LG 02/14/2007

**Requirements for the Mechanical Structure of HFT detector for STAR**

We are attempting to design a inner vertex tracking detector for the
STAR experiment at RHIC and the Brookhaven National Laboratory. The
detector will be placed in the vertex region of the STAR detector set
and is to be used to detect displaced vertices in interactions in the
STAR detector. The typical vertex of interest is approximately 100
microns from the main interaction vertex. A complete description of the
intent may be found in our proposal. (insert link here)

[Existing Strawman Design.]{.underline}

We have constructed a straw man design for the detector and support
structure. This is intended to show a design that incorporates our
present thinking on a solution to the design problem delineated by the
requirements listed below. We intend this to be a starting point to show
what we are thinking but not a limiting constraint on the development of
an alternate design if one meets the requirements. While many aspects of
the current strawman design are the result of calculation followed by
modeling and prototyping, we have not performed all of the calculations,
modeling and testing required to know if this design even meets all of
the requirements,. Solidworks files are available to show this design as
well as the existing detector placement. (insert howard's link here)

[Existing constraints:]{.underline}

The sensor technology choice is CMOS Pixel for the sensors. 30 micron
square pixels with a sensor size of \~2 x 2 cm. These detectors can be
thinned using commercial processes to 50 microns thickness but residual
stresses in the silicon cause it to exhibit curl at this thickness. (add
more details and examples)

The sensors will dissipate a maximum of 100 mW / cm2. This heat must be
removed and the positional tolerances maintained. Due to the stringent
radiation length requirements, we expect this cooling to be forced air.
Some preliminary tests have been performed to gauge the effectiveness of
this type of cooling. Please see
<http://www.lbnl.leog.org/ladder_thermal_imaging.pdf>

The detector envelope is determined by the existing detector set, the
new beam pipe design, and the negotiated integration envelopes of the
new inner tracking detectors. We intend to integrate the design of both
the new beam pipe and the new inner tracking detectors with the HFT.
Some preliminary information on the inner tracking detectors may be
found here.<http://rnc.lbl.gov/~jhthomas/public/review/kelsey.ppt>

The inner and outer layer separation is set by the tracking needs and
the mechanical and coverage requirements. We have simulated the case for
the existing system design with a inner tracing radius of 2.5 cm and a
staggered outer radii of 6.5 and 7.5 cm. Ladder assembly complexity and
the cost of sensors and electronics associated with a design are also a
concern.

The HFT will be part of a larger inner tracking upgrade that has other
detector layers installing outside of our detector. There will be
interface and integration issues that must be addressed.

The design of a new beam pipe is an additional constraint. We currently
envision a 0.5 mm Beryllium beam pipe in the eta coverage, but
indications are that a thicker (up to \~ 1 mm) may be allowable. The
design of this new beam pipe and the attendant support structure may be
integrated into the design of the HFT support structure.

The current design of the new beam pipe has an outer radius of 2.0 cm.

A support consisting of a carbon composite cone already exists and is in
place. This support may be used, modified or discarded as the design
proceeds.

[Requirements:]{.underline}

Requirement: The radiation length per detector layer shall be less than
0.5 percent in the required eta coverage with a desirable value of 0.25
percent. Electronics and additional supporting structures outside of the
eta coverage are allowed but must have minimized radiation length. Some
preliminary radiation length studies have been done and may be found
here.
<http://www.lbnl.leog.org/update_radlength_for_proposal_2005_11_17.pdf>

Requirement: The structure shall consist of two layers of detectors with
a separation optimized to the constraints given in the study linked
below. <http://rnc.lbl.gov/~jhthomas/public/HFT/HFTLayerLocation.pdf>

Requirement: The eta coverage is required to be the same as the outer
tracking detectors. Specifically, -1 \< eta \< 1.

Requirement: Each layer of the detector system shall be hermetic in
pixel coverage except in instances where sensor edges shall meet (we
expect the gap between the lithography and the edge of the silicon to be
20-50 microns). If using a staggered ladder arrangement for layers, the
sensitive pixel regions in one ladder shall overlap those in adjacent
ladders by 100 microns.

Requirement: The detector must be removable and replaceable on a short
timescale whilst the STAR detector system is in place. We expect to have
4 copies of the detector. The detector must be removable and replaceable
with another HFT while the STAR detector remains in place in the hall
and without breaking vacuum in a short timescale (1 day to 1 week)

Requirement: The positions of the pixel cells must be known and
maintained to 10 (???) microns internally to the HFT detector.
Appropriate design and metrology must be employed to accommodate this
requirement. The location accuracy of the HFT detector as a whole in the
STAR detector coordinate system is set by the tracking capabilities of
the surrounding detectors and shall be 100 microns.

Requirement: Each APS sensor contains 410K pixels and is read out by 2
differential pair traces and requires additional digital control signals
running to and from each sensor. In our current design, we have 10
sensors per ladder. This communication must be carried to each sensor on
the ladder with a cable. Currently we expect to use a 4 layer Kapton
flex cable with aluminum conductors. At the end of the flex cable,
provision must be made to attach wires that connect to the readout
electronics. This cable arrangement must be integrated into the
mechanical package in a way that preserves the 10 micron positioning
requirement. Some preliminary work has been done on this problem and the
results can be seen here.

Requirement: All tooling, fixturing and assembly jigs, insertion and
support fixturing, and other hardware required to construct the HFT are
considered part of the design.