Determination of In Situ Trap Properties in CCDs Using a "Single-Trap Pumping" Technique

The science goals of space missions from the Hubble Space Telescope through to Gaia and Euclid require ultraprecise positional, photometric, and shape measurement information. However, in the radiation environment of the space telescopes, damage to the focal plane detectors through high-energy protons leads to the creation of traps, a loss of charge transfer efficiency, and a consequent deterioration in measurement accuracy. An understanding of the traps produced and their properties in the CCD during operation is essential to allow optimization of the devices and suitable modeling to correct the effect of the damage through the postprocessing of images. The technique of "pumping single traps" has allowed the study of individual traps in high detail that cannot be achieved with other techniques, such as deep level transient spectroscopy, whilst also locating each trap to the subpixel level in the device. Outlining the principles used, we have demonstrated the technique for the A-center, the most influential trap in serial readout, giving results consistent with the more general theoretical values, but here showing new results indicating the spread in the emission times achieved and the variation in capture probability of individual traps with increasing signal levels. This technique can now be applied to other time and temperature regimes in the CCD to characterize individual traps in situ under standard operating conditions such that dramatic improvements can be made to optimization processes and modeling techniques.