Radiation Detection with Diamond

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Radiation Detectionwith Diamond

• Adam Edwards
• Stanford University

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Outline

• The Future
• Chemical Vapor Deposition
• Diamond vs. Silicon
• Diamond Based Radiation Detectors
• SVT Radiation Protection and Monitoring
• BaBars Diamonds
• New Phenomena
• Diamond Applications on HEP and Beyond
• The Future

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The Future
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The Future
Diamond is the new silicon.
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The Future

• Silicon based technologies have their limitations
• Operating speed and component density are limited
  by power consumption and heat transfer.
• Diamond based electronics have several
  advantages.
• Operate at higher speed, current, power, density.
• More durable against radiation, heat, and
  chemicals.
• The quality of man-made diamond wafers is rapidly
  improving.
• Chemical Vapor Deposition (CVD) diamond wafers
  are commercially available.
• Polycrystalline CVD quality has vastly improved
  and single-crystal CVD is now becoming available.

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CVD

• The manufacturing methods are proprietary black
  boxes.
• This is diamond after all!
• Substrates are made of Ti, Si, SiC, C...
• Hydrocarbons are disassociated in a plasma and
  the carbon rains down forming diamond

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CVD
top
bottom
100 Microns
2 Microns
top

• Many diamond seeds are produced
• The (1,1,1) orientation grows preferentially, and
  the grain size increases as the crystal is grown.

• bottom

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CVD

• Man-made diamond
• Polycrystalline Thousands of fused crystal
  grains with irregular grain boundaries.
• Single-crystal wafers grown on top of diamond
  substrate. Size is still small.

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Diamond vs. Silicon
Property
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Diamond vs. Silicon
Property
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Diamond vs. Silicon
Property
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Diamond vs. Silicon
Property
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Diamond vs. Silicon
Property
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Radiation Hardness

• Diamond radiation hardness measured with
  different incident particles.
• Photons (keV and MeV) no change even after 1000
  Mrad
• Efficiency starts dropping after O(1015/cm2)
  pions and protons
• Dark currents stays at O(nA/cm2)

Normalized Efficiency
Normalized Efficiency
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Radiation Detection

• CVD diamond with no doping can be used in lieu of
  silicon diodes for radiation detection.

Metals can be sputtered or evaporated directly
onto the diamond surface to form ohmic electrodes
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Radiation Detection

• Diamond can be thought of as a solid state
  version of an ionization chamber.

• Diamonds high resistively gives low leakage
  current under high bias.
• Ionizing radiation and the bias field create a
  measurable current.
• Strong lattice structure is resistant to
  radiation damage.

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Radiation Detection

• Diamond can be AC coupled (like SVT) to detect
  single particles.

• Electrodes are easily made in any configuration.
• Large area pads, strips, pixel detectors
• No doping or implantation
• electrodes can be chemically removed and left
  with usable diamond
• Your detector is diamond hard!

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Quality

• Quality Charge Collection Distance (CCD)

• Freed electrons and holes can become trapped and
  recombine before they reach the electrodes
• CCD is the average distance an e-h pair moves
  apart
• Imeasured
• Igenerated ? (CCD/Thickness)

CCD increases with applied electric field and is
observed to saturate at 1V/?m
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Quality

• CCD has been steadily improving.

• Single crystal CVD diamond promises full
  collection efficiency.

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Pumping

• Diamond based radiation detectors need to be
  pumped before their response is linear
• This is done through pre-irradiation with 100s
  to 1,000s of Rads.
• This process fills long lived traps due to grain
  boundaries, impurities, ect.
• Once pumped, the diamond remains pumped
• Sunlight and UV light are known to empty pumped
  traps.

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Single Crystal

• Sizable (5mm x 5mm) single crystal diamond has
  been produced by Element 6

• No traps due to grain boundaries
• Collects full charge (higher carrier lifetime and
  mobility)

CCD saturates at 0.2 V/?m
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Our experience with polycrystalline diamond for
radiation protection and monitoring
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Radiation Monitoring in BaBar

• Reverse-biased (50V) Si PIN diodes, 1cm x 1cm x
  300µm active area, near innermost SVT electronics.

• DC coupled readout, monitors total (leakage
  radiation) current
• Large leakage current subtraction with
  temperature corrections (thermistors)
• Trigger beam dump before silicon SVT is damaged

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Radiation Monitoring in BaBar

• The PIN diodes are themselves damaged by
  radiation and this has made their use
  increasingly difficult.

Temperature and radiation variations. 1nA signal
current 5mRad/s
Leakage currents have increased by
1000x Suppressed Zero
Diamonds to the rescue...
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pCVD Diamonds in BaBar

• In Fall 2002, two diamond based radiation sensors
  were installed inside BaBar for testing and proof
  of principle.

• These sensors went from idea to installation in 2
  weeks.
• Wires are directly soldered onto Ohmic contacts.
• They are insulated with Kapton tape, electrically
  shielded with copper tape

Diamond Sensor
1cm
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pCVD Diamonds in BaBar
7 hours of radiation monitoring. Diamond closely
follows diode and beam current. Also, two abort
situations are seen by the diamond.
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pCVD Diamonds in BaBar

• Side-by-side comparison of the same radiation
  event seen by a diamond sensor and a silicon
  sensor.
• Diamond signal qualitatively matches the silicon
  signal.

90 correlation between diamond and diode abort
signals
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pCVD Diamonds in BaBar
A scope picture was taken of a short burst of
radiation during an injection spike in
BaBar. The measured rise time for the diamond
signal is 20ns and limited by amplifier
bandwidth. This response is much faster then the
required 10 µs.
diamond sensor
PIN diode
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pCVD Diamonds in BaBar

• Diamonds work reliably in BaBar for two years and
  met all radiation monitoring requirements.
• All is not perfect. We have discovered some
  quirks (not quarks) to our diamond sensors.

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Remnant Currents

• After radiation ends, a small decreasing current
  still remains in the diamond sensors.
• These remaining currents can be explained by low
  energy charge traps being thermally emptied.
• Remnant current decays non-exponentially. The
  curve can be fit with I ? 1/vt

Log Scale
sec.
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Erratic Dark Currents (EDC)

• During operation, BaBar has a 1.5T magnetic field
  that is ? to the electric bias in the diamond
  sensors. There is no dark current.
• Without any radiation, and with BaBars 1.5T
  magnetic field off, the dark currents from the
  two diamond sensors installed inside BaBar become
  erratic and large. (First seen Feb. 03)

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Test pCVD Diamonds

• Four additional diamond sensors were made for
  further lab testing.
• Found that different channels on the same diamond
  developed EDC at different times and at different
  current levels.

1 Channel
4 Channels
Solder connections for cables
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Erratic Dark Currents

• When subjected to the same magnetic field as in
  BaBar (E?B), the EDC are suppressed.

EDC, spanning 5 orders of magnitude, are seen
here being suppressed by a 1.5T magnetic field.
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Erratic Dark Currents

• Changing magnetic field magnitude shows
  suppression occurs between 0.1 and 0.6 T.

• Changing the orientation of magnetic field shows
  that EDC is suppressed only by the field
  perpendicular to the electric bias.

pA
Tesla
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Erratic Dark Currents

• Lowering the applied voltage can also eliminate
  the EDC.
• Different channels cease having EDC at different
  voltages.
• No EDC has ever been seen at a 100V bias or below.

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Erratic Dark Currents

• In BaBar there is always a 1.5T magnetic field ?
  sensor bias when operating. EDC has no adverse
  effects on our radiation monitoring.
• Experiments where a magnetic field is not present
  or not adequate to suppress EDC, operation at
  lower voltages will suppress EDC.
• However, this will lower the operational CCD of
  the diamond.

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Radiation Protection and Monitoring Upgrade

• Reiterate Our operational experience with
  polycrystalline CVD diamond has been great.
• We are now producing 12 new pCVD diamond based
  sensors to be installed inside BaBar.

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CVD Diamond Applications

• IR and microwave windows
• High-power microelectronics
• High-speed microelectronics
• High-resolution dosimetery for radiation therapy
• ATLAS - pixel module
• ATLAS - beam monitoring and protection system
• BaBar inspired radiation monitoring _at_ CDF Belle
• ILC - tracking and calorimetery

• - The first and best in HEP

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Conclusions
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Conclusions
The future is Now!