Anderssen, E

Greiner, L

Matis, H S

Ritter, H G

Sun, X

Szelezniak, M

Wieman, H H

CERN Document Server

2009 JINST 4 P05015PUBLISHED BY IOP PUBLISHING FOR SISSARECEIVED: December 23, 2008ACCEPTED: March 24, 2009PUBLISHED: May 20, 2009PIXEL 2008 INTERNATIONAL WORKSHOPFERMILAB, BATAVIA, IL, U.S.A.23–26 SEPTEMBER 2008STAR PIXEL detector mechanical designH.H. Wieman,1 E. Anderssen, L. Greiner, H.S. Matis, H.G. Ritter, X. Sun andM. SzelezniakLawrence Berkeley National Laboratory,Berkeley, CA 94720, U.S.A.E-mail: hhwieman@lbl.govABSTRACT: A high resolution pixel detector is being designed for the STAR [1] experiment atRHIC. This device will use MAPS as the detector element and will have a pointing accuracy of∼ 25microns. We will be reporting on the mechanical design required to support this resolution. Theradiation length of the first layer (∼ 0.3% X0) and its distance from the interaction point (2.5 cm)determines the resolution. The design makes use of air cooling and thin carbon composite structuresto limit the radiation length. The mechanics are being developed to achieve spatial calibrations andstability to 20 microns and to permit rapid detector replacement in event of radiation damage orother potential failures from operation near the beam.KEYWORDS: Particle tracking detectors; Instrumentation and methods for heavy-ion reactions andfission studies1Corresponding author.c© 2009 IOP Publishing Ltd and SISSA doi:10.1088/1748-0221/4/05/P050152009 JINST 4 P05015Contents1 Introduction 12 Mechanical design 22.1 Silicon support design 32.2 Detector cooling 43 Mechanical design simulation and analysis 43.1 Ladder support structure 43.1.1 Control of gravity sag 53.1.2 Control of thermal expansion induced deformation 53.1.3 Ladder cooling analysis 54 Conclusion 71 IntroductionAn inner PIXEL detector is being designed and prototyped for the STAR experiment at RHIC.This is part of the STAR [1] Heavy Flavor Tracker (HFT) upgrade project. A main function ofthis detector is improved detection of D mesons through topological reconstruction within the hightrack environment of Au on Au collisions [2].The mechanical design has been driven by the following design goals:• Minimize multiple coulomb scattering, particularly at the inner most layer• Locate the inner layer as close to the interaction region as possible• Allow rapid detector replacement• Provide complete spatial mapping of the pixels from the beginningThe first two goals, multiple coulomb scattering and minimum radius, set the limit on pointingaccuracy to the vertex. This defines the efficiency of D and B mesons detection.The third goal, rapid detector replacement, is motivated by recognition of difficulties encoun-tered in previous experiments with unexpected detector failures. This third goal is also motivatedby the need to replace detectors that are radiation damaged from operating so close to the beam.The fourth goal, complete spatial mapping, is important to achieve physics results in a timely fash-ion. The plan is to know at installation where the pixels are located with respect to each other towithin 20 microns and to maintain the positions throughout the operation.The PIXEL detector will be made of Monolithic Active Pixel Sensors based on standardCMOS silicon [3]. The silicon will be thinned to 50 microns and air cooling will be used tominimize multiple coulomb scattering.– 1 –2009 JINST 4 P05015Table 1. Some features of the HFT PIXEL design.Pointing resolution (13⊕ 19 GeV/p·c) µmLayers Layer 1 at 2.5 cm radiusLayer 2 at 8 cm radiusPixel size 18.4 µm× 18.4 µmHit resolution 10 µm rmsPosition stability 6 µm (20 µm envelope)Radiation thickness per layer X/X0 = 0.31%Number of pixels 436 MIntegration time (affects pileup) 0.2 msRadiation tolerance 300 kRadFigure 1. Pixel detector mechanics showing detector barrel, support structures and insertion parts plusinterface electronics boards.2 Mechanical designThe pixel detector (see table 1) consists of two concentric barrels of detector ladders 20 cm long.The inner barrel has a radius of 2.5 cm and the outer barrel has an 8 cm radius. The barrels separateinto two halves for assembly and removal. In the installed location both barrel halves are supportedwith their own 3 point precision kinematic mounts located at one end close to the detector barrel.During installation, support is provided by the hinge structures mounted on a railed carriage. Someparameters of the design are listed in figure 1.The pointing resolution indicated in table 1 is based on our original 30 micron pixel size andthe specified radiation length. The stability requirement was originally set so as to not compromisepointing resolution when we were using the larger pixels. We have subsequently gone to a smallerpixel size to improve charge collection, but have not changed the stability requirement to be consis-– 2 –2009 JINST 4 P05015Figure 2. Thin wall carbon support beam (green) carrying a single inner barrel ladder and three outer barrelladders. The beam in addition to supporting the ladders provides a duct for conducting cooling air and addedsurface area to improve heat transfer to the cooling air.tent. The reason for this choice is that the multiple coulomb scattering term is the dominant factorand any attempt to improve the stability would have required increased mass, further compromisingthe multiple coulomb scattering term.2.1 Silicon support designThe thinned silicon detector chips are arranged in ladders composed of a single row of 10 chips.The thinned silicon chips are bonded to a flex aluminum Kapton cable which is in turn bondedto a thin carbon composite structure. All electrical connections from the chips to the cable aredone with a single row of wire bonds along one edge of the ladder. The carbon composite layer isquite thin. It’s function is just to allow ladder handling and heat conduction. The primary stiffnessand support of the ladders is provided by a thin carbon composite support beam which carriesone inner ladder and three outer ladders as shown in figure 2. This beam which is an adaptationof the ALICE pixel detector design provides a very stiff support while minimizing the radiationlength budget. Significant stiffness is required to control deformations from gravity, cooling airforces and differential expansion forces from both thermal and humidity variations. The compositebeam carries a single inner ladder and three outer ladders. Ten of these modules form the two barrellayers. The beam in addition to its support function provides a duct for cooling air and adds coolingsurface to increase heat transfer from the silicon chips. By making the beam from high strengthand high thermal conductivity carbon fiber the wall thickness can be as thin as 240 microns andstill satisfy strength and heat transfer requirements.– 3 –2009 JINST 4 P05015Figure 3. Pixel detector cooling air path. The air flows down the center of the sector modules and returnsback over the detector ladders on the sector modules and into the larger ISC volume where it is ducted backto the air cooling unit.2.2 Detector coolingCooling of the detector ladders is done with forced air to minimize radiation length. The pixelchips dissipate a total of 160 watts or 100 mW/cm2 and an additional 80 watts is required for thedrivers at the end of the ladders. In addition to the ladder total of 240 watts some fraction of this isrequired for voltage regulators and latch up electronics that are off the ladder but reside in the aircooled volume. Cooling studies (see next section) show that air velocities of 8 m/s are required overthe detector surfaces and a total flow rate of 280 cfpm is sufficient to maintain silicon temperaturesof less than 12 deg C above the air temperature. The temperature of the cooling air will be 24 degC, slightly above the ambient temperature in the STAR experimental hall.The detector cooling path is shown in figure 3. Air is pumped in through the support beam. Abaffle in the Inner Support Cylinder (ISC) forces the air to return back over the detector surfacesboth along the beam pipe and along the ISC.3 Mechanical design simulation and analysisA number of mechanical design studies have been carried out to find designs that can meet re-quirements of stability and cooling. The work reported here has been carried out by either AREScorporation or us.3.1 Ladder support structureThe mechanical design of the pixel support system must meet stringent position stability require-ments while also minimizing radiation length. The basic support design analyzed is pictured infigure 2.The issues investigated are:• Ladder backing stiffness required to hold thinned silicon flat against it’s tendency to curl• Support strength to control gravity sag– 4 –2009 JINST 4 P05015• Support strength to control deformation from air flow pressures• Control of thermal expansion induced deformation• Control of moisture expansion induced deformation• Support strength to handle insertion loadsWe report on a subset of this list in the space allotted.3.1.1 Control of gravity sagThe most critical component for controlling deformation from a variety of sources is the sectortube shown in figure 2. This structure is in the tracking path and thus requires the most attentionto radiation thickness. Analysis of the sector tube control of gravity sag has been carried out by usand by the ARES corporation [4]. The ARES analysis, included details of the composite weave,and showed that a sector tube could be fabricated with a thickness of 120 microns that more thansatisfies our 20 micron stability requirement giving a gravitational displacement of less than 6microns. We have performed a similar analysis, but with an isotropic modulus representation ofthe composite. This work shows a 5 micron deformation for the detector elements, but adding anend lip reinforcement on the support beam reduces the gravitational detector displacement to 0.6microns. These results show that gravity induced distortions are not a problem with this design.3.1.2 Control of thermal expansion induced deformationOne of the greatest potential sources of deformation is differential expansion resulting from changesin temperature between powered on and power off. It is planned to spatially calibrate or map thedetector structures in a vision coordinate machine with the power off and the structure should notdeviate from the map while powered on during operation by more than 20 microns. This require-ment was one of the main reasons for choosing the current design with its large moment of inertiaand consequently large stiffness. In analyzing the ladder support beam it was found that the mainissue requiring control is the bimetal thermostat effect from differential expansion. The problemis the result of the very large coefficient of thermal expansion (CTE) of the kapton cable com-pared to the rest of the structure. A thermal expansion analysis of ladders plus support [5] showsthat by using a very compliant adhesive (3M 200MP) the Kapton cable is largely decoupled fromthe structure greatly reducing the thermal induced bending. Simulation results give a maximumdeformation of 9 microns, well inside of the 20 micron requirement envelope.3.1.3 Ladder cooling analysisAir cooling has been chosen for the pixel detector in order to minimize multiple coulomb scatteringand a number of studies have been carried out to optimize the air cooling design. Original tests [6]with a heated ladder in a fan driven air stream indicated that ladders with heat loads of 100 mW/cm2could be successfully cooled with a moderate air velocity. A study performed by the ARES Cor-poration found that an airflow velocity of 8 m/s through the sector beam was sufficient to limit thesilicon temperature rise above ambient to 13 deg C. To accomplish this, however, additional coolingfins were required under the outer layer of silicon. These fins add mass and complicate construc-tion, but it was found that by using air flow on both sides of the structure acceptable cooling can be– 5 –2009 JINST 4 P05015Figure 4. Stream lines showing the cooling air flow. The flow direction is from inside to outside. The colorcode shows air velocity.achieved without added fins. We have done CDF modeling [7] to demonstrate the two flow coolingperformance. The cooling simulation was run for one sector out of the 10 sectors in the completepixel cylinder. The modeled air flow path is shown in figure 4.In this case an input air velocity of 8 m/s was used. This results in a maximum silicon temper-ature rise of 12 deg C above the 24 deg C ambient temperature (see figure 5) which is acceptable.It is interesting to note that the inside ladders next to the beam pipe cool more effectively than theoutside ladders. This is because the surface area of the support beam sides provides a significantfraction of the cooling.The total air flow in this case for the full pixel detector barrel is ∼ 280 CFPM and the temper-ature rise in the air for the total power of the ladders, 240 W, is 1.5 deg C. This is a very modest andacceptable rise in the air temperature.The rapid air velocity, 8 m/s, has the potential of vibrating the detector structure as well as staticdeformation which could compromise position resolution. At the original time of this presentationwe were working on a wind flow test to investigate this issue. These tests have since been completedusing a capacitive probe to measure vibration and displacement. We observed multiple vibrationmodes peaking at around 135 Hz with an RMS amplitude of 7 microns. This is right at the limit ofour position stability requirement.– 6 –2009 JINST 4 P05015Figure 5. Surface temperature of silicon ladders. The maximum temperature increase above ambient is12 deg C. The cooling air flows across both the inner and outer surfaces. The air enters from the left on theinside of the support beam, turns around at the right and exits on the left.4 ConclusionWe have a workable mechanical design for a very low mass inner pixel detector for STAR whichhas been shown through analysis to meet requirements of stability and cooling. Prototype workis proceeding.AcknowledgmentsThis work received support in part from the US Department of Energy — Contract no. DE-AC02-05CH11231, Office of Nuclear Physics.References[1] J. Adams, M.M. Aggarwal, Z. Ahammed, J. Amonett, B.D. Anderson, D. Arkhipkin et al.,Experimental and theoretical challenges in the search for the quark-gluon plasma: the STARCollaboration’s critical assessment of the evidence from RHIC collisions, Nucl. Phys. A 757 (2005)102.[2] S. Kleinfelder, S. Li, F. Bieser, R. Gareus, L. Greiner, J. King et al., A proposed STAR microvertexdetector using Active Pixel Sensors with some relevant studies on APS performance, in Proceedings ofthe International Workshop on Semiconductor Pixel Detectors for Particles and Imaging — PIXEL2005, Bonn, Germany, September 5–8 2005, Nucl. Instrum. Meth. A 565 (2006) 132.– 7 –2009 JINST 4 P05015[3] L. Greiner et al., Sensor development and readout prototyping for the STAR Pixel detector, at Pixel2008 International Workshop, Batavia, IL, U.S.A., September 23–26 2008.[4] ARES Corporation, 557 Oppenheimer Dr, Los Alamos, NM 87544, www.arescorporation.com.[5] http://www-rnc.lbl.gov/∼wieman/Thermal deformation study.pdf.[6] http://www.lbnl.leog.org/ladder thermal imaging.pdf.[7] http://www-rnc.lbl.gov/∼wieman/ReportFullPath 001.htm.– 8 –

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