Preliminary Report of Effects of Low Temperature Radiation Exposure Using Front Iluminated 107409-10-6

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Radiation Tolerance of High-Resistivity LBNL CCDs
Kyle Dawson for the SNAP Collaboration
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Its a SNAP! The SuperNova/Acceleration Probe

• A proposed large wide-field telescope with a
  state of the art camera and spectrograph to
  measure
• thousands of supernovae going back 10 billion
  years (z1.7), with exquisite control of
  systematic errors
• a high fidelity weak lensing map to study the
  growth of structure

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LBNL CCDs for Imaging and Spectroscopy on SNAP

• Standard CCDs used in astronomy are thinned,
  back-illuminated n-channel devices with 10-20 um
  epitaxial depletion region
• LBNL CCDs are thick, back-illuminated
  fully-depleted p-channel devices
• higher QE up to 1000 nm
• no fringing at long wavelengths
• small, controllable point spread function (PSF)
  through application of high bias voltage
• p-channel has improved radiation tolerance
• Patents Issued
• U.S. Patent 6,259,085 Fully Depleted Back
  Illuminated CCD, Jul. 10, 2001.
• U.S. Patent 6,025,585 Low-resistivity
  photon-transparent window attached to
  photosensitive silicon detector, Feb. 15, 2000.

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SNAP Orbit

• L2 lagrange point

1,500,000 km
L2
340,000 km
1,500,000 km
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5 Years at L2 with 1/1/2014 Launch (SPENVIS 95
Upper Limit)
Minimum shielding 9 mm Al --gt equivalent dose of
4x109 protons/cm2. Average shielding 38 mm Al
--gt equivalent dose of only 7x108 protons/cm2.
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Cave 4A Beamline at 88Cyclotron
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Irradiation at 88-inch Cyclotron

• Measure effects of radiation exposure on 10.5 mm
  pixel SNAP CCDs
• All exposures using 12 MeV protons
• Irradiate full CCDs at room temperature, two
  doses
• 1x1010 protons/cm2
• 5x1010 protons/cm2
• Irradiate single CCD while maintained at -140C
  inside dewar, three doses
• 5x109 protons/cm2
• 1x1010 protons/cm2
• 2x1010 protons/cm2
• Measure Charge Transfer Efficiency using Fe 55
• Evolution of dark current and hot pixels in cold
  irradiated device over period of 7 weeks
• Effect of anneal to room temperature

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Even with threshold as low as 100 e-, negligible
contamination from hot pixels Compare to ACS,
1.8x10-3 at 600e-, Sirriani et al
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Dark Current vs Time
BEFORE ANNEAL 9 mm shielding 90
e/px/hr 38 mm shielding 16 e/px/hr Compare to
zodiacal of greater than 240 e/px/hr in all
filters Anneal provides another factor of 5
improvement, with time constants t059.6 /- 4
and t1288 /- 32
ANNEAL
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Parallel CTE vs Dose
For 9 mm Shielding --gt CTE 0.999978 after 5
years For 38 mm Shielding --gt CTE 0.999996
after 5 years Compare to n-channel E2V CTE
0.99986 at 2x109 protons/cm2 equivalent dose
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Serial CTE vs Dose
9 mm Shielding --gt CTE 0.999991 after 5
years 38 mm Shielding --gt CTE 0.999998 after 5
years
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Conclusions
Generation of hot pixels is negligible in SNAP
CCDs, even at doses as large as 2x1010
protons Dark Current Generation expected to
remain significantly lower than zodiacal
background More than an order of magnitude
improvement over n-channel devices in degradation
of parallel CTE Parallel and serial CTE expected
to remain greater than 0.999978 five years into
mission
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Trap Filling and CTE
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Parallel CTE vs Time
ANNEAL
Slight decline with time, additional tests have
shown parallel CTE to be improved by trap-filling
dark current, consistent with trend observed above
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Serial CTE vs Time
ANNEAL
Little variation before anneal, factor of two
decline following anneal. Evidence for change in
trap population? New trap behavior is still
under investigation..
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