Marco Sirianni
TIPS 07.15..2004
Radiation damage in ACS CCDs:
And comparison with STIS and WFPC2
M. Sirianni
M. Mutchler, T.Wheeler, D. Van Orsow
ACS – Detectors
Marco Sirianni
TIPS 07.15..2004
WFC FPA:
A
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
STIS FPA:
Same as HRC, different AR coating
T = - 83 °C
WFPC2 FPA:
4x Loral 800x800 Thick Frontside CCDs
15 mm pixel size - MPP
1 amp readout
T = - 88 °C
D
WFC-2
B
WFC-1
2x SiTe 2048x4096 Thinned Backside CCDs
15mm pixel size - MPP (integration only)
Site VIS-AR Coating - 4 amps readout
T = -77 °C
3 mm minichannel
C
CCD Degradation
Marco Sirianni
TIPS 07.15..2004
HST is in a low earth Orbit with periodic transits through the SAA.
CCDs degrade due to Ionizing and displacement damage
CCD parameters that degrades on orbit:
Parameter
Dark Current
Ionizing
Damage
Displacement
Damage
(Surface)
(Bulk)
Hot Pixels
Full Well
Capacity
Voltage Shift
CTE degradation
MPP mode greatly reduces the impact of Ionizing Damage
Marco Sirianni
TIPS 07.15..2004
Radiaton damage: effects on ACS
Do we see signs of degradation for ACS ?
• YES (first signs already during SMOV)
• BIAS ( hot columns)
• Read Noise
• Dark Rate
• Hot Pixels
• CTE
WFC Bias Frame
Marco Sirianni
TIPS 07.15..2004
March 2002
May 2004
Marco Sirianni
TIPS 07.15..2004
Read Noise WFC
Marco Sirianni
TIPS 07.15..2004
Read noise Jump WFC- A
• Only WFC-Amp A
• Same amplitude at
Gain=1 and Gain=2
• Occurred on June 2003
• No apparent anomaly in
Telemetry data
• SAA transit just before
RN Jump
• Initial sensitivity to anneal
process
• Possible cause: radiation
damage
• Stabilized to ~ +0.5 e-
Read Noise HRC
Marco Sirianni
TIPS 07.15..2004
No anomaly in HRC read noise.
Marco Sirianni
TIPS 07.15..2004
Read Noise : comparison
STIS:
RN jump Dec 1999 (~ 0.5e-)
Some instability after it
Only at Gain=1
Side-1 failed in May 2001
No anomaly
reported for
WFPC2
Marco Sirianni
TIPS 07.15..2004
Dark Current Variation
Dark Current is expected to increase:
Anneals have no impact on dark current rate
Dark Current : comparison
Marco Sirianni
TIPS 07.15..2004
STIS
21.6
May 2001
Dec 1999
14.4
e-/pix/hr
Side-2
7.2
Dark current growth: e-/pix/hr per year
Predicted
(rad. Test)
Observed
Temp.
Current rate
(e/pix/hr)
WFC
HRC
STIS
WFPC2
WF3
1.5 (-81 C)
n.a
n.a
n.a
1.4 (-83 C)
1.4-2.0
1.8
3.3 (side 1)
2.2 (side 2)
2.0 (0-5 yr)
~ 0 after
-77 C
-81 C
-83 C / (?)
-88 C
12.5/11.0
13
21.5
27.3
0.2
Hot Pixels
Marco Sirianni
TIPS 07.15..2004
• Population evolves with time
• Mitigation: annealing the CCD (at ambient temperature - monthly).
• Most of the pixels anneal with the first cycle, few more in following cycles
• Hot pixels not annealed in 6-7 cycles became permanent
Mar 02
HRC : (section 255 x 256)
permanent hot pixels growth:
(Sequence of CR-free dark
Frames after each anneal cycle)
Nov 02
May 03
Nov 03
Mar 04
Hot Pixel Annealing
Marco Sirianni
TIPS 07.15..2004
A
Anneal day
Daily
Hot Pixel
growth
B
C
Permanent
Hot pixels
growth
Annealing
rate
(A - B) / ( A - C)
Annealing Rate :
constant with time
depends on the threshold
Annealing rate
Marco Sirianni
TIPS 07.15..2004
Anneal Rate
100
WFC (-77/+20)
Anneal Rate (%)
90
Hot pixels anneal
better than warm
pixels
HRC (-81/+20)
80
70
60
50
Rate comparison:
Should take into account:
-threshold, Top and Tann
Shielding, pixel size
c
40
30
20
10
0
> 0.02
> 0.04
> 0.06
> 0.08
> 0.1
Hot pixel Threshold (e-/pix/sec)
HRC vs STIS
Instrument
Temp
(CCD/ann.)
Threshold
(e-/pix/sec)
Anneal rate
Source
STIS
orbit
-83 / +5
> 0.1
~ 80 %
~ 75 %
Hayes et al.1998
Kim Quijano et al. 2003
WFPC2
orbit
-88 / +22
> 0.02
variable
~ 80 %
Koekemoer et al. 2003
WFC3
ground
-83 / +30
>0.01
>0.02
>0.044
67 - 80 %
~ 80 %
93 - 97%
Polidan et al. 2004/2005
Marco Sirianni
TIPS 07.15..2004
Permanent Hot Pixel Growth
HRC
WFC
# of permanent hot pixels increase linearly with time
Hot pixel growth - comparison
Marco Sirianni
TIPS 07.15..2004
STIS
SIDE2
> 0.1
> 1.0
> 0.02 e-pix/sec
Hot Pixel Growth:science impact
Marco Sirianni
TIPS 07.15..2004
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
> 0.02
1.60
1.54
2.99
(0.30--0.11)
>0.04
0.78
0.52
>0.06
0.46
0.29
>0.08
0.30
0.21
>0.10
0.23
0.17
0.36
>1
0.03
0.02
0.08
Hot Pixels have greater impact in STIS than WFC and HRC
Hot pixels are not fully stable, noise > shot noise
Best Solution: dither the observations
CTE monitoring
Marco Sirianni
TIPS 07.15..2004
ACS WFC1 parallel EPER amp A
“INTERNAL’ TESTs:
HRC EPER (S/P)
FPR (S/P)
TV3
0.99999
SMOV
Oct-02
0.99998
Mar-03
0.99997
CTE per pixel
WFC EPER (S/P)
FPR (S)
1.00000
Apr-03
0.99996
May-03
0.99995
Oct-03
0.99994
Apr-04
0.99993
0.99992
0.99991
First signs of degradation after
one month in orbit (SMOV)
0.99990
100
1000
10000
Signal (e-)
ACS HRC parallel FPR amp C
1.00000
Lost signal = f(signal,background,
Position, time) (Riess et al. 2004)
First signs of degradation but
still not a serious problem for
science.
CTE per pixel
“EXTERNAL” Test
0.99990
TV3
SMOV
Oct-02
May-03
Oct-03
"Apr 04"
0.99980
0.99970
0.99960
0.99950
0.99940
10
100
1000
log (signal)
10000
CTE degradation: trend
Marco Sirianni
TIPS 07.15..2004
EPER PAR (1620 e-)
1
0.99999
0.99998
0.99997
At each signal level CTE
degrades linearly
CTE
0.99996
WFC-2 AMP D
0.99995
WFC-1 AMP A
0.99994
Linear (WFC-2 AMP D)
0.99993
0
10
20
30
Months Since Launch
40
Comparison of results is problematic
- different tests measure different aspects of CTE (deferred vs trapped charges)
- strong dependence on Temp and clocking rates
We compare not the absolute value (0.9999??) but the monthly CTE
degradation rate.
Dmag from external test converted to CTE figure for s=1620e- b=1e-.
CTE degradation rate
Marco Sirianni
TIPS 07.15..2004
Monthly CTE degradation rate at 1620 e-:
Camera
Test
Temp
Direction CTE deg.
HRC
Eper
FPR
-81 C
Parallel
-8 x 10-7
-2 x 10-6
WFC
Eper
External
-77 C
Parallel
- 7 x 10-7
- 6 x 10-7
WFC
Ground
EPER
Fe55
-81 C
Parallel
-4 x 10-6
-6 x 10-6
WFC
EPER
-77 C
Serial
-6 x 10-8
WFC
Ground
FPR
Fe55
-81 C
Serial
-5 x 10-7
Note
s=1620 b=1eDifferent clocking
Same clocking
WFC : agreement between internal and external tests.
P and S DCTE on-orbit better than ground prediction.
ground predictions too negative or just not a fair comp?
is it the effect of the minichannel?
What is the role of the non-MPP readout?
Marco Sirianni
TIPS 07.15..2004
Conclusions
ACS CCDs have been exposed to radiation for two years
The damage is visible in terms of
- increased dark current
as expected
- hot pixel growth
comparable (≤) to other HST CCDs
- CTE degradation
≤ than predicted
- (Read noise jump - WFC1 A)
So far the damage has minimum impact on science
What next: keep monitoring and possibly discover WFPC2
longevity secrets
We are building a unique database of information:
it that can be used to predict future scientific capabilities
of HST SI and for other missions.
External 2 CTE
Marco Sirianni
TIPS 07.15..2004
magn = -2.5Log(countsn )
countsn = counts0 ⋅CTE
Dmag = mag0 - magn
n
Dmag = n ⋅ 2.5Log(CTE)
CTE = 10
Ê Dmagˆ
Á
˜
Ë 2.5⋅n ¯
Marco Sirianni
TIPS 07.15..2004