atlas upgrades research and development a. introduction · 2007. 9. 17. · responsible for the...

MCPHERSON,R.A. PIN 103470 1

ATLAS Upgrades Research and Development

A. Introduction

The CERN Large Hadron Collider (LHC) will be the premier project in high-energy particlephysics, and maintaining Canada’s central role in the ATLAS collaboration at the LHC wouldkeep us at the forefront of international particle physics. While the LHC and ATLAS are onlynow completing initial accelerator and detector commissioning with first physics results expectedin 2008, it is already essential to start research and development of new detector systems forupgrades of the ATLAS detector. The currently approved LHC program ends in 2014, by whichtime current planning anticipates that ATLAS will accumulate significant data sets that will allowthe search for the mechanism for electroweak symmetry breaking that gives mass to elementaryparticles, the ”Higgs boson”, as well as signs of anything beyond the Standard Model of particlephysics. By that time, several of the ATLAS detector subsystems will be reaching their radiationdamage limit and need replacement, and even with a fully working detector the scientific gain ofcontinued running at the same data-taking rate would be modest. In order to extend the reachof the LHC physics program, it is intended to increase the luminosity of the LHC by an order ofmagnitude along with a potentially small increase in the beam energy in 2015 (the ”sLHC”), whileexploring the possibility of larger energy increases in the future. The increase in instantaneousluminosity for the sLHC will require significant detector upgrades using technologies capable ofhandling the higher rates and radiation doses. Research and development programs for these newtechnologies are already well underway, aimed at starting significant major construction projectsin about 2010 for installation around 2014-2015. Letters of Intent by collaboration membersinterested in ATLAS upgrades are due by the end of 2007 making it critical that Canadiangroups invest effort in this direction now.

Developing the expertise in the technologies required for the sLHC will involve the training ofuniquely and highly qualified personnel with skills at the leading edge of world-wide particle de-tector and readout electronics knowledge. For example, diamond pixel detectors may be the mostradiation hard semiconductor particle detector technology currently foreseen, Silicon-Germaniumheterojunction Bipolar Transistor technology may prove a significant advance in low-noise, high-rate, rad-hard electronics, and technological possibilities for a new, very high rate calorimeteroperating near the LHC beam axis would also be state of the art. Projects like these, and thedevelopment of expertise in application specific integrated circuit (ASIC) design and field pro-grammable gate array logic programming, will require the training of personnel at the cuttingedge of instrumentation.

Canadian ATLAS upgrade efforts include both detector subsystems built by Canadians andsubsystems which are relatively new for Canadian participation. In initial ATLAS construction,Canadian detector contributions were concentrated primarily on the Liquid Argon calorimeter(LAr) endcaps, including the hadronic endcaps (HEC), forward calorimeters (FCAL), and LArendcap feedthroughs and front-end board digital ASICs. The Canadian ATLAS group receivedabout $15M in NSERC funding for capital equipment for current ATLAS detector subsystems.The R&D for upgrades requested here would set the stage for a capital equipment request in 2010for significant contributions to the construction of the next phase of the ATLAS experiment. We


note that ATLAS was endorsed at the highest priority level in the recent NSERC SubatomicPhysics long range planning exercise, and that report also recommended significant investmentsin particle detector R&D that are commensurate with this request.

The LAr endcap calorimeters, particularly in the FCAL region near the beam axis, are subjectto very high rates and radiation damage. One Canadian upgrade R&D effort will concentrateon testing the effect of liquid argon purity on the signal collection at high rates in the endcapcalorimeters, with a goal of increasing the maximum luminosity at which they can usefully operate.At the rates expected in the sLHC, the liquid argon in the FCAL region may boil, and this wouldcripple the functionality of the entire LAr endcap calorimeter system; a second Canadian upgradeproject will be investigating technologies for very high rate calorimeters sitting in front of theforward LAr region, shielding the LAr calorimeters from the high sLHC fluxes. The HEC hasGaAs preamplifiers which may not survive sLHC radiation doses, and Canadians seek to performelectronics R&D and radiation tests on possible replacement technologies.

Canadians also seek to join the ATLAS inner detector upgrade efforts. The ATLAS innerdetector will need to be completely redesigned for the sLHC era for both occupancy and radiationdamage reasons. This will likely require removing the straw-tubes and replacing them with siliconstrips, replacing the current silicon tracker with pixels, and replacing the pixels with a technologycapable of higher doses. One major Canadian effort is the R&D of diamond sensors for a newATLAS pixel detector, following along major Canadian involvement with diamond detector R&D.New Canadian investigators also seek involvement with the electronic readout of the upgradedATLAS inner detector, which will need to handle the order of magnitude higher luminosity.

While more investigators are likely to be involved in detector upgrade projects by the timemajor construction starts, currently the involvement is more limited, including several investiga-tors who are relatively new to ATLAS-Canada. The investigators currently directly involved inATLAS upgrade R&D are listed in Table 1. We do not anticipate personnel working full-time onATLAS upgrade R&D. We also see this as an opportunity for our students and postdocs to con-tribute significantly to hardware development while also taking data and doing physics analysison a running experiment.

B. Canadian Plans for ATLAS Upgrades

(i) Forward Liquid Argon Calorimetry

The Canadian Group was and remains a key player in ATLAS calorimeter design, construction,calibration, installation, and commissioning. They have a dominant role in the HEC and FCALcommunities. They are now fully involved also in the upgrade activities. For example, they areresponsible for the design of the test equipment, experimental area layout, and the requested testbeam properties.

The development of the present ATLAS forward calorimeters was a major challenge. TheLHC rate of about 25 p-p interactions per beam crossing, most of which give backgrounds in theforward calorimetry, creates problems for the design team. At sLHC this situation is worse by


Investigators Upgrade Project ATLAS UpgradeFraction Fraction

David Asner(*) Carleton Pixels 75% 50%David Axen UBC High-rate calorimetry 100% 30%Colin Gay UBC Tracker readout 100% 50%Leonid Kurchaninov TRIUMF HEC electronics 30% 100%Claude Leroy Montreal HEC elect./High-rate calo 100% 30%Jean-Pierre Martin Montreal HEC electronics 40% 50%Robert McPherson Victoria/IPP Pixels/High-rate calo 100% 20%Gerald Oakham Carleton High-rate calorimetry 100% 30%Chris Oram TRIUMF High-rate calorimetry 100% 30%Robert S. Orr Toronto Pixels 100% 20%Wendy Taylor York Tracker readout 75% 50%William Trischuk Toronto Pixels 100% 30%

Table 1: ATLAS-Canada investigators interested in detector upgrade R&D, along with theirfraction of research time spent on ATLAS and the fraction of their ATLAS time expected to bespent on upgrade R&D. Also listed is their principal R&D project. (*)Asner is will be stronglyramping his ATLAS fraction when his term as CLEO co-spokesperson finishes.

a factor of ten. Given that making modifications to the end-cap calorimeters would require atleast a two year shutdown, the present sLHC upgrade programme, while planning for the worst,concentrates on investigating scenarios not requiring alterations to the equipment in the cryostats.

If the calorimeters do not operate satisfactorily four main scenarios are available:

1. Continue without the affected regions.

2. Replace the calorimeter with one that will operate satisfactorily.

3. Add a dopant to the Liquid Argon, such as ethylene, to increase the ion mobility.

4. Add a detector in front of the affected portion of the detector to shield the present calorime-ter from the high rate.

The Canadian effort is focused on scenarios 3 and 4.

(a) Heat Loads

The heat load from the deposit of energy in the beam into the Forward Calorimeter will heatthe liquid argon. The present best estimates of the temperature rise are in the 3 to 4 degreerange, which is uncomfortably close to the temperature range between boiling and freezing. Thebubbles from boiling will cause shorts if they get into the active region of the detector wherethe high voltage is present. The Canadian group, from Carleton, Alberta and TRIUMF, have


Figure 1: Temperature profile in the FCAL region, with 45 Watts heat load.

historically taken sole responsibility for heat flow modeling in the complex structure of the end-cap calorimeters. We have extended these studies to the sLHC environment. Figure 1 showsthe result of a recent sLHC modeling for the FCAL. When the beam turns on these studies willbe compared with the many temperature monitoring probes in the cryostat. Studies to reduceheating by adding a warm calorimeter in front of the FCAL are on-going.

(b) Mobility Measurements

The major initial design criteria for the ATLAS end-cap calorimeters was to operate in a regimewhere the charge of the argon ions in the liquid argon gap was less than the charge residenton the electrode that forms the active gap. As the ion mobility is not well known, conservativeestimates were used in the design so the calorimeters operate reasonably safely within this regimefor the maximum LHC intensities. With the sLHC increasing ion density by a factor of ten thereis now a real possibility that the calorimeters will, unless changes are made, no longer operatein this regime. In addition other problems might arise. For instance calorimeter materials underirradiation out-gas and this might pollute the liquid argon or affect the surfaces of the gap andspoil the calorimeter operation. This was tested for by the Montreal Group for LHC intensities.Given the possibility of undesirable unanticipated effects, a programme of a full system check ofsmall samples of the end-cap calorimeters was started. Three small cryostats were placed in a50 GeV proton beam at Protvino, with a variable intensity over a wide range, so that a full studyof operation from LHC instantaneous intensities to sLHC 10 year integrated doses is possible.Figure 2 shows a diagram of the setup. The experiment will take first beam in November 2007. Itis anticipated we will get one week of beam time to establish the operation of our equipment andmake our first preliminary measurements. Further beam time is anticipated each year for 3 years.The programme aim is to establish the present working intensity limits of the calorimeters, and ifthese will limit the operation of ATLAS at the sLHC investigate possible solutions. The Canadianteam plans to study the effect of beam produced impurities on the operation of the liquid argongap. For this we need argon purification equipment (requested in this grant) to purify the argonand then investigate the purification rate required to remove any adverse effects of beam inducedimpurities. If the argon ion density proves to be limiting operation we will investigate if this can


Figure 2: Layout of the cryostats and absorbers in the 50 GeV beam line at Protvino.

be usefully changed by doping the argon with impurities such as ethylene. Both liquid argon andkrypton have been doped with ethylene and the electron drift velocity has been seen to increaseby about a factor of two for argon (see [1]) and seven for krypton (see [2]). However measurementson Ar+ mobility as a function of impurities are not reported.

(c) High Rate Calorimeter

In the event that either the argon boils or the rate is too high in the FCAL for satisfactoryoperation, a factor of about 2.5 reduction can be obtained by placing a new small warm calorimeterin front of the FCal region. This would shadow the FCal, and the new calorimeter would operateat very high rates. We are investigating options for suitable active detectors in this calorimeter.Presently two options are under consideration: GOSSIP [3] and diamond detectors. We proposeto study the operation of the two most suitable detectors in the 50 GeV beam at Protvino undersLHC conditions, and request funds to purchase two sample detectors for this purpose.

References

[1] “Electron drift velocity and characteristics of ionization of alpha and beta particles in liquidargon doped with ethylene for LHC calorimeter”, V. Vuillemin et al., Nucl. Instr. and Meth.A316 (1992) 71.

[2] “Effect of CH4 addition on excess electron mobility in liquid Kr” A. F. Borghesani, M. Fole-gani, P. L. Frabetti and L. Piemontese, J. Chem. Phys. 117 (2002) 5794.

[3] “The Charge Signal Distribution of the Gaseous Micropattern Detector Gossip”, F. Hartjes,2006, www.nikhef.nl/i56/Hartjes Gossip 10-10-06-1.ppt


(ii) Hadronic Endcap Electronics

The Hadronic Endcap Calorimeter (HEC) uses the concept of active pads. The signals fromindividual pads are summed longitudinally to form the related read-out channel: each signalis fed into a preamplifier and finally 4, 8, or 16 preamplifier output signals are fed into thesumming amplifier. This is achieved using integrated GaAs electronics operated in liquid argon.The operation of cryogenic GaAs amplifiers, positioned close to the pad electrodes, provides anoptimum signal-to-noise ratio. The preamplifier and summing boards (PSB) equipped with GaAsASICs are installed on the outer radius of the calorimeter wheel inside the cryostat with liquidargon.

The GaAs preamplifiers presently used in ATLAS were manufactured by TriQuint in 3 µmMESFET technology some 12 years ago. Tests performed at the IBR-2 reactor in Dubna, Russia,[1] have shown that the characteristics of this ASIC start to degrade for neutron fluence exceeding3 ×1014 n/cm2 and remain unchanged for γ-dose up to 50kGy. Both boundary values are wellabove the radiation levels expected in the final ATLAS environment at LHC; however, luminosityten times higher is foreseen at sLHC and the present HEC cold electronics would be operated atits limit.

The radiation hardness of the IC has to be re-tested against neutrons, γ and charged hadronsat fluences close to those expected at sLHC. In particular, neutron irradiation will be performedat a facility located at the NPIASc-CR cyclotron at Rez [2] where large neutron fluences of fastneutrons can be achieved through (d,n) or (p,n) reactions. The neutron average energy is 7 MeVand 14 MeV for deuteron and proton beams, respectively. The neutron flux is about 1.5×1015/cm2

accumulated per day for samples of 2 cm2 area and 1.2×1014/cm2 per day for 25 cm2 area.

In parallel, alternative options of HEC cold electronics will be investigated. The most straight-forward solution is to redesign ASIC in 0.15 µm technology currently available at TriQuint [3]. Itis expected that such an amplifier will be much more radiation hard because of the smaller featuresize. The main concern for this technology is the level of excess flicker noise which can becomedominated in sub-µm devices. This option requires extensive R&D efforts to optimize schematics,chip layout, packaging, etc. Spice and physics layer models for low-temperature operations mustbe created. ASIC design and prototyping will require specific functional and stability tests bothat room and cryogenic temperatures.

Another option for HEC cold electronics is SiGe hetero-junction technology available now fromIBM and other manufacturers [4]. This technology is very attractive because of high frequencyand low-noise performance with extremely low power consumption. It is known that such devicesare operational at cryogenic temperatures and are radiation stable. A R&D similar to that ofGaAs option will be required to validate SiGe ASIC for HEC cold electronics.

One more option for HEC is to use the same or similar to warm amplifiers as used for otherATLAS LAr calorimeters. This option will require design of passive summation scheme (to replacePSBs), optimization of warm amplifiers for HEC-specific conditions and development of analogfiltering and digital processing of HEC signals.

We plan to start an extensive R&D program including the following steps:


• Measure the radiation hardness of the present GaAs amplifiers

• Study the radiation hardness of new technologies irradiating discrete components

• Irradiation of new HEC ASICs prototypes

• Setup working place in TRIUMF for ASIC design and simulations

• Establish test setup at TRIUMF for full scale studies of HEC amplifiers including measure-ments at cryogenic temperatures.

• Organize and support a test facility at CERN for validation tests of HEC amplifiers

The R&D will be performed in a combined effort of the groups from Kosice, Montreal, MPI-Munich, Prague, NPI-Rez, and TRIUMF. The measurement equipment will be provided by allthe groups involved.

References

[1] J. Ban et al., Cold Electronics for the liquid argon hadron end-cap calorimeter of ATLAS,Nucl. Instr. and Meth. A556 (2006), 158.

[2] J. Ban et al., Proposal to measure the Radiation Hardness of the ATLAS-HEC Cold Elec-tronics up to the highest sLHC neutron fluxes, MPI note 2007.

[3] http://www.triquint.com/company/mfg/

[4] http://www-01.ibm.com/chips/techlib/techlib.nsf/products/SiGe BiCMOS

(iii) Pixel Detector Upgrade

The ATLAS Pixel detector will be upgraded in two steps. The current B layer sits at a radiusof 5.5 cm from beamline and is only expected to survive an exposure equivalent to 10 fb−1 ofATLAS data-taking before it will cease operation. ATLAS plans to replace this layer, or add alayer at a smaller radius, in 2012. The replacement modules will take advantage of advances inradiation hard sensor technology since the baseline sensor decision was made in 2002. Beyondthis the entire pixel detector will be replaced as part of the sLHC tracker replacement. This iscurrently envisaged for 2015.

Canadians have been leading the development of particle detectors based on artificial ChemicalVapour Deposited (CVD) diamond for the last decade. Over the last three years this sensormaterial has been metalised with detector patterns and successfully bump-bonded to ATLAS Pixelfront-end I readout chips, (the version that currently populates the full ATLAS pixel detector.Several single chip (approximately 1 × 1 cm2 sensors) have been produced and one full, sixteenreadout chip, module has been built (see fig 3 a) and tested. Figure 3 b) shows the correlationbetween charged particle trajectories in a testbeam and the hits recorded in the diamond module.


We are currently in the middle of a programme of irradiation that will eventually see these devicesreceive fluences in excess of 1015 protons per cm2 – approximately the lifetime dose expected by aB-layer replacement over a three to four-year period while ATLAS accumulated several 100 fb−1.

a) b)

Figure 3: a) A photograph of the fully assembled diamond pixel module prototype. The sixteen(silver) silicon readout chips can be seen emerging from below the ... b) The spatial correlationbetween testbeam tracks measured by an external telescope and hits found in the ATLAS diamondpixel module prototype. The module is found to be in excess of 98% efficient at detecting minimumionising particles and has a position resolution of 15 µm.

The current irradiations must stop at that level, though other diamond sensors have surviveddoses in excess of 1016 protons per cm2, because the current generation of ATLAS Pixel readoutchips will not survive beyond 1015. In parallel with the sensor and module development describedhere a new, more radiation tolerant, readout chip is being developed for the sLHC era pixeldetector.

In order to demonstrate that we can scale up from the production of a few diamond pixelmodule prototypes to the production of a B layer replacement (approximately 200 parts) or afull sLHC pixel detector system (currently envisaged to have somewhere between 800 and 1500modules, a group of six institutions (Bonn, Carleton, CERN, Ljubljana, Ohio State and Toronto)submitted a proposal, submitted a proposal to the ATLAS upgrade steering group to assemble,irradiate and test ten (10) additional diamond pixel module prototypes. In addition to scalingup diamond sensor production we propose to further establish the industrial production (sensorgrowth as well as metalisation and bump-bonding) of these modules. These ten modules wouldbe the subject of intensive testbeam study to understand their performance as well as extensiveadditional irradiations to understand their properties at least to the point where the current FEIreadout chips cease operation.

Diamond pixel sensors offer an additional important opportunity as the basis of a trackingdetector. To survive silicon pixel sensors must operate at a temperature of -20 ◦ (or lower) ortheir leakage current will swamp the readout electronics with noise and cause thermal run-away– to the point where they can no longer be depleted. Diamond, due to its much higher resistivitytypically has bias currents that are nA (or lower) compared to the 10’s or even 100’s of µA in acomparable silicon sensor. Due to the high band-gap in diamond these sensors can be operated at


room temperature. This puts considerably less strain on the detector cooling system. Indeed thecurrent pixel detector layers are 3.5 % X0 thick. 2.5 % of this material comes from the coolingchannels necessary to keep the silicon cold. Given the much lower cooling capacity needed fordiamond sensors we are planning to prototype a support mechanics that should be less than 1%of a radiation length thick (per layer) and still remove the power produced by the pixel readoutchips.

Once prototyping had been completed and the final technical choices have been made, majorconstruction will start. As it took five years to assemble the current ATLAS pixel detector. Withtwo years of R&D remaining prior to making similar technical choices in early 2010 it is anticipatedthat the somewhat simpler B layer replacement can be ready for installation in 2012 while theassembly of a much more ambitious sLHC pixel detector replacement could be completed by2015. However it is crucial that the R&D proceed apace so that the designs can be finalised in atimely way. Canadian researchers have been leaders in the development of radiation hard, CVDdiamond particle detectors over the last decade – including projects in e+e− beam safety andATLAS beam conditions monitoring that have been NSERC supported. This provides an idealopportunity to build on that reputation while also providing challenging detector developmentprojects for young Canadian researchers to work on as ATLAS and the LHC embark on theirdata-taking era.

(iv) Inner Detector Readout Electronics

(a) Overview

The total replacement of the ATLAS Inner Detector tracking system requires not just new detectorsensor elements, but a replacement of the electronics systems as well.

The final structure and technology of the Inner Detector replacement will not be made for2-3 years. The main area of the tracking detector will be a form of single-sided silicon sensors,similar to the ones currently employed in ATLAS. The outermost central tracker, the TransisitionRadiation Tracker (TRT) is base upon straw tubes, and cannot handle the track density that willbe present with instantaneous luminosities above the LHC high-luminosity target of 1034. TheTRT detector extends to a radius of 1m, and the only viable technology for replacing this tracker,which entails a very large surface area detector, is silicon strip detectors. A current straw-manlayout of the inner tracker replacement calls for 5 layers of pixel detectors of some form, followedby 5 layers of silicon strip detectors very similar to the current ATLAS Semiconductor Tracker(SCT). While the technology for the pixel sensors is still under active development, as evidencedby other parts of this grant proposal, it is almost certain that the outer tracker elements will besilicon strips of some form. For this part of the tracker, significant R&D is still required in thereadout electronics, which is the focus of this proposal.

The replacement detector will have between 5-10 times more front-end channels. However,the space available for services such as cooling, power distribution and data readout remains thesame as for the current detector. For this reason a new electronics architecture along with newcustom ASICS are needed to concentrate the control and readout systems, with higher speedlinks handling the large increase in bandwidth required.


(b) Hardware

The current ATLAS Semiconductor Tracker (SCT) is readout using a custom ASIC, the ABCDchip. Each chip handles 128 channels, with a total of ≈50k chips in the full system. Each stripis connected to its own preamplifier, whose output it passed to a comparator. Channels above aprogrammable threshold are marked as hit (i.e. a binary readout for each channel), and the datais stored in a pipeline for up to 3µs awaiting the Level 1 trigger decision.

This chip is already being updated for the upgraded Silicon tracker. A new version (ABC-Next) has been submitted to be fabricated using a 130nm BiCMOS process (the current ABCDchip was built with a 250nm feature size). Since the upgraded tracker will require more than250k such chips, a change in system architecture that simplifies these front-end chips as much aspossible is needed to reduce costs, power consumption, and increase reliability.

The heirarchical system consists of several front-end ABCN chips connected to a ModuleController Chip (MCC). This chip would perform many functions that are common to a set ofABCN chips. For example, the clock PLL, command decoding and distribution, and slow controlfor the module can be performed in the MCC on a per-module basis. Since there will be roughly10 times fewer MCC than ABCN chips, this simplifies the system considerably.

Each Module Controller would also act as a data concentrator – the data readout of ≈10chips would flow through this chip. Several MCC would then be aggregated by one SuperModuleController (SMC) which communicates to the main back-end DAQ system via optical links.

We propose to participate in the design R&D for the Module Controller Chip (MCC) part ofthe system. The R&D phase of the project has two basic stages and participation would placeATLAS-Canada in a strong position to lead a major piece of the readout system productionin several years time. In addition, the project will develop new HQP capable of participatingin custom ASIC design projects. As costs for such projects comes down, and detector channelcounts and density continue to increase, such expertise will be very useful to a wide range ofprojects within the community.

The MCC will be a custom ASIC implemented in 130nm or 90nm CMOS technology. Since thisis a new chip design, and since ASIC submissions are time-consuming and somewhat expensive,we propose a two stage R&D approach. First, we develop the logical design elements needed tointerface to the ABCN chip and Super Module controller. These will be implemented and testedin a relatively inexpensive FPGA-based test board. Once the design features have been developed,refined and debugged, we will move the design to a custom ASIC using standard libraries.

The funding for overall project management, engineering time for integrating the firmwareblocks into the project, travel, and testing manpower will come out of existing sources. Theadditional RTI funding will allow us to leverage these resources towards participation in one ofthe major upgrade projects of the next 10 years of the ATLAS collaboration.

atlas upgrades research and development a. introduction · 2007. 9. 17. · responsible for the...

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