Marco Sirianni (ESA/STScI)

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Radiation Damage in HST Detectors

• Marco Sirianni (ESA/STScI)
• Max Mutchler (STSci)

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Detectors on HST
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CCDs on HST
ACS/WFC FPA 2x SiTe 4048x2096 Thinned
Backside CCDs 15mm pixel size - MPP
(integration only) Site VIS-AR Coating - 4 amps
readout T -77 C 3 mm minichannel
ACS/HRC FPA 1x SiTe 1024x1024 Thinned
Backside CCDs 21 mm pixel size - MPP - Site NUV
AR Coating 1 amp readout
T - 81 C 3 mm
minichannel
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HST Radiation Environment

• LEO - alt. 580 Km, incl. 28.41 - 14.99 rev/day

7/9 orbits/day are SAA free 6/8 orbits/day
are SAA impacted LEO are quite shielded orbits
still
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Radiation Damage

• MPP devices are mainly sensitive to displacement
  damage

• Vacancies migrate until a stable configuration
  is
• reached mainly
• P-V centers
• V-O centers
• V-V centers

• Any new energy level in the bandgap acts as
  emission/trapping site
• Direct impact on
• - Dark Current increase
• - hot pixels ( Field-enhanced dark spikes )
• - CTE degradation

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Single Event effect

• On June 2003 one of the four ACS/WFC amplifier
  showed a jump in read noise 1 e- rms in
  amplitude.
• The change occurred during a SAA transit and
  stabilized to 0.6 e- after few anneal cycles.

STIS suffered of a similar problem six months
before The failure of the side-1 electronics.
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Dark Current variation
As expected the dark rate increases linearly with
time
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Dark Current comparison
Dark rate increase e-/pix/hr/yr
WFC HRC STIS WFPC2 WF3
Predicted (rad. Test) 1.5 (-81 C) n.a n.a n.a 1.4 (-83 C)
Observed 1.8 2.1 3.3 (side 1) 2.2 (side 2) 2.0 (0-5 yr) 0 after
Temp. -77 C -81 C -83 C / (lt -83 C) -88 C
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Hot Pixels ACS HRC
Pre flight dark frame - selected 256x256 pix
region
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Hot Pixels ACS WFC
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Hot Pixels ACS WFC
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Annealing of defects

• In order to remove contamination from the
  detector windows WFPC2 is heated once a month to
  22 C
• It has been noticed a reduction of hot pixels
  after the CCD warm-up (up to 80)
• All known traps anneal at much higher temperature
  (150-320 C)
• STIS and ACS also warm up the CCDs once a month
• to anneal hot pixels.

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Hot pixel annealing
Annealing Rate constant with time depends
on the threshold same rate for 24,12,6hr
soak same rate at -10 C
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Annealing comparison
Instrument Temp (CCD/ann.) Threshold (e-/pix/sec) Anneal rate Source
STIS -83 / 5 gt 0.1 80 75 Hayes et al.1998 Kim Quijano et al. 2003
WFPC2 -88 / 22 gt 0.02 variable 80 Koekemoer et al. 2003
WFC3 ground -83 / 30 gt0.01 gt0.04 80 97 Polidan et al. 2004
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Life of a hot pixels
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Permanent hot pixels
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Permanent hot pixels..
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Permanent hot pixels..
Permanent hot pixel growth ( of total number of
pixels / year)
Threshold e-/pix/sec WFC HRC STIS WFPC2
temp - 77 C - 80 C - 83 C - 88 C
Dark curr. 0.003 0.004 0.006 0.008
gt 0.02 1.60 1.54 2.99 (0.30--0.11)
gt0.04 0.78 0.52
gt0.06 0.46 0.29
gt0.08 0.30 0.21
gt0.10 0.23 0.17 0.36
gt1 0.03 0.02 0.08
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Annealing lesson learned

• We still do not why 0-20 C annealing is
  effective
• Only field-enhanced hot pixels are effected,
  there is no measurable impact on the uniform dark
  rate.
• The anneal rate depends on the dark rate of the
  pixel
• 24, 12, or 6 hr at 20 give the same annealing
  rate
• The same improvement is seen at -10 C (24-48hr)
• Evidence of reverse annealing
• Compete anneal is rare

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CTE monitoring

• WFPC2/STIS/ACS empirical correction for point
  source photometry
• Measurement of charge loss as a function of
• signal - position - background and epoch

• ACS EPER and FPR (only serial for WFC)
• Every six months at several signal levels
• Every month at the Fe55 level (1620e-)

• Different temperature (from -76 C to -88 C)
• different clock rate (parallel clock rate from
  20 to 60 Hz)
• different shielding (different trap
  population)

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CTE measurement
We can investigate CTE degradation as a function
of time/signal level. Ex WFC -A EPER
And predict the impact of science data in the
next few years
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CTE trend
At all signal levels CTE degradation is
linear with fluence
WFC PARALLEL EPER
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CTE Degradation rate
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Conclusion

• In 15 years more 22 detectors have flown on HST
• Different architectures in the same radiative
  environment
• Unique possibility
• To try to understand what is really going
• To provide information for the development/operati
  on of future space detectors
• Huge archive but data collection and analysis did
  not followed any standard.

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Conclusions

• We have started with CCDs analysis
• Dark rate increase and Hot pixel generation is
  quite well understood - they are a concern, but
  not an issue.
• CTE is an issue, little mitigation is possible
• WFPC2 saturation to radiation is a mystery
• We have better understanding of annealing
  effectiveness, but we still do not why it occurs
• We will post the data of the analysis on a
  dedicated web page at STScI.