on the constraints on: min radius for B-layer, max radius
for layer 2, max outer/inner disk radius, disk position along z.

Do we need a special section , or subsection, on the B-layer replacement
constraints?

The disk are physically constrained in inner and outer radii and z
position. The physical inner radius of the disks should not be less than
the physical inner radius of barrel layer 1. This constraint insures
that any device that will fit within barrel layer 1 will also pass
through the disks. The physical outer radius of the disk s must leave
sufficient space between the disks and the inner surfaces of the global
support frame for the routing of services and for disk support and
alignments structures. At present these limitations are taken as a
minimum physical inner radius of 95 mm and a maximum physical outer
radius of 206 mm.

The z positions of the disks are constrained by the necessity to rout
barrel services between the physical end of the barrel and the first
disk and by the necessity to keep the maximum z extent of the pixel
detector less than +/- 780 mm. The requirement of service space between
the physical end of the barrel region and the first disk also imposes a
maximum radius constraint on barrel layer 2 since disk 1 approaches the
barrel as the radius of barrel layer 2 increases. At present disk 1 is
constrained to have an absolute z position of approximately 490 mm or
greater. The last or fifth disk z position is constrained to be no
greater than 770 mm from the interaction point to fit within the pixel
detector volume.

# MODULE ENVELOPES

# BARREL LAYOUT

## Basic assumptions

## Acceptance losses calculation method

## Module layout on stave

## Stave layout on phi (Stave azimuthal layout)

# DISK LAYOUT

## Basic assumptions

The pixel disks continue two pixel hit coverage beyond the pixel
barrels. The region extends from approximately an eta of 1.8 to an eta
of 2.5. Pixel disks are sized in radial extend and positioned in z so
that continuous two hit coverage is provided in this region. Since there
is no tilting of disk modules there is little or no dependence of
coverage on particle transverse momentum.

## Acceptance losses calculation method

Losses due to small v-shaped gaps in acceptance at the outer active
radii of each disk have been estimated using a Monte Carlo simulation
which tracks particles. Particles which do not register at least two
hits are counted as lost. The losses in acceptance from eta of
approximately 1.8 to eta of 2.5 (the disk region) are approximately 0.3%
largely independent of the transverse momentum of the particles.

## Module layout on disk

The physical and active dimensions of disk modules are the same as
barrel modules. Modules are attached to both sides of disk structures
whose thickness, for the baseline design, is 5.2 mm. Each disk is
divided into sectors of six modules each for fabrication and service
modularity. The modules on one side of a disk are offset in azimuth by
360/(nr modules) degrees from the modules on the other side to insure
active area overlap. The twelve o'clock position on a disk lies halfway
between a module on one side and the other. The number of modules per
disk and their inner active radii are given in the table following
section 6.4. Drawings X and Y show the 66 module (11 sector) and 54
module (9sector) disks respectively. The inner active radii quoted below
are to the center of the module's active inner radial side. The fact
that modules on one face of a disk can not overlap constrains the disk
layout such that there are small v-shaped gaps in active coverage of
each disk at the outer radii. The acceptance losses due to these gaps
are given in section 6.2.

## Disk layout in z 

The z position of disks 1 and 2 are determined by the end of acceptance
coverage of barrel layers 1 and 2 respectively. Disk 3 begins acceptance
coverage where disk 1 ends and so on until coverage of two pixel hits is
extended to an eta of 2.5. The nominal thickness of the disks is assumed
to be 5.2 mm, hence the modules on the side of disk facing the IP are
2.6 mm closer to the IP and those on the opposite side are 2.6 mm more
distant from the IP.

Disk Nr Modules Active Inner Radius Disk Center Z Position

1 66 121.4 mm 490 mm

2 66 121.4 591

3 66 121.4 651

4 54 99.2 710

5 54 99.2 770

# GLOBAL LAYOUT OF PIXEL ACTIVE REGION

Here a drawing should provide all geometrical information to locate each
module active area in the pixel detector volume.

I can ask to Genoa people to prepare such a drawing.