Proposal for FY08 R&D

Aimed at Technical Research and Development needed prior to construction
of the HFT Detector for STAR

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Proposal for FY08 R&D

Aimed at Technical Research and Development needed prior to construction
of the HFT Detector for STAR

The STAR HFT detector has been under study for several years. It
consists of a pixel detector (PIXEL) and the Intermediate Silicon
Tracker (IST). With the exception of last year, past R+D efforts have
been focused on the pixel detector. Since the efforts are all related to
the development of the HFT detector, the previous two separate requests
for R&D funds for PIXEL (LBL) and IST (MIT) are combined into one
request with two distinct activities.

**Summary:** We propose to carry out technical research and development
needed pior to the construction of a Heavy Flavor Tracker (HFT) for
STAR. The concept for the HFT detector in the inner layers, the pixel
detector, is a new and innovative detector concept that will enable STAR
to do the direct topological reconstruction of open charm hadrons.

**Scientific Motivation:** The primary motivation for this work is to
lay the technological foundation for the future construction of the STAR
HFT, an upgrade which will iextend STAR's capability to measure heavy
flavor production by the measurement of displaced vertices. This is a
key measurement for the continuing heavy ion and p-p physics programs at
RHIC. Heavy quark measurements will facilitate the heavy ion program as
it moves from the discovery phase to the systematic study of the dense
medium created in heavy ion collisions. The primary physics topics to be
addressed by the HFT include open charm measurements and the study of
thermalization, flow, and heavy quark energy loss.

One of the most exciting prospective measurements with the pixel
detector is to perform a measurement of the elliptic flow of D mesons
down to very low p~T~ values. It is a generally accepted hypothesis that
elliptic flow is established in the partonic phase. If charm quarks,
with a mass much larger than the temperature of the system, undergo
elliptic flow then the flow has to arise from many collisions with the
abundant light quarks. Thus, flow of charm quarks can be taken as a
probe for frequent re-scatterings of light quarks and is an indication
of thermalization that can be reached in the early stages of heavy ion
collisions at RHIC. We believe that proof of thermalization constitutes
the last step towards the establishment of the QGP at RHIC and this
measurement requires a very thin detector.

**Detector Concept:** The proposed HFT pixel detector will reside inside
the HFT strip detector and it will surround the interaction vertex. It
will exploit all of STAR's unique features including the full azimuthal
coverage of the TPC and tracking from the lowest to the highest p~T~.
The HFT pixel detector has two tracking layers composed of monolithic
CMOS detectors using 30 μm × 30 μm square pixels. These critical
innermost tracking layers lie at radii of 2.5 cm and 8.0 cm,
respectively, and these layers are active over 20 cm in Z and have
\~ 100 million pixels. The HFT pixel detector will provide tracking
information for decaying particles that are displaced by a hundred
microns, or less, from the primary vertex. In this respect, the STAR HFT
pixel is unique. No other silicon detector at RHIC combines this kind of
extreme pointing accuracy and effectively measures all particles down to
150 MeV/c; it is these feature that enables the HFT pixel detector to do
the direct topological reconstruction of open charm hadrons. The silicon
chips for the detector will be thinned to 50 μm and will be mounted on
low mass carbon fiber structures to minimize pointing errors generated
by multiple Coulomb scattering.

The IST will serve as an intermediate tracking layer placed between the
existing SSD and the Pixel layers. It will provide the necessary
pointing resolution for tracks found in the TPC and SSD, and will
provide redundancy crucial for the pointing towards the pixels. The
layers have to be light mass, and stiff.

## PIXEL Detector R+D effort:

The R+D work related to a future pixel detector covers three distinct
areas that are crucial:

Sensor development

Readout development

Development of lightweight structures

The sensor development covers initial fabrication costs in test quantity
for a Phase 1 active pixel sensor at Institut Pleuridiscipline Hubert
Curien ( IPHC) in Strasbourg and some testing including a testing board
at LBL. The phase 1 sensor is a digital readout chip. A preliminary
document describing the Phase 1 sensor attributes can be found here
<http://rnc.lbl.gov/hft/hardware/docs/Ultimate_development_with_m8.doc>.
This also covers the development of a probe card and data acquisition
system for the testing of Phase 1 sensors. We hope to base this data
acquisition system on our new readout system using some items developed
before. We assume that we will have access to an automated probe station
and are pursuing two alternatives through LBL and Purdue.

The readout board covers the design and prototype fabrication of the
next generation of readout boards. The design features all digital
inputs and outputs and tests will show whether this prototype can be
used in the future for the readout of the final detector. The boards are
expected to be a combination of a Xilinx Vertex-5 development board and
a custom motherboard / IO board. The R&D funds for this part of the
project will also cover the cost of the development of a low radiation
length aluminum conductor flex cable used to carry signals to and from
the sensors on the carriers. The effort is expected to include both a
FR-4 and copper conductor version for extensive testing before placing
the aluminum conductor cable order.

The structures that will be needed in the future must be ultra-light to
minimize the material budget, and ultra-stable for precision positioning
and re-positioning. Consequently R&D must be performed to find materials
and structural designs that will work. The various mechanical components
to be developed are as follows:

The sector beam which is a light weight carbon composite structure that
will support 4 detector ladders. Mechanically this is one of the most
critical and demanding elements. It will be paper thin to minimize
multiple coulomb scattering and it must be mechanically stable over a
range of temperatures to provide the desired 10 micron position
resolution and it must be sufficiently stiff to resist vibration induced
by the cooling air.

A ladder. This is a strip of thinned silicon detector chips bonded to
flex aluminum kapton cable which is in turn bonded to thin sheet of
carbon composite. The design challenge is to develop bonding methods
that produce reliable very thin flat structures while minimizing
material that would contribute to multiple coulomb scattering. This will
require developing specialized tooling and vacuum chucks to handle
assembly and wire bonding of thin delicate structures.

The pixel placement system is a mechanism for quick turn around
replacement of detector elements. This system will allow insertion and
withdrawal of the detector on rails without disturbing the rest of the
STAR system and will have precision docking to preserve detector
alignment. Development of a reliable system that meets the desired
levels of position reproducibility will require several levels of
prototyping and R&D.

A component of these development efforts is the handling of service
requirements for cooling air a cabling.

The focus of the electronics and mechanical R&D effort is to develop
structures that can satisfy the demanding requirements of the PIXEL
detector. The development program will involve design and evaluation of
a variety of components which will eventually result in a small number
of prototype modules that will be installed in STAR in the summer of
2010. These modules should be close to production versions that can be
duplicated to make a full detector system. The larger supporting
structures for the full detector system should be the final elements and
will be installed at that time.

The deliverables for 2008 are:

-Phase 1 detector sensors

-Readout electronics for phase 1 sensors

-Mechanical support for PIXEL ladder

The Pixel R&D efforts will be carried out by the LBL group.

**Strip Detector - IST R&D Efforts**

The IST R&D items for 2008 are to develop a ladder structure and a
readout system. These studies will lead to a stable mechanical support
structure, to an efficient readout system and will validate our air
cooling system. We will also develop assembly and manufacturing
procedures.

We propose to develop a fully functional prototype IST ladder with the
necessary readout electronics and an air cooling system. We will have to
produce 1 silicon sensor batch. This will give us 11 sensors, enough to
fully equip a ladder. We will develop and produce 2 full ladder cores,
100 APV25-S1 readout chips, new kapton cable/hybrid prototypes and 2
readout Control Units. We will attempt to test the ladder inside of the
STAR TPC, preferably close to the final IST position. This means that
there is a need for proper mechanical support structures and a reliable
air cooling system. A research scientist, one graduate student and a
technician will work roughly 50% of their time on this project. The
technician will be paid out of the MIT Hadronic Physics Group funds. An
electronics engineer will do the design of the kapton cables/hybrids and
of the Control Units. A mechanical engineer will design the ladders and
the support structure. To be able to bond the readout chips to sensor
and hybrids our bonding machine has to be kept on an annual service
contract. All prototype sensors will have to be tested. Our
semi-automatic probe station will be 10 years old and will need to be
refurbished.

The IST R&D efforts will be carried out by the MIT group.

The **deliverables** of the IST FY2008 R&D are:

\- 1 full IST prototype ladder (70% complete)

\- 1 readout system able to readout the IST prototype ladder (100%
complete)

**Integration Research**

The integration efforts in support of HFT as well as connections to STAR
for next year include Beam Pipe design studies, researching heat and
cooling concerns as well as the support cylinder/cone studies.

The integration research will be carried out by LBL, BNL and MIT.

**Cost Summary:**

The costs associated with the work on this project are shown in Table 1.
In summary, the costs for FY08 is \$1,461K for all efforts; Pixel, IST,
and Integration Research.

**Table 1: FY08 R&D Cost Summary**

  --------------------------------------------- --------------- -------------
                                                **Costs**       **Costs**

  **HFT R&D total**                             **1,461,085**   

  **Pixel R&D Efforts**                                         **820,479**

  **Procurements**                              125,079         

  **Chip Development**                          240,480         

  **Labor**                                     454,920         

  **Strip Detector - IST R&D Efforts**                          **538,206**

  **Procurements**                              **257,679**     

  **Labor**                                     280,527         

  **Integration Research**                                      **62,400**

  **Labor**                                     62,400          

  **Oversight**                                 **40,000**      **40,000**
  --------------------------------------------- --------------- -------------