Radiation Concerns in HEP: The Switched Capacitor Array Controller in ATLAS

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Radiation Concerns in HEPThe Switched Capacitor
Array Controller in ATLAS

• Norm Buchanan Douglas Gingrich
• Centre for Subatomic Research
• University of Alberta

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Outline

• Historical background and introduction.
• Radiation damage/concerns at HEP facilities.
• LHC and ATLAS radiation issues
• SCA controller
• Summary

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In the Beginning

• Radiation effects on humans were earliest
  concern.
• Nuclear weapons development along with nuclear
  reactors led to more detailed studies.
• Space environment concerns (eg. Van Allen Belts)
• HEP

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Radiation at HEP Facilities

• Early particle accelerators (over 10 MeV) began
  to induce radioactivity in surrounding materials
• Injection/Extraction points and beam stops were
  most susceptible.
• Cooling periods were calculated for personnel
  safety.
• Increases in beam energy and new detector
  technologies raised new concerns
• In the 1980s hadron colliders were approaching
  energies of a TeV and luminosities of 1032
  cm-2s-1.
• Semiconductor detectors and sensitive front-end
  electronics were/are susceptible to accelerator
  radiation fields.

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Cooling Curves
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Radiation Dose Projections
(data from Report of Task Force on Radiation
Levels in SSC D. Groom)
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Types of Radiation

• Electrons, Photons and Muons
• Contribute to total ionizing dose
• To a lesser extent can cause displacements
• Photons can initiate nuclear interactions (Tg
  gt10MeV)
• Hadrons
• Neutrons, protons, pions
• Primarily contribute to nuclear interactions
  (e.g. spallation)
• Thermal neutrons can be captured 10B(n,a)7Li
• Total dose contributions for charged hadrons
• Heavy ions not a concern

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Neutron Map (ATLAS)
(data courtesy M. Shupe URL http//isnwww.in2p3.f
r/atlas/andrieux/mshupe.html)
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Radiation Damage

• TID from charged particles and photons can cause
  trapped charge in insulators e.g. FETs.
• Nuclear fragments from spallation reactions can
  cause displacements affecting material
  composition and causing ionization.
• Activation of nuclei.
• Activation can cause background.

Charged hadron damage cascade in a solid.
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Radiation Concerns at ATLAS

• LHC is pp collider with s1/2 14 TeV.
• Luminosity 1034 cm-2s-1 and sinel 80 mb.
• ATLAS made up of sub-detectors inner detector,
  calorimeter (hadronic and electromagnetic), and
  muon tracker.
• High radiation field throughout detector
  especially in inner detector.
• Mainly concerned with radiation coming from pp
  interactions 2 orders of magnitude higher doses
  than other contributions.

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Damage Concerns at ATLAS

• Inner Detector
• High flux of charged and neutral hadrons from
  interaction region and neutron backscatter from
  calorimeter.
• Damage to semiconductor detectors (displacements)
  and background (eg. neutron capture in 10B leads
  to signal as much as 30 times greater than MIPs).
  Readout electronics also susceptible.
• Calorimeters
• Primary concern is readout electronics. Lower
  radiation field than inner detector but enough to
  cause damage to electronic components.
• Concern for pollution of liquid argon also
  investigated.
• Muon Detector
• Radiation field from shower tails penetrating
  calorimeter will contribute to background.

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Example SCA Controller in ATLAS LAr Calorimeter

• SCA controller will direct and control dataflow
  prior to readout.
• Strict requirements on speed, power consumption,
  and size. Design puts restrictions on
  architecture.
• Will be implemented in a microelectronics logic
  device.
• Will reside in electronics crates in crack
  between tile calorimeters (4 m from interaction
  region).
• Over 10 years of operation must survive
• 680 Gy total ionizing dose
• 4 x 1011 neutrons/cm2
• 7 x 109 protons/cm2 (Pions have equivalent
  fluence)

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Effects of TID on Controller

• TID causes trapped charge in SiO2 which changes
  the electronic properties of device
  (increases Icc)
• Tested prototype (FPGA)
• Irradiated with 60Co source (Eg 1.25MeV)
• Observed current increase at approx. 250 Gy.
• Devices failed (ceased to operate in correct
  manner) after approx. 400 Gy.

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Single Event Upsets

• Unlike permanent damage caused by TID hadrons can
  cause transient effects in digital devices called
  Single Event Upsets or SEUs.
• As a charged particle passes through a sensitive
  element within the device charge can be quickly
  collected and change the state of a transistor
  resulting in a bit flip.
• Protons and neutrons will generally interact with
  the nuclei in the sensitive element causing
  nuclear fragments to deposit the charge. Heavy
  ions can cause SEUs directly.

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Single Event Upset Rate

• To determine upset rate the SEU cross-section
  (sE) must be determined for the device in
  question.
• The particle spectrum (f(E)) of the environment
  where the device will operate must be known or
  estimated.
• The upset rate is then given as

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SEU Cross-section (protons)
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Particle Spectra in Electronics Region
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Controller Prototype Upset Rates
All upset rates include a safety factor of 3.5
to account for uncertainties in simulation of
particle spectra and possible variation in device
characteristics, such as lot differences.
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Future Work

• We plan to examine various commercial devices in
  terms of TID and SEU rates.
• A radiation tolerant microelectronics technology
  is currently in the prototype stage and will
  require study under irradiation.
• The data did not fit well with current models and
  work is being done to develop a consistent model
  to alleviate some of the cost and time of
  testing.

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Summary

• Radiation concerns have become prevalent in HEP
  collider and detector design. The next possible
  hadron collider will have more than double the
  radiation fields the LHC (and detectors) will
  have.
• Radiation effects nearly all detector components
  to some degree.
• Electronics design present some of the most
  interesting challenges under these conditions.
• As with other aspects of HEP, our knowledge of
  design in the radiation environment will migrate
  into other fields.