Radiation Tolerance of SNAP CCDs Koki Takasaki for the SNAP Collaboration

1. Specifications for SNAP CCDs * Expected to deteriorate with irradiation ** First order estimates. Simulations in progress to determine precise values Expected Dose at L2 Orbit for SNAP 5-year Extended Mission CTE degradation The SNAP satellite will be placed in orbit at L2 Lagrange point, approximately 1.5x106 km from the earth. At this point, the dose is dominated by solar protons. The figure left shows the expected NIEL damage as a function of shielding thickness assuming launch date of 1/1/2014 with 5-year mission. Assuming NIEL factor of 8.9x10-3 MeV/g/cm2 for 12.5 MeV protons, the equivalent dose is 3.4x109 protons/cm2 for 9mm Al shielding and 5.6x108 protons/cm2 for 40mm shielding. Parallel CTE ~10 times better than n-channel E2V CCD Expected parallel CTE after 5 years of operation is 0.999 978 in worst case (minimum expected shielding of 9mm Al) Displacement Damage Dose (MeV g(Si)-1) Worst Case 9mm Shielding LBNL Nominal Case 40mm Shielding WFC3 Serial CTE Expected serial CTE after 5 years of operation is 0.999 991 in worst case (WFC3 CCDs show negligible degradadtion of serial CTE) Results from SPENVIS: www.spenvis.oma.be Spherical Al Shield Thickness (mm) Hot Pixels Evolution of Dark Current with Time Dark current decreases exponentially with time constant of ~60 hours, leaving 90 e-/pix/hr with worst case. Expected zodiacal background level is 500 e-/hr at 400nm and 1300 e-/hr at 1000nm. Dark current of 90 e-/hr will increase the RMS contribution from the background by only 9%. The fraction of hot pixels is expected to be 2.3x10-5 in the worst case Threshold=100e-/px/10min Irradiation at LBNL 88-inch cyclotron • 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 55Fe • Evolution of dark current and hot pixels in cold irradiated device over period of 7 weeks • Effect of anneal to room temperature Radiation Tolerance of SNAP CCDs Koki Takasaki for the SNAP Collaboration To observe high redshift supernovae and perform weak lensing measurements, SNAP requires radiation-tolerant visible detectors with high resolution, low noise, low dark current, and high quantum efficiency from 400 to 1000 nm. Expected CCD Performance after 5-year operation at L2 point • Effect of Anneal to Room Temperature • Dark Current • Decreased by a factor of 5 • Parallel CTE • CTI (1-CTE) increased by factor of 1.2~1.4 • Serial CTE • CTI increased by factor of 2~3 • Hot Pixels • The number of hot pixels became negligible (1.9x10-5→ 1.1x10-6 in the worst case) See Dawson et al, Proceedings of IEEE, 2007 for additional information. Image of the Dumbbell Nebula taken by the WIYN collaboration using an LBNL CCD Note the red background stars which would not be visible using standard CCD technology SNAP is supported by the U.S. Dept. of Energy, Office of Science, under contract DE-AC02-05CH11231 http://snap.lbl.gov