SDW Marco Sirianni Marco Sirianni (ESA/STScI) Max Mutchler (STSci) Radiation Damage in HST Detectors

SDW Marco Sirianni Detectors on HST CCD:(16+2) WFPC yr WFPC yr STIS 1 7 yr ACS 3 3 yr WFC3 2 0 yr IR: (3+1)) NICMOS 3 8 yr WFC3 1 0 yr UV-MCP: (3+1)) STIS 2 7 yr ACS 1 3 yr COS 1 0 yr 1990 : WFC +FOC+FOS +GHRS+HSP 1993 : WFPC : STIS + NICMOS 2002 : ACS ????: WFC3 +COS HST Instruments:

SDW Marco Sirianni CCDs on HST ACS/WFC FPA: 2x SiTe 4048x2096 Thinned Backside CCDs 15  m pixel size - MPP (integration only) Site VIS-AR Coating - 4 amps readout T = - 77 °C 3 m minichannel WFC-1 A B WFC-2 C D ACS/HRC FPA: 1x SiTe 1024x1024 Thinned Backside CCDs 21  m pixel size - MPP - Site NUV AR Coating 1 amp readout T = - 81 °C 3 m minichannel STIS FPA: Same as HRC, different AR coating T = - 83 °C WFPC2 FPA: 4x Loral 800x800 Thick Frontside CCDs 15  m pixel size - MPP 1 amp readout T = - 88 °C

SDW Marco Sirianni HST Radiation Environment LEO- alt. ~580 Km, incl rev/day ~ 7/9 orbits/day are SAA free ~ 6/8 orbits/day are SAA impacted LEO are quite shielded orbits… still

SDW Marco Sirianni 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

SDW Marco Sirianni 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.

SDW Marco Sirianni Dark Current variation WFC - 76 C e-/pix/hr per year HRC - 81 C e-/pix/hr per year As expected the dark rate increases linearly with time

SDW Marco Sirianni Dark Current comparison Side-2 May 2001 Dec e-/pix/hr STIS WFCHRCSTISWFPC2WF3 Predicted (rad. Test) 1.5 (-81 C)n.a 1.4 (-83 C) Observed (side 1) 2.2 (side 2) 2.0 (0-5 yr) ~ 0 after Temp.-77 C-81 C-83 C / (< -83 C)-88 C Dark rate increase: e-/pix/hr/yr

SDW Marco Sirianni Hot Pixels ACS HRC Pre flight dark frame - selected 256x256 pix region

SDW Marco Sirianni Hot Pixels ACS WFC Pre Flight 1 Yr 2 Yr 3 Yr

SDW Marco Sirianni Hot Pixels ACS WFC 0,1,2,3 yrs Field enhanced dark spikes

SDW Marco Sirianni 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 ( C) STIS and ACS also warm up the CCDs once a month to anneal hot pixels.

SDW Marco Sirianni Hot pixel annealing Anneal day Daily Hot Pixel growth Permanent Hot pixels growth Annealing rate (A - B) / ( A - C) A B C Annealing Rate : constant with time depends on the threshold same rate for 24,12,6hr soak same rate at -10 C

SDW Marco Sirianni Annealing comparison InstrumentTemp (CCD/ann.) Threshold (e-/pix/sec) Anneal rateSource STIS-83 / +5> 0.1~ 80 % ~ 75 % Hayes et al.1998 Kim Quijano et al WFPC2-88 / +22> 0.02 variable ~ 80 %Koekemoer et al WFC3 ground -83 / +30>0.01 >0.04 ~ 80 % ~ 97 % Polidan et al. 2004

SDW Marco Sirianni Life of a hot pixels

SDW Marco Sirianni Permanent hot pixelsWFC years %

SDW Marco Sirianni Permanent hot pixels.. SIDE2 > 0.1 > 1.0 STIS

SDW Marco Sirianni Permanent hot pixels.. Threshold e-/pix/sec WFCHRCSTISWFPC2 temp- 77 C - 80 C- 83 C- 88 C Dark curr > ( ) > > > > > Permanent hot pixel growth (% of total number of pixels / year)

SDW Marco Sirianni 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

SDW Marco Sirianni CTE monitoring Different temperature (from -76 C to -88 C) different clock rate (parallel clock rate from 20 to 60 Hz) different shielding (different trap population) 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 Fe 55 level (1620e-)

SDW Marco Sirianni 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

SDW Marco Sirianni CTE trend At all signal levels CTE degradation is linear with fluence WFC PARALLEL EPER

SDW Marco Sirianni Single case: –Signal 1620e- –Low backg. 1e- –1000 transfers  mag converted into CTE CTE Degradation rate Eper TEST (ACS): –Signal 1620e-

SDW Marco Sirianni 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/operation of future space detectors Huge archive but data collection and analysis did not followed any standard.

SDW Marco Sirianni 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.