Determination of in situ trap properties in charge coupled devices using a single-trap “pumping” technique

The science goals of missions from the Hubble Space Telescope through to Gaia and Euclid require ultra-precise positional information. However, in the radiation environment of the space telescopes, damage to the CCD focal plane detectors through high energy protons leads to the creation of traps, a loss of charge transfer efficiency and a consequent deterioration in the positional accuracy. An understanding of the traps produced and their properties in the CCD during operation is essential to allow optimisation of the devices and suitable modelling to correct the effect of the damage through the post-processing of images. The technique of “pumping” single traps has allowed the study of individual traps to a new level of detail and accuracy that cannot be achieved with other techniques, such as Deep Level Transient Spectroscopy, whilst also locating each trap to the sub-pixel level in the device. Outlining the principles used, we have demonstrated the technique for the A-centre, the most influential trap in serial read-out, 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 characterise individual traps, localised to sub-pixel accuracy, such that dramatic improvements can be made to optimisation processes and modelling techniques.