April 10, 2000

US ATLAS PIXEL MECHANICS REVIEW

**10 APRIL 2000**

The Design Review committee congratulates the Pixel Team for their hard
work in preparing the design review data package, and the excellent and
comprehensive presentations. The review committee was very impressed by
the amount of very good technical work that has been completed since the
last review in March 1999. It is obvious that the people involved in the
mechanics design have a good understanding of the requirements for the
Pixel System, and have a reasonable plan for completing the design.

**General Comment**

1.  The Pixel Mechanics has assembled a high quality team that has
    completed some exemplary work. They need some more help yet in both
    design and technical support in order to complete the effort on
    time.

2.  We congratulate the Pixel Group for recognizing the need for a
    cabling and services mock-up. This effort is essential for
    determining whether sufficient space is allocated for these systems.
    We are encouraged by the adjacent and interfacing subsystems also
    participating in like effort.

3.  The testing done to date lends a good deal of confidence in the
    modeling efforts.

4.  The conservatism in delta P and delta T in analysis and testing
    efforts is commendable. This provides the necessary information for
    assessing the impact of inevitable increases as designs mature.

### Pixel Mechanics Issues 

1.  Radiation hardness testing on materials and adhesives should be
    completed before the PRR.

2.  The decision regarding C-C or aluminum tubes should be made before
    the PRR. Consider radiation effects in the final decision.

3.  Scheduling is aggressive and optimistic for the next 9-12 months.
    More engineering is necessary to achieve that tight scheduling, and
    effort should be taken to assure the additional resources are
    provided.

4.  Assembly and installation needs to be well defined before the PRR.
    There is a potential for these efforts to impact the Pixel Mechanics
    design.

5.  Several decisions need to be made in the near future:

-   C-C or aluminum cooling tubes;

-   Cooling methodology (CRITICAL ITEM);

-   Final performance requirements (hard/soft need to become all hard);
    and

-   Pixel patch panel size and location.

We recommend early identification of decision-making criteria and
strictly working toward those criteria to minimize schedule impact.

6.  Coolant joints remain a significant concern from the last review of
    March 1999. There is a need for analysis and test efforts to assess
    structural integrity, radiation hardness, and thermal robustness
    before the PRR.

7.  There is a need to identify a methodology for assessing the affects
    of the services strain relief on the structure.

### Status Update for March 1999 Review

The items from the March 1999 were reviewed for progress. These items
are listed below with an assessment of their status as a result of this
review.

## ATLAS LEVEL ISSUES

1.  *The ATLAS Project Engineer must define guidelines for acceptable
    resonant frequencies.*

This has been done. A resonant frequency of \~90-100 Hz is being used
for design.

2.  *The Pixel Project Engineer responsibilities are being transferred
    and this may impact the schedule. CAD integration is important and
    must be continued.*

This effort is being continued, and is being augmented by mockups.

3.  *The ATLAS Inner Detector needs Configuration Control. There is
    significant potential for interference and incompatibility between
    the various components of the Inner Detector.*

Configuration control is claimed, and is in use at CERN for baseline
designs. However, designs which are not baselined still suffer from the
potential for interferences. Wider and formal participation in the
mock-up should prevent problems with irreversible designs.

4.  *A decision must be made soon regarding the cooling system fluid.
    The different cooling systems may have an effect on the vibrations
    transmitted through the cooling lines, and the different fluids may
    present compatibility issues with Pixel components.*

This issue remains open and is very critical. See above.

## PIXEL DESIGN ISSUES

1.  *There is a need to define thermal barrier requirements for Pixel
    detector. This may be an ATLAS issue. The current design is
    anticipating using heater strips to prevent condensation. This will
    drive up the power consumption for the overall detector.*

This is in process, but is only in a conceptual design stage. The
decision to use a thermal barrier has been incorporated in the design.

2.  *The electrical design currently has no margin for an increase in
    power requirements. This is not advisable this early in the design
    effort.*

Margins have been added for the electrical design impact and are
included in verification testing of the mechanics.

3.  *There did not seem to be any extra cooling capacity in the Pixel
    thermal management system.*

The review indicated consideration is being given to various operating
scenarios in the decision making process for the Pixel thermal
management system.

4.  *The sector cooling tube joint is viewed as a concern by the
    committee. There is a risk to the Pixel project schedule and the
    sector design development if the proposed tubing joint prototype is
    not successful.*

The cooling tube is evolving for best thermal performance, but the joint
is not well addressed. The joint represents a concern to the committee
and has the potential to seriously impact the Pixel mechanics design.

5.  *Final design for B-Layer depends upon finalization of bake-out
    jacket design, beam pipe support design and vacuum system design. To
    this end, the B-layer design requires close coordination with beam
    tube and bake-out jacket design. The NEG changes discussed in the
    review (moving NEG inboard) is viewed as good for B-layer design.*

The B-layer design is still evolving. There is a beam tube design review
scheduled for mid-May, after which a refinement to the B-layer design is
anticipated. This issue is still open.

6.  *The B-layer installation method (insertion tool versus rails) must
    be defined by ATLAS now. This has a significant impact on B-Layer
    and Inner Detector design and is viewed as the very next step to be
    resolved before B-Layer and adjacent systems mechanical design may
    continue. Several systems may need modification if the rail option
    is adopted. The insertion tool may need to be developed further.*

The Pixel installation method has been defined, but the B-layer
installation is separate from the overall installation. We did not see
good definition of the installation for the B-layer, therefore this
issue remains open and is critical.

7.  *B-layer cable routing must be developed by ATLAS before more of the
    Pixel System mechanics can be designed, and must be done in
    conjunction with the installation and beam tube vacuum development.
    It may be very difficult to have cables connected to each end if the
    barrel is removable.*

This issue is being resolved. Cabling will exit from just one end of the
B-layer. Detail design of services supports is to be completed.

8.  *During the review it was stated that the alignment tolerance is 25
    -- 50 μm, but this was not critical because Pixel systems can rely
    upon x-ray for alignment calibration. However, the viability of the
    x-ray alignment is not well known for Pixels.*

The Pixel system will be installed and aligned using x-rays.

9.  *The Committee recommends the Pixel Project evaluate the
    repeatability of distortion on sectors under temperature cycling
    (hysterisis).*

This issue is still under evaluation.

10. *The global support progress is encouraging. The decision to go to a
    flat panel configuration as opposed to a truss structure should save
    both time and construction costs. A continued concern is the effect
    of services (which include both cables and cooling tubes) on the
    mechanical stability of the system. A strain relief system must be
    well thought out to avoid putting oblique forces on this precision
    structure.*

This issue is still under investigation, although significant progress
has been made since the March 1999 review. The strain relief effects
remain a concern of the committee. See above.

11. *ATLAS needs to determine whether the flat side of the omega section
    will warp and potentially contribute to the dimensional
    instability.*

This issue applies to deliverables outside the scope of US ATLAS. The
review indicated this issue is not resolved.

12. *The interconnect flex hybrid should be connected to the silicon
    with a flexible adhesive to avoid distortion. The CTEs for Kapton
    and silicon differ substantially. There is a potential for
    distortion of the silicon if a rigid adhesive is used. This should
    be evaluated.*

This issue was not addressed at this review.

13. *There is a lot of work done on 3-D modeling for the Pixel System.
    This is a good approach, and permits some early evaluation of
    interference. However, mock-ups also should be constructed to ensure
    all potential interference is identified.*

A full-scale mockup effort is underway. Wider and formal participation
in the mock-up should prevent problems with irreversible designs.

14. *The current disk design is a continuous disk. This means the disks
    can only be removed by removing the beam tube. Where is the first
    vacuum flange? Are the rings captive on a welded system? When is the
    pipe made up? Is this consistent with the other schedules? Is there
    any beam line commissioning scheduled before the detector is
    installed? Disk removal options must be evaluated before the disk
    design is finalized.*

This aspect of the design has not changed. However, other systems in the
ATLAS design have a similar impact on this operational aspect of the
detector, and appears to be accepted by the detector management.

## PIXEL PROGRAMMATIC ISSUES

1.  *Final design reviews must be conducted before PRR. Design must be
    finalized before PRR (see below).*

This is the intention of the collaboration. However, it was noted at
this review that the schedule is aggressive and optimistic. There are
some issues that impact the Pixel mechanics design that will not be
resolved before some FDRs. These have the potential to adversely impact
the Pixel mechanics design.

2.  *This review indicated the stave and cooling tube back-up designs
    will be pursued right up to PRR. We need to make the decision before
    PRR. The purpose of the PRR is not design blessing, but a
    determination of readiness to proceed to production (design is
    complete, drawings completed, validation testing is completed, QA
    program in place, etc.). If the design cannot be finalized in time
    for the PRR, then perhaps we need to recommend later dates for CERN
    PRRs.*

This issue remains open and just as critical.