Mitigating Radiation-Induced Aging in the WFC3/UVIS Channel CCD Detectors

John W. MacKenty, mackenty@stsci.edu, & Jay Anderson, jayander@stsci.edu

The cumulative exposure to ionizing radiation in the space environment degrades the scientific performance of the CCD detectors onboard Hubble. After three-plus years in flight, the CCDs in the ultraviolet-optical (UVIS) channel of Wide Field Camera 3 (WFC3) are showing significant damage. The effect is easily seen in the charge trails behind hot pixels and cosmic rays. Nevertheless, recent advances in mitigation techniques promise to recover much of the CCD’s scientific performance, but they will require observers to make some changes in their observing and data-reduction strategies.

The CCD detectors on Hubble operate by converting incoming photons into electrons, collecting the electrons in each pixel during the science exposure, and then transferring those electrons across the detector array when the device is read out. The transfer process moves each pixel’s electrons down along the columns and then across in a transfer register to the amplifier located in the corner of the detector array. When the detectors were manufactured these transfers were extremely efficient, and for the longest transfer of 2051 shifts down the column for the WFC3/UVIS detectors, over 99% of the charge collected in a pixel would be successfully read out.

In space, the flux of energetic particles continuously damages the silicon lattice of a CCD, creating both “hot” pixels and charge traps. The hot pixels represent sites of excess charge generation, which can be removed from observations by dithering (shifting the scene slightly on the detector between exposures). Fortunately, the large majority (>80%) of new hot pixels can be repaired by warming the CCDs to approximately room temperature (“annealing”). This step, which is repeated every four weeks, has proven successful in limiting the growth of the population of hot pixels, which now comprise only about 1% of all pixels. Furthermore, the Institute obtains daily dark calibration images, which, when combined to produce seven-day running average dark frames, provide fairly good identification of hot pixels. When these dark files become available—which is typically 1–2 weeks after observations are initially delivered—observers are encouraged to re-process datasets to flag and remove hot pixels.

Unfortunately, the damage from radiation exposure also results in the accumulation of charge traps. This effect appears to be cumulative and irreversible. Traps redistribute electrons from one pixel to another during the readout process, which results in obvious charge trails. After several years of exposure to the space environment, there are enough traps to make this a serious issue for observers.

Charge traps degrade the efficiency with which charge is transferred from pixel to pixel during the readout of the CCD array. At the time of launch, the initial charge-transfer efficiency (CTE) of the WFC3 CCDs was 0.999996. After three years, the CTE is about 0.9995 for charge packets with ~50 e¯. This implies that our worst-case transfer across 2051 pixels would recover only about 36% of the initial charge collected in the pixel. In practice, things are not quite this bad. First, a significant fraction of the charge that fails to transfer from one pixel to the next is released within a couple of hundred milliseconds (a few pixel shifts). This is seen directly in the “charge trails” that follow hot pixels, cosmic rays, and bright stars. Hence, a measurement of the flux from a star within an aperture of several-pixels radius will capture a sizable fraction of the charge that was originally in that star. Second, the traps can hold only a finite number of electrons within a pixel. Thus, the presence of charge in the preceding charge packets—from the pixels in the column between the pixel of interest and the readout register—will fill some of those traps and make the transfer of charge significantly more efficient. While this improves the CTE, its value for each specific transfer, and thus the photometric calibration of every source in the image, depends upon both the morphology of the source and the distribution of electrons—from sources, cosmic rays, and hot pixels—in the detector column between the source and the transfer register. This poses an interesting calibration challenge!