# ATLAS PIXEL LOCAL SUPPORT REQUIREMENTS

# 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 is 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.

The common requirements for both barrel staves and disk sectors are
summarized in Table 1. The other requirements for barrel staves and disk
sectors are summarized in Table 2 and Table 3, respectively.

  ----------------------------------- -----------------------------------
  **Item**                            **Requirement**

  *Normal Thermal Conditions*         

  ∆T from coolant to face of local    ≤15
  support(^O^C)                       

  Temperature uniformity on local     ≤7
  support(^O^C)                       

  Meet all requirements after T       20 cycles
  cycles(+20^o^C ↔ -20^o^C)           

  Minimum temperature                 -25^O^C

  *Normal Pressure Conditions*        

  Maximum normal operating            4.0
  pressure(bar absolute)              

  Pressure cycling of ± 0.5 bar about Nnnnnn cycles
  2 bar absolute                      

  *Radiation Conditions*              

  Total dose                          500,000 Gy(50,000,000 Rads)

  *Transient and Fault Conditions*    

  Once-per-lifetime pressure fault    8 bar absolute
  for one hour                        

  Once-per-lifetime temperature fault ??^O^C
  for ??                              

  *Miscellaneous Conditions*          

  Conducting particles from carbon or Not allowed
  other materials                     

  Corrosion from all sources          Prevent

  Erosion from fluid flow             Prevent

  Repair after complete assembly      Not required

  Surface characteristics             Compatible with module attachment
  ----------------------------------- -----------------------------------

Table 1 Common requirements of barrel staves and disk sectors. See text
for explanation of conditions.

  ----------------------------------- -----------------------------------
                                      

                                      
  ----------------------------------- -----------------------------------

Table 2 Requirements applying only to barrel staves.

  ----------------------------------- -----------------------------------
  **Item**                            **Requirement**

  *Envelopes and As-Built Tolerances* 

  Faceplates as cut(mm)               ± 0.250

  Thickness of sector not including   ≤ 5.0
  support buttons(mm)                 

  Faceplate planarity(mm)             ± 0.075

  *Survey Tolerances*                 

  Reference targets on faceplates to  ± 0.010
  mounting holes(mm)                  

  Reference targets front-to-back     ± 0.025
  sides(mm)                           

  *Stability Tolerances*              

  Z (out-of-plane)(mm)                ± 0.10

  Y(R) in-plane(mm)                   ± 0.065

  X(φ) in-plane(mm)                   ± 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 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 typically related to the rms by the
formula tolerance = x (12)^1/2^ rms(\*\*\*this needs to be agreed
\*\*\*). Tolerances are expressed in the local coordinate system
appropriate to survey measurements so that Quality Control procedures
may be applied.

## Barrel Staves

### Geometry 

The geometry of the barrel stave is given in Figure 1 (or Figs. ?? since
may need to define local coordinate system at typical module location
and coordinate system for stave) .

Figure 1 Barrel stave geometry and summary of key requirements.

### Envelopes and As-Built Tolerances

### Survey Tolerances

### Stability Tolerances

## Disk Sectors

### Geometry 

The geometry of the disk sector is given in Figure 2 along with a
summary of the key requirements.

![](media/image1.wmf){width="6.493055555555555in"
height="4.868055555555555in"}

Figure 2 Geometry of disk sector and summary of key requirements.

### Envelopes and As-Built Tolerances

The as-built dimensions of the faceplates of the sector have a tolerance
of + 0.25 mm 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 neighboring (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 ± 0.075 mm.

### Survey Tolerances and Requirements

The location of every pixel module on sector will be measured. The
reference coordinate system on the sector for these measurements is
defined by the location of the mounting buttons. The origin of the
coordinate system corresponds to the center of the precision hole in one
of the mounting buttons. For practical reasons, this coordinate system
is referenced to optical survey targets. Four of these are mounted on
each faceplate for redundancy. The location in the X-Y plane and in Z of
fiducial marks(at least four) on the backside of the silicon sensor on
each pixel module are measured relative to this coordinate system and
recorded.

The two sides of the sector may be referenced by using the through holes
in the mounting buttons. Again for convenience, two small tooling balls
that are visible from either side of the sector will be precisely
located at the inner radius of the sector. These will provide a
secondary means of referencing the modules on the front and back sides
of the sector.

Each optical survey target on a sector is required to be referenced to
the coordinate system defined by the mounting buttons within a tolerance
of ± 0.010 mm.

Each of the tooling balls on a sector is required to be referenced to
the coordinate system defined by the mounting buttons within a tolerance
of ± 0.025 mm.

### Stability Tolerances and Requirements

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 and requirements of the disk sectors cannot be
separated easily from system stability requirements. 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 φ 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 φ resolution. Thus [total]{.underline} stability tolerances in
Z and φ motion are about √12 x 0.140 mm = 0.485 mm and √12 x 0.008 mm =
0.028 mm, respectively.

The [total]{.underline} stability requirements are comprised of module
attachment, sector, disk support ring and disk-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 four contributions. The stability of module
attachment will be excellent and is ignored here. 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,
although all forces from services 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 nominal locations in each case may be added in quadrature. Under
this assumption, we have chosen to allocate the \"stability budget\"
such that the tolerance for Z(φ) sector stability is about 0.10(0.006)
mm. This is estimated from the total stability budget of about
0.485(0.028) mm 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.

Practically, the sector stability is assessed by measuring motions under
thermal and pressure changes, including 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 disk sectors is measured and
judged to be acceptable for peak motion above 100 Hz.

# 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 outside the pixel detector
volume. The local supports are designed for a maximum power of 50W per
disk sector and 107W per stave, except for the B-layer in which case the
maximum power is 134W.[^2]

The primary thermal requirement of the local supports is to achieve an
operating temperature of the pixel silicon detectors of ≤ -6^0^C. The
operating temperature of the silicon detectors must by maintained below
-6^0^C in order to achieve an acceptable lifetime after irradiation.[^3]
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 minimum coolant temperature of -25^o^C, the allowed
temperature gradient from the average (local) coolant to the face of the
local supports should be ≤ 15^O^C.

## Maximum Operating Temperature

The maximum normal operating temperature of the pixel silicon sensors is
-6^0^C.

## Minimum Operating Temperature

The minimum normal operating temperature of the pixel silicon sensors is
-20^o^C(to be confirmed).

## Temperature Uniformity 

In this section we give the requirements for temperature uniformity of
the pixel silicon sensors under power. The uniformity of temperature of
the pixel silicon sensors on a single disk sector is 7^o^C. The
uniformity of temperature of modules on a single barrel stave is
7^o^C(??). The uniformity of temperature of pixel silicon sensors on two
disk sectors cooled in series is 14^o^C. The uniformity of temperature
of pixel silicon sensors on two staves(not B-Layer) cooled in series is
14^o^C. The uniformity of temperature of pixel silicon sensors on two
staves(B-Layer) cooled in series is also 14^o^C.\*\*\*Need to confirm
these specs\*\*\*

## Temperature Cycling

All requirements must be met after temperature cycling of the local
supports and the interfaces to them from +20^o^C to -20^o^C and return
for a minimum of 20 cycles.

# Material Requirements

It is desirable to minimize the amount of material as measured in
radiation lengths(X~o~) 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~o.~

# Normal Operating Pressure Requirements

Cooling by evaporation of C~3~F~8~ is assumed.[^4] 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.

The fluid channels within the Local Supports and the connections to them
must be leak tight to 4 bar absolute under pressure cycling of +/- 0.5
bar about a mean pressure of 2 bar absolute for nnnn cycles.

# Radiation Requirements

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

# Transient and 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 a
once-per-lifetime single system failure resulting in partial or complete
loss of coolant with power on that raises the average surface local
support temperature to ??^o^C for one(1) minute??.

The design of the local supports and interfaces to them will meet all
requirements after operation at -25^o^C i.e. operation without power.

# 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.

[^1]: Overview of the ATLAS Pixel System - Final Design Review.

[^2]: This should refer to note but which one?

[^3]: Pixel Technical Design Report, CERN/LHCC/98-13, May 1998. See
    Chapter 4 for background information.

[^4]: Need reference to description of cooling system.