1. SPIE Astronomical Instrumentation June 26, 2008 Physics of reverse annealing in high- resistivity ACIS Chandra CCDs Catherine E. Grant (MIT) Bev LaMarr, Gregory Prigozhin, Steve Kissel, Stephen Brown, Mark Bautz

2. Catherine Grant (MIT) June 26, 2008 Talk Outline (Brief) Description of ACIS CCDs Flight experience with irradiation/annealing First ground experiment in 2002 Experimental setup in 2005 Data analysis results –Sources of systematic errors Summary & future plans

3. Catherine Grant (MIT) June 26, 2008 ACIS CCDs Framestore-transfer High-resistivity float-zone silicon Depletion depth: 50-75  m 24  m pixels 40  sec/pix image-to- framestore transfer rate Four output nodes 10 5 pix/s 3.2 sec nominal frame time

4. Catherine Grant (MIT) June 26, 2008 1999 flight experience with irradiation and annealing Displacement damage in imaging array Charge transfer inefficiency (CTI) ~ 1-2 x 10 -4 at 6 keV No damage in framestore and serial-transfer arrays No damage to back-illuminated CCDs Believed to be due to soft protons (~200 keV) scattered by mirror during radiation belt passages After focal plane was warmed from –100°C to +30°C for 8 hours, CTI increased by 34%

5. Catherine Grant (MIT) June 26, 2008 Laboratory experiment 2002 Designed to duplicate flight experience Low-temperature irradiation –CCD at –100°C; 120 keV protons 8-hour +30°C annealing cycle CTI increased by 150% –Much larger than flight increase (34%) Possible causes: variations between CCDs, different irradiating particle spectrum, ? See Bautz, et al. 2005, IEEE Trans. Nucl. Sci, 52(2), 519

6. CXC ACIS Page 6 Proposed “Model” for CTI Increase from Annealing One possible model  Reverse annealing of carbon impurities causes CTI increase during bakeout.  Expect chip-to-chip variations in carbon concentration to cause variations in CTI increase.  Measurements of carbon concentration show much smaller variation than required by differences between 2002 laboratory & flight results. Schematic of silicon lattice changes during irradiation & bake Si-Si- C -Si-Si | | | | | Si-Si-Si- P -Si | | | | | Si-Si-Si-Si-Si -Si-Si-Si-Si | C | | | Si-Si-Si- P -Si | | | | | Si-Si-Si-Si-Si -Si-Si-Si-Si | | | | | Si-Si-Si-CP-Si | | | | | Si-Si-Si-Si-Si Pre-irradiation: P impurites intentionally implanted C impurities benign CTI perfect Post-irradiation, pre-bake: Si displaced (some CTI increase) C impurites displaced to benign locations; don’t affect CTI C can’t migrate at low temp. CTI somewhat greater Post-irradiation, post-bake: C migrates during bake & bonds to P (or C) causing additional traps CTI increases due to bake

7. Catherine Grant (MIT) June 26, 2008 Laboratory experiment 2005 Designed to better explore parameter space and understand why flight and ground experience differed Six front-illuminated CCDs –5 from ACIS backup focal plane –1 from 2002 experiment (only 2 quadrants were used) Four proton energies Three types of annealing cycle –8-hour +30C (like flight and 2002) –Long duration +30C anneal (over 100 hrs) –Multi-T isochronal (1-hr at 0°C, +10°C, +20°C, +30°C)

8. Catherine Grant (MIT) June 26, 2008 Experiment Details: Irradiation 2 MeV van de Graaff accelerator at GSFC radiation lab Proton energies of 100, 120, 180 and 400 keV Dosimetry via surface barrier detector between accelerator and CCDs Dose chosen to cause (pre-anneal) CTI ~ 10 -4 CCDs irradiated cold (–100°C) and powered off

9. Catherine Grant (MIT) June 26, 2008 Experimental Details: Camera Camera holds two CCDs side by side Framestore shielded Slit-shaped baffle confined beam (3.5mm x 12 mm)

10. Catherine Grant (MIT) June 26, 2008 Experimental Details: Beamline CCD chamber Accelerator 55 Fe source Gate valve Dosimeter Flexible bellows

11. Catherine Grant (MIT) June 26, 2008 Irradiation Pattern Pivoting CCD chamber Beam confined by slit Beam fits within one readout quadrant CCD aligned by directly imaging low- flux proton beam

12. Catherine Grant (MIT) June 26, 2008 Experimental Procedure Cool CCD to –100°C; measure CTI Irradiate CCD quadrant (x 4) –Select energy; align proton beam with low-flux CCD images –Irradiate quadrant (CCD power off) periodically monitoring flux w/ beam monitor Measure CTI Perform annealing cycle (one of three types) Cool CCD to –100°C; measure CTI

13. Catherine Grant (MIT) June 26, 2008 CTI Measurements Fractional CTI increase due to annealing: –R =  CTI annealing /  CTI irradiation Two (known) sources of systematic error: –Temperature variations –Post-irradiation CTI relaxation Correct where possible; increase error budget to compensate

14. Catherine Grant (MIT) June 26, 2008 Temperature Correction CTI strongly dependent on temperature Assume post-annealing CTI is constant –Where possible, use already-annealed quads to track temperature and correct CTI Size of correction, 1% to 25% in CTI

15. Catherine Grant (MIT) June 26, 2008 Post-Irradiation CTI Relaxation In the hour after irradiation, CTI decreases Apply correction to data taken immediately after irradiation

16. Catherine Grant (MIT) June 26, 2008 Anomalous Annealing Results Averaged over all CCD quadrants Weak dependence on proton energy Result similar to flight, 2002 experiment is highly discrepant

17. Catherine Grant (MIT) June 26, 2008 Long Duration Annealing Anneal at +30°C for increasingly longer intervals Maximum CTI increase after 8 hours

18. Catherine Grant (MIT) June 26, 2008 Multi-T Isochronal Annealing Test sensitivity of annealing CTI increase to temperature 1-hour each at 0°C, +10°C, +20°C, +30°C CTI only increases after T anneal reaches +30°C A puzzle! - CTI initially decreases

19. Catherine Grant (MIT) June 26, 2008 Why was 2002 different from 2005? Cannot be due to: –Proton energies (dependence is too weak) –CCD variations one CCD was irradiated/annealed in both ground experiments - no significant difference from other five CCDs in 2005 Temperature differences? Possibly –Camera setups, temperature sensor position different –CTI measurements indicate CCD was ~5°C warmer in 2002 than 2005 –If CTI temperature dependence is different pre- and post-annealing, R is also dependent on temperature

20. Catherine Grant (MIT) June 26, 2008 Summary & Future Plans Six CCDs irradiated cold by soft protons Room temperature annealing increases CTI Fractional increase due to annealing ~ 0.2 Time constant for annealing CTI increase less than 8 hours CTI increase requires T anneal ≥ +30°C Plan to study charge in trailing pixels –May help explain isochronal annealing CTI decrease –May better validate physical model