---------------------------------------------------- ----------------------- ---------------- ------------
  ![](media/image1.wmf){width="0.9326388888888889in"   ATLAS Pixel Local                        
  height="0.975in"}                                    Supports Requirements                    

  ATLAS Project Document No:                           Institute Document No.  Created :        Page: **1
                                                                                                of** **14**

  ATL-IP-EP-0005                                                               Modified :       Rev. No .:
                                                                               **13/11/2023**   **2**
  ---------------------------------------------------- ----------------------- ---------------- ------------

+-----------------------+-----------------------+-----------------------+
| ATLAS PIXEL LOCAL     |                       |                       |
| SUPPORT REQUIREMENTS  |                       |                       |
|                       |                       |                       |
| Abstract              |                       |                       |
|                       |                       |                       |
| The design            |                       |                       |
| requirements for the  |                       |                       |
| ATLAS Pixel System    |                       |                       |
| Local Supports are    |                       |                       |
| presented. The Local  |                       |                       |
| Supports are barrel   |                       |                       |
| staves and disk       |                       |                       |
| sectors. Requirements |                       |                       |
| common to both are    |                       |                       |
| given and the         |                       |                       |
| requirements applying |                       |                       |
| only to staves or     |                       |                       |
| sectors are also      |                       |                       |
| provided.             |                       |                       |
+-----------------------+-----------------------+-----------------------+
| Prepared by:          | Checked by:           | Approved by:          |
|                       |                       |                       |
| E. Anderssen, S.      |                       |                       |
| Cuneo, M. Gilchriese, |                       |                       |
| M. Olcese             |                       |                       |
+-----------------------+-----------------------+-----------------------+
| Distribution List     |                       |                       |
+-----------------------+-----------------------+-----------------------+

+---------+---------+---------+----------------------------------------+
| History |         |         |                                        |
| of      |         |         |                                        |
| Changes |         |         |                                        |
+---------+---------+---------+----------------------------------------+
| Rev.    | Date    | Pages   | Description of changes                 |
| No.     |         |         |                                        |
+---------+---------+---------+----------------------------------------+
| 1       | 1/25/01 |         | General update based on final local    |
|         |         |         | support design.                        |
| 2       | 6/27/01 |         |                                        |
|         |         |         | Some missing requirements included,    |
|         |         |         | some changes to editing, slight        |
|         |         |         | changes to thermal requirements,       |
|         |         |         | change power load according to DMS     |
|         |         |         | technology, updated stability          |
|         |         |         | tolerances according to new layout,    |
|         |         |         | stability tolerances for the B-layer   |
|         |         |         | included.                              |
+---------+---------+---------+----------------------------------------+

Table of Contents

1 Introduction [5](#introduction)

2 Dimensional Requirements [6](#dimensional-requirements)

2.1 Barrel Staves [7](#barrel-staves)

2.1.1 Geometry ad reference system [7](#geometry-ad-reference-system)

2.1.2 Envelopes and As-Built Tolerances
[8](#envelopes-and-as-built-tolerances-1)

2.1.3 Survey Tolerances [8](#survey-tolerances)

2.2 Disk Sectors [8](#disk-sectors)

2.2.1 Geometry [8](#geometry)

2.2.2 Envelopes and As-Built Tolerances
[9](#envelopes-and-as-built-tolerances-2)

2.2.3 Survey Requirements [9](#survey-requirements)

3 Thermal Requirements [9](#thermal-requirements)

3.1 Introduction and Background [9](#introduction-and-background)

3.2 Maximum Operating Temperature [9](#maximum-operating-temperature)

3.3 Minimum Operating Temperature [9](#minimum-operating-temperature)

3.4 Temperature Uniformity [9](#temperature-uniformity)

4 Normal Operating Pressure Requirements
[10](#normal-operating-pressure-requirements)

5 Temperature and Pressure Cycling
[10](#temperature-and-pressure-cycling)

6 Transient Conditions Requirements
[10](#transient-conditions-requirements)

7 Material Requirements [10](#material-requirements)

8 Radiation Requirements [10](#radiation-requirements)

9 Fault Condition Requirements [10](#fault-condition-requirements)

9.1 Pressure fault [10](#pressure-fault)

9.2 Temperature fault [11](#temperature-fault)

10 Stability Tolerances and Requirements
[11](#stability-tolerances-and-requirements)

10.1 Introduction [11](#introduction-1)

10.2 Stave Requirements [11](#stave-requirements)

10.3 Disk Sector Requirements [12](#disk-sector-requirements)

11 Miscellaneous Requirements [12](#miscellaneous-requirements)

11.1 Conducting Particles [12](#conducting-particles)

11.2 Corrosion [12](#corrosion)

11.3 Erosion [12](#erosion)

11.4 Repair [12](#repair)

11.5 Surface [12](#surface)

11.6 Impact of Ambient Atmosphere [12](#impact-of-ambient-atmosphere)

11.7 Leak rate of cooling connections
[12](#leak-rate-of-cooling-connections)

# Introduction

This note summarizes the requirements for the Local Supports of the
ATLAS Pixel System. A general description of the ATLAS Pixel System and
the definition of Local Supports are given elsewhere.[^1] There are two
types of Local Supports: barrel staves and disk sectors. These may have
different dimensional, survey and stability requirements, but have
common thermal, material, operating pressure, transient or fault
tolerance and miscellaneous requirements. Requirements given here apply
to barrel layers 1 and 2. Special B-layer requirements are under
development.

The common requirements for both barrel staves (including the B-layer)
and disk sectors are summarised in Table 1. The other requirements for
barrel staves and disk sectors are summarized in Table 2 and Table 3,
respectively. Details of the requirements are provided in subsequent
sections.

+----------------------------------+-----------------------+----------+
| **Item**                         | **Requirement**       | **To be  |
|                                  |                       | checked  |
|                                  |                       | on full  |
|                                  |                       | prod     |
|                                  |                       | uction** |
+----------------------------------+-----------------------+----------+
| ###### Normal Operating T        |                       |          |
| hermal Conditions at Full Power  |                       |          |
| {#normal-operating-thermal-condi |                       |          |
| tions-at-full-power .unnumbered} |                       |          |
+----------------------------------+-----------------------+----------+
| T from coolant to face of local  | 15 ^O^C               | X        |
| support                          |                       |          |
+----------------------------------+-----------------------+----------+
| Temperature uniformity on local  | 13 ^O^C               | X        |
| support                          |                       |          |
+----------------------------------+-----------------------+----------+
| **Normal Operating Pressure      |                       |          |
| Conditions**                     |                       |          |
+----------------------------------+-----------------------+----------+
| Operating pressure at full Power | 2.0 bar abs           |          |
+----------------------------------+-----------------------+----------+
| Operating pressure in Stand-by   | 4.0 bar abs           |          |
| condition                        |                       |          |
+----------------------------------+-----------------------+----------+
| Pressure cycling of 0.1 bar      | 10^7^ cycles          |          |
| about operating pressure         |                       |          |
+----------------------------------+-----------------------+----------+
| Maximum pressure drop across     | 170 mbar              |          |
| stave/sector at Full Power       |                       |          |
+----------------------------------+-----------------------+----------+
| ###### Transient an              |                       |          |
| d Fault Conditions {#transient-a |                       |          |
| nd-fault-conditions .unnumbered} |                       |          |
+----------------------------------+-----------------------+----------+
| Meet all requirements after T    | 30 cycles             |          |
| cycles(+20^o^C -25^o^C)          |                       |          |
+----------------------------------+-----------------------+----------+
| Minimum temperature of Local     | -35^o^C               |          |
| Supports and Modules on start-up |                       |          |
| of cooling                       |                       |          |
+----------------------------------+-----------------------+----------+
| Once-per-lifetime pressure fault | 8 bar abs             | X        |
| for one hour                     |                       |          |
+----------------------------------+-----------------------+----------+
| Temperature runaway due to loss  | no thermal/structural |          |
| of coolant with module on for    | damage                |          |
| one hour once per lifetime       |                       |          |
+----------------------------------+-----------------------+----------+
| ###### Radiation Conditions {#r  |                       |          |
| adiation-conditions .unnumbered} |                       |          |
+----------------------------------+-----------------------+----------+
| Total dose                       | 500,000 Gy(50,000,000 |          |
|                                  | Rads)                 |          |
+----------------------------------+-----------------------+----------+
| ######                           |                       |          |
| Material {#material .unnumbered} |                       |          |
+----------------------------------+-----------------------+----------+
| Radiation length(active)         | 0.7 %X~0~             |          |
| region - goal                    |                       |          |
+----------------------------------+-----------------------+----------+
| ######                           |                       |          |
| Miscellaneous Conditions {#misce |                       |          |
| llaneous-conditions .unnumbered} |                       |          |
+----------------------------------+-----------------------+----------+
| Dynamic stability as supported   | 100Hz                 |          |
+----------------------------------+-----------------------+----------+
| Conducting particles from carbon | Not allowed           |          |
| or other materials               |                       |          |
+----------------------------------+-----------------------+----------+
| Corrosion from all sources       | Prevent               |          |
+----------------------------------+-----------------------+----------+
| Erosion from fluid flow          | Prevent               |          |
+----------------------------------+-----------------------+----------+
| Repair after complete assembly   | Not required          |          |
+----------------------------------+-----------------------+----------+
| Surface characteristics          | Compatible with       |          |
|                                  | module attachment     |          |
+----------------------------------+-----------------------+----------+
| Impact of ambient atmosphere     | Compatible with dry   |          |
|                                  | nitrogen with up to   |          |
|                                  | about one-part-per    |          |
|                                  | thousand of C~3~F~8~  |          |
+----------------------------------+-----------------------+----------+
| Maximum leak rate of each        | 10^-7^ cc/atms of He  | X        |
| cooling connection               |                       |          |
+----------------------------------+-----------------------+----------+

Table 1. Common requirements of barrel staves (including B-layer) and
disk sectors. See text for explanation of conditions.

+---------------------------------+-------------------------+----------+
| **Item**                        | ##### *Requirement* {#r | **To be  |
|                                 | equirement .unnumbered} | checked  |
|                                 |                         | on full  |
|                                 | \(mm\)                  | prod     |
|                                 |                         | uction** |
+=================================+=========================+==========+
| ###### Envel                    |                         |          |
| opes and As-Built Tolerances (C |                         |          |
| ommon) {#envelopes-and-as-built |                         |          |
| -tolerances-common .unnumbered} |                         |          |
+---------------------------------+-------------------------+----------+
| Max deviation from nominal      | 0.25                    | X        |
| shape (along Z                  |                         |          |
+---------------------------------+-------------------------+----------+
| Max deviation from nominal      | 0.25                    | X        |
| shape (along X)                 |                         |          |
+---------------------------------+-------------------------+----------+
| Max lateral tilting of each     | 0.05                    | X        |
| step(Y) displacement            |                         |          |
+---------------------------------+-------------------------+----------+
| Single step planarity           | 0.05                    | X        |
+---------------------------------+-------------------------+----------+
| ###### Stability Toler          |                         |          |
| ances (layer 1&2) {#stability-t |                         |          |
| olerances-layer-12 .unnumbered} |                         |          |
+---------------------------------+-------------------------+----------+
| Z                               | 0.11                    |          |
+---------------------------------+-------------------------+----------+
| Y(R)                            | 0.042                   | X        |
+---------------------------------+-------------------------+----------+
| X()                             | 0.014                   |          |
+---------------------------------+-------------------------+----------+
| ###### Stability To             |                         |          |
| lerances (B-layer) {#stability- |                         |          |
| tolerances-b-layer .unnumbered} |                         |          |
+---------------------------------+-------------------------+----------+
| Z                               |                         |          |
+---------------------------------+-------------------------+----------+
| Y(R)                            |                         | X        |
+---------------------------------+-------------------------+----------+
| X()                             |                         |          |
+---------------------------------+-------------------------+----------+

Table 2. Requirements applying only to barrel staves. See text for
explanation of conditions.

+---------------------------------+-------------------------+----------+
| **Item**                        | ##                      | **To be  |
|                                 | ##### Requirement {#req | checked  |
|                                 | uirement-1 .unnumbered} | on full  |
|                                 |                         | prod     |
|                                 | \(mm\)                  | uction** |
+---------------------------------+-------------------------+----------+
| ###### Envelopes and As-Bui     |                         |          |
| lt Tolerances {#envelopes-and-a |                         |          |
| s-built-tolerances .unnumbered} |                         |          |
+---------------------------------+-------------------------+----------+
| Faceplates as cut               | 0.25                    | X        |
+---------------------------------+-------------------------+----------+
| Thickness of sector not         | 5.0                     | X        |
| including support buttons       |                         |          |
+---------------------------------+-------------------------+----------+
| Faceplate planarity             | 0.075                   | X        |
+---------------------------------+-------------------------+----------+
| #                               |                         |          |
| ##### Stability Tolerances {#st |                         |          |
| ability-tolerances .unnumbered} |                         |          |
+---------------------------------+-------------------------+----------+
| Z (out-of-plane)                | 0.10                    |          |
+---------------------------------+-------------------------+----------+
| Y(R) in-plane                   | 0.065                   |          |
+---------------------------------+-------------------------+----------+
| X() in-plane                    | 0.006                   |          |
+---------------------------------+-------------------------+----------+

Table 3. Requirements applying only to disk sectors. See text for
explanation of conditions.

# Dimensional Requirements

The dimensional requirements for barrel staves and disk sectors are
covered in this section. Survey and stability requirements are also
included here. Whenever possible tolerances are given. These represent
hard limits that the design and fabrication of the Local Supports must
meet. Some of these tolerances (i.e. those related to stability) are
derived from the impact of misalignments on track reconstruction in the
Pixel System, which are more naturally expressed in terms of
root-mean-square (rms) deviations. In these cases, the tolerance is
related to the rms by the following formula: tolerance range = +/- rms
deviation x (12)^1/2^ where deviation means difference of a given
dimension from the undeformed to the deformed conditions due to a given
action or load. Tolerances are expressed in the local coordinate system
appropriate to survey measurements so that Quality Control procedures
may be applied.

## Barrel Staves

### Geometry ad reference system 

The geometry of the barrel stave is given in Figure 1.

![](media/image2.png){width="5.447222222222222in"
height="8.030555555555555in"}

Figure 1. Barrel stave geometry.

The adopted local reference system for the geometrical quality control
and survey is defined according to the following criteria:

-   Right-handed Cartesian co-ordinate system

-   x-z plane is defined by two reference planes at the ends of the
    stave (marked with "D" in Figure 1)

-   z axis is parallel and shifted by 6 mm towards the stave centreline
    with respect to the projection on x-z plane of the line joining the
    centres of the two reference balls located on the reference planes
    (marked with "C" in Figure 1)

-   z=0 is placed in the middle of the straight line joining the centres
    of the two reference balls.

-   orientation of axes are shown in figure 1

### Envelopes and As-Built Tolerances

The envelopes and as built tolerances are shown in Table 2. The total
allowable deviation in Y from the nominal shape (longitudinal combined
with lateral tilting) is such to allow for adequate clearance in-between
adjacent staves. The tolerance in X also arises from the need to allow
adequate clearance.

### Survey Tolerances

Both reference balls and planes, which define the stave co-ordinate
system, are always accessible with a touch probe, also when the modules
are mounted. Each module will be surveyed relative to this reference
system. The two reference balls and the two reference planes will be
also used at later assembly stages to survey the position of each stave
relative to the external support structures.

## Disk Sectors

### Geometry 

The geometry and definition of the local co-ordinate system of the disk
sector are given in Figure 2. The x-axis is defined by the two extreme
mounting buttons. The y-axis is perpendicular to the x-axis at the
center of the mounting button shown. The z-axis is defined in a
right-handed co-ordinate system from the average surface of the x=y=0
mounting button.

![](media/image3.wmf){width="6.763888888888889in"
height="5.472222222222222in"}

Figure 2. Geometry of disk sector.

### Envelopes and As-Built Tolerances

The envelopes(22.4 mm wide) of the pixel modules are 1.0 mm apart at the
inner radius. The as-built dimensions of the faceplates of the sector
have a tolerance of + 0.25mm to allow adequate clearance at the inner
radius of the sectors when the sectors are mounted on disk support ring.
The as-built dimensions of the faceplates of the sector have a tolerance
of -0.25 mm so that carbon-carbon material is always present under all
modules. The thickness of the sector, not including mounting
buttons/washers, shall not exceed 5.0 mm. The thickness of the sector is
limited by material considerations well before reaching any interference
limits with neighbouring (in Z) disks. The Z positions of modules on the
sectors are referenced to a plane defined by the (average) Z values of
the faces of the three mounting buttons on the face of the sector
mounted to the disk support. The deviation from planarity of each
faceplate relative to this plane shall not exceed 0.050 mm. This is set
by the thickness of the material used for module attachment.

### Survey Requirements

The location of every pixel module on sector will be measured using an
optical coordinate measurement machine. The fiducial marks in the corner
of each sensor will be measured relative to a coordinate system defined
by the mounting buttons on the sector. These mounting buttons are, in
turn, referenced to the corresponding mounting buttons on the disk
support ring. It is also planned to survey each sector after mounting on
a disk support ring and to reference this survey to disk ring mounts.

# Thermal Requirements

## Introduction and Background

The temperature of the pixel silicon sensors is determined by the heat
load generated by the integrated circuit electronics attached to the
sensors, leakage currents in the sensors, heat flow from the surrounding
power and cooling services and heat flow from/to outside the pixel
detector volume. Since the heat transfer efficiency (convection) to the
surrounding environmental gas is very low, the temperature of the
sensors is driven by the heat dissipation efficiency of the cooling
structures. The local supports are designed for a maximum power of 39W
per disk sector and 83W per stave, except for the B-layer in which case
the maximum power is 105W.

The primary thermal requirement of the local supports is to achieve an
operating temperature of the pixel silicon detectors of 0^0^C. The
operating temperature of the silicon detectors must by maintained at or
below -0^0^C in order to achieve an acceptable lifetime after
irradiation.[^2]

The temperature of coolant at the local support inlet is driven by the
pressure drop in the stave or sector. A maximum coolant temperature
difference between inlet and outlet of 2.5 °C is a reasonable compromise
between allowable pressure drops in the local support, and the maximum
allowable temperature difference between coolant and modules. This
corresponds to a maximum pressure drop in a stave of around 170 mbar.
Two local supports are currently assumed to be cooled in series.
Therefore, given a minimum temperature at the outlet of the local
support of --25°C, the maximum temperature at the inlet of the first
local support will be --20°C.

*The temperature of the silicon sensors is determined by the (local)
coolant temperature, the temperature gradient between the average
(local) coolant temperature and the face surface of the local support
and the temperature gradient between the face of the local support and
the sensor, through the coupling adhesive or grease, the electronics
integrated circuits and bump bonds. Assuming a maximum inlet coolant
temperature of -20^o^C and a maximum temperature gradient through the
adhesive layer of 5°C the allowed temperature gradient from the average
(local) coolant to the face of the local supports should be 15^O^C.*

The local pressure (boiling conditions) determines the temperature of
the coolant in the stave. The minimum practical temperature at the inlet
of the compressor and the pressure drops in the return pipe limit the
outlet temperature of the stave (or sector). We assume the minimum inlet
pressure of the compressor to be above the atmospheric (to avoid
contamination of the coolant in case of leaks). Assuming a minimum
pressure drop in the return pipes (consistent with an acceptable piping
layout and sizes) of 650 mbar, the minimum temperature that can be
realistically achieved at the outlet of the local support is --25°C.
During the start up of the cooling system lower temperatures could be
temporarily reached in the local support, a minimum transient coolant
temperature of --35°C is assumed.

## Maximum Operating Temperature

The maximum normal operating temperature of the pixel silicon sensors is
0^0^C.

## Minimum Operating Temperature

The minimum normal operating temperature of the pixel silicon sensors is
-25^o^C, but during transient conditions the minimum temperature may
reach -35^O^C.

## Temperature Uniformity 

The temperature uniformity under power is given by three contributions:
uniformity of the heat transfer efficiency from the local support
cooling channel to the face surface (5^o^C); uniformity of the
module-to-support adhesive interface (5^o^C); and the temperature
distribution along the local support cooling channel (5^o^C for two
local supports in series). Thus the desired uniformity of temperature of
the pixel silicon sensors on a [single]{.underline} local support is
about 13^o^C.

# Normal Operating Pressure Requirements

Cooling by evaporation of C~3~F~8~ is assumed. The typical operating
pressure within a local support is 2 bar absolute. The normal operating
pressure within a local support will not exceed 4 bar absolute. The
fluid channels within the Local Supports and the connections to them
must be leak tight to 4 bar absolute under normal operating conditions
and after fault conditions.

Pressure cycling around the normal operating pressure of 2 bar absolute
is assumed to be 0.1 bar. However, more experience with the baseline
evaporative cooling system is required to better establish this
requirement and the corresponding number of cycles expected over the
life of the detector

# Temperature and Pressure Cycling

All requirements must be met after pressure and temperature cycling of
the local supports and the interfaces according to the following table
(Table 4):

+------------------+--------------+------------+----------------------+
| TRANSIENT MODE   | PRESSURE     | T          | NUMBER OF CYCLES     |
|                  | CYCLE (bar)  | EMPERATURE |                      |
|                  |              | CYCLE (°C) | (LIFETIME)           |
+------------------+--------------+------------+----------------------+
| Single module    | \-           | 20         | 750 (\*)             |
| On-Off           |              |            |                      |
+------------------+--------------+------------+----------------------+
| Cooling circuit  | 3            | 30         | 750                  |
| On/Off           |              |            |                      |
+------------------+--------------+------------+----------------------+
| Detector warm-up | 3            | 45         | 30                   |
+------------------+--------------+------------+----------------------+

> (\*) It is expected to have one On/Off transient per day over the
> whole detector, the worst case for an individual module would be
> realistically to have 1 cycle per week.

Table 4. Local support temperature and pressure cycling conditions.

# Transient Conditions Requirements

Transient conditions are those conditions that occur under normal
operating circumstances during e.g. start-up or shutdown of the cooling
or power system. The types of transients and their frequency are given
in Table 5.

+-------------+-------------+-------------+-------------+-------------+
| TRANSIENT   | EXPECTED    | Tmax (°C)   | Tmax (°C)   | p (bar) IN  |
| TYPE        | FREQUENCY   | ON LOCAL    | ON GLOBAL   | LOCAL       |
|             |             | SUPPORTS    | SUPPORTS    | SUPPORTS    |
|             |             |             |             | AND COOLING |
|             |             |             |             | PIPES       |
+-------------+-------------+-------------+-------------+-------------+
| Local       | **daily**   | **20**      | **0**       | **0**       |
| module      |             |             |             |             |
| power       |             | (-25 to     |             |             |
| ON/OFF      |             | --5)        |             |             |
| (cooling    |             |             |             |             |
| always ON)  |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Cooling     | **weekly**  | **30**      | **0**       | **3**       |
| circuit(s)  |             |             |             |             |
| ON/OFF      |             | (-35 (\*)   |             |             |
|             |             | to --5)     |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Detector    | **6         | **45**      | **30**      | **3**       |
| warm-up     | months**    |             |             |             |
|             |             | (-25 to     | (-10 to     |             |
|             |             | +20)        | +20)        |             |
+-------------+-------------+-------------+-------------+-------------+

(\*) minimum expected temperature during the start up of the cooling
system

Table 5. Temperature and pressure transients.

# Material Requirements

It is desirable to minimize the amount of material as measured in
radiation lengths (X~0~) for the local supports. The desired goal for
the radiation length of a local support, averaged over the (active)
region covered by a pixel module is to be \<0.7% X~0~.

# Radiation Requirements

All requirements must be met after a total dose of 5x10^5^ Gy(5 x 10^7^
Rad).

# Fault Condition Requirements

## Pressure fault

The design of the local supports and interfaces to them will assume a
once-per-lifetime single system failure resulting in a maximum pressure
of 8 bar absolute for a duration of one hour.

## Temperature fault

The design of the local supports and interfaces to them will assume that
no damage both structural (loss of integrity), geometrical (loss of
shape, distortions) and thermal (loss of cooling efficiency) occurs in
case of single system failure resulting in partial or complete loss of
coolant with power on (one hour T runaway once per lifetime).

# Stability Tolerances and Requirements 

## Introduction

The ultimate alignment of the pixel modules will be done with charged
particle tracks. In principle, it is possible to do this (there are
enough tracks) on about a daily basis. However, this imposes severe
requirements on the software reconstruction process and it is desirable
that the system be as stable as possible and meet other requirements.
The stability tolerances of the local supports cannot be separated
easily from system stability requirements. The stability tolerances for
the barrel staves may be, and are, different than for the disk sectors
because of the different geometry and support of the two structures.

The [total]{.underline} stability requirements are comprised of module
attachment, local support, intermediate support structures (disk rings
and barrel shells) and intermediate supports-to-global support frame
stability requirements. It is not possible to entirely separate these
from each other from current (or future) measurements or finite-element
analyses. Thus some judgement must used to apportion the \"stability
budget\" among these contributions. The stability of module attachment
will be excellent and is ignored here.

Practically, the local support stability is assessed by measuring
motions under thermal and pressure changes, including the ability to
meet requirements after transient and fault conditions. If these motions
are less than the tolerance allocation, then the stability requirements
are judged to be met. Similarly, the frequency response of the local
supports is measured and judged to be acceptable for peak motion above
100 Hz(including support conditions).

The reference conditions for stability measurements are ultimately
defined by the ability to align elements of the system with particle
tracks. The system must be stable on a timescale long enough to perform
this type of alignment. We assume that transient and fault conditions
occur rarely enough that alignment by tracks would be possible
afterwards. A reference condition for the stability requirements is a
temperature cycle of 20°C, although tests are typically performed over a
larger temperature range. This value (20^O^C) arises from the module
on/off conditions given in Table 4. The reference condition for pressure
stability is 0.1 bar about 2 bar absolute, which we expect to be the
variation during stable operation. However, measurements of stability
under pressure variations are typically done over a much wider range of
pressures.

## Stave Requirements

The intrinsic resolution for the barrel is assumed to be 0.010 mm in and
0.080 mm in Z. The stability tolerance is calculated to keep the
degradation of the intrinsic resolution to less than 20%. Therefore the
stability requirements are 0.0066mm(rms) in and 0.053 mm(rms) in Z for
the barrel system. Displacements of a module in r affect the and Z
stability and are added in quadrature. Since the expected stave
stability in is very high (better than 0.005 mm(rms), this tighter limit
helps in relaxing the stability requirement in r(most difficult
direction for the stave to be kept stable). In Z direction the worst
location for combined r-Z stability is about 100 mm (assuming five
supports for the stave) away from the most sensitive location (end of
the stave), where the expected r stability should be an order of
magnitude better than the one at the worst location. However, the
uniformity of the active thickness of the silicon sensor becomes
important at this level as does the actual distribution of hit pixels in
Z.. This is particular important for the B-layer, which covers a larger
angular range.

Assuming a stability budget for the stave to be about 80% of the total
stability budget, the stability tolerance for a stave have been
calculated, and are given in Table 6.

  -------------------- ---------------- ---------------- ----------------
                       R (mm)           \(mm\)           Z (mm)

  **Layer 1 and 2**                                      

  Total barrel module  0.015            0.005            0.04
  stability (rms)                                        

  Stave stability(rms) 0.012            0.004            0.032

  Stave stability      0.042            0.014            0.11
  tolerances                                             

  **B-layer**                                            

  Total barrel module  0.015            0.005            0.04
  stability (rms)                                        

  Stave stability(rms) 0.012            0.004            0.032

  Stave stability      0.042            0.014            0.11
  tolerances                                             
  -------------------- ---------------- ---------------- ----------------

Table 6. Stave stability requirements summary.

## Disk Sector Requirements

The intrinsic resolution in the disk region is assumed to be 0.012 mm in
and 0.100 mm in R. An rms deviation of 0.140 mm in the Z location of a
module at the most sensitive location in the disk system (disk closest
to the interaction point at outer radius) results in a degradation of
the resolution in R by about 20%. Measurements in the more accurate
coordinate are even less sensitive to a local distortion (rotation about
the local Y axis of a module) and thus deviations in Z of a sector by
roughly a factor of three. However, motions of the sector in the plane
of the faceplate of a sector must be less than an rms of about 0.008 mm
to have less than a 20% effect on the intrinsic resolution. The
[total]{.underline} stability requirements for modules on a disk sector
are summarized in Table 7.

The stability of disk sectors in Z and in the plane of the faceplates
can be and has been estimated via measurements under the operating
conditions. Similarly, the stability of a complete disk has been and can
be roughly measured before final assembly, although all forces from
service connections cannot be imposed. Stability of a disk within the
global support frame has not been measured and will not be easily
measured with precision. Thus it makes sense to apportion the
\"stability budget\" with tighter restrictions on the sector, then the
disk support ring and finally the attachment of the disk ring to the
global support frame. We also assume the deviations from initial
locations in each case may be added in quadrature. The sector stability
requirement is estimated from the total stability budget by assuming the
contribution from the disk ring/disk is twice the sector and the
contribution from the disk-to-frame stability is four times the sector
contribution. The tolerances are summarized in Table 7 under these
(arbitrary) assumptions.

  -------------------- ---------------- ---------------- ----------------
                       R (mm)           \(mm\)           Z (mm)

  Total disk module    0.066            0.008            0.140
  stability(rms)                                         

  Sector               0.0144           0.0017           0.0306
  stability(rms)                                         

  Sector stability     0.050            0.006            0.106
  tolerances                                             
  -------------------- ---------------- ---------------- ----------------

Table 7. Disk sector stability requirements summary.

# Miscellaneous Requirements

We summarize here miscellaneous requirements common to both staves and
sectors.

## Conducting Particles

The Local Supports shall be coated or impregnated to prevent conducting
particles from being liberated, which might cause electrical breakdown
in the pixel modules or connections to them.

## Corrosion

The materials in the Local Supports and connections to them shall be
such as to prevent corrosive effects that would prevent the Local
Supports from meeting the requirements. This includes corrosive effects
that might be induced by galvanic action between metallic and carbon
materials, by coolant contact with coolant channels or connections to
them, including adhesives, and environmental effects (e.g. humidity).

## Erosion

The materials in the Local Supports and connections to them shall be
such as to prevent erosion of material that might be caused by passage
of the coolant fluid.

## Repair

Repair of barrel staves and disk sectors after assembly is not required.

## Surface

The surface of the local supports is required to meet the interface
requirements for pixel module attachment. The surface must be clean of
volatile residues and residual particles. There is no specification for
surface roughness.

## Impact of Ambient Atmosphere

The ambient operating atmosphere around the local supports will be dry
nitrogen with up to about one-part-per thousand of C~3~F~8~. The
construction of the local supports shall be such as to meet all
requirements under such a condition.

## Leak rate of cooling connections

The local support cooling circuit is made of a continuous tube embedded
in the support structure and connected to a fitting at each end.

The max acceptable leak rate for the local supports depends on the
acceptable leak of the entire cooling circuit in between the two
pressure regulators located on the external platforms.

The max leak rate of one cooling circuit is limited by the max
acceptable vacuum leak during the system filling.

Assuming:

-   a max acceptable vacuum leak per circuit of 0.01 mbar/s

-   the leak rate of the tubes at least 100 less than that of the
    fittings

-   standard fittings to have a leak rate at least 100 smaller than
    custom fittings

-   all the non standard fittings inside the pixel detector volume to
    have the same leak rate

This gives a maximum leak budget per cooling connection (i.e. stave tube
to end fitting) of 10^-7^ cc/atms of He

[^1]: ATL-IP-EM-0026, Pixel Local Supports Overview - Final Design
    Review.

[^2]: Pixel Technical Design Report, CERN/LHCC/98-13, May 1998. See
    Chapter 4 for background information.