Abstract-The science goals of space missions from the Hubble Space Telescope through to Gaia and Euclid require ultra-precise 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 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 in high detail 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 Acentre, 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 in situ under standard operating conditions such that dramatic improvements can be made to optimisation processes and modelling techniques.

The Wide Field InfraRed Survey Telescope-Astrophysics Focused Telescope Asset (WFIRST-AFTA) mission is a 2.4-m class space telescope that will be used across a swath of astrophysical research domains. JPL will provide a high-contrast imaging coronagraph instrumentone of two major astronomical instruments. In order to achieve the low noise performance required to detect planets under extremely low flux conditions, the electron multiplying charge-coupled device (EMCCD) has been baselined for both of the coronagraph's sensorsthe imaging camera and integral field spectrograph. JPL has established an EMCCD test laboratory in order to advance EMCCD maturity to technology readiness level-6. This plan incorporates full sensor characterization, including read noise, dark current, and clock induced charge. In addition, by considering the unique challenges of the WFIRST space environment, degradation to the sensor's charge transfer efficiency will be assessed, as a result of damage from high-energy particles such as protons, electrons, and cosmic rays. Science-grade CCD201-20 EMCCDs have been irradiated to a proton fluence that reflects the projected WFIRST orbit. Performance degradation due to radiation displacement damage is reported, which is the first such study for a CCD201-20 that replicates the WFIRST conditions. In addition, techniques intended to identify and mitigate radiation-induced electron trapping, such as trap pumping, custom clocking, and thermal cycling, are discussed.

The charge transfer efficiency of a CCD is based on the average level of signal lost per pixel over a number of transfers. This value can be used to directly compare the relative performances of different structures, increases in radiation damage or to quantify improvements in operating parameters. This number does not however give sufficient detail to mitigate for the actual signal loss/deference in either of the transfer directions that may be critical to measuring shapes to high accuracy, such as those required in astronomy applications (e.g. for Gaia's astrometry or the galaxy distortion measurements for Euclid) based in the radiation environment of space.Pocket-pumping is an established technique for finding the location and activation levels of traps; however, a number of parameters in the process can also be explored to identify the trap species and location to sub-pixel accuracy.This information can be used in two ways to increase the sensitivity of a camera. Firstly, the clocking process can be optimised for the time constant of the majority of traps in each of the transfer directions, reducing deferred charge during read out. Secondly, a correction algorithm can be developed and employed during the post-processing of individual frames to move most of any deferred signal back into the charge packet it originated from.Here we present the trap-pumping techniques used to optimise the charge transfer efficiency of p-and n-channel e2v CCD204s and describe the use of trap-pumped images for on-orbit calibration and ground based image correction algorithms.

As electrons are transferred through a radiation damaged Charge Coupled Device (CCD), they may encounter traps in the silicon in which they will be captured and subsequently released. This capture and release of electrons can lead to a 'smearing' of the image. The dynamics of the trapping process can be described through the use of Shockley-Read-Hall theory, in which exponential time constants are used to determine the probability of capture and release. If subjected to a hostile radiation environment, such as in space where the dominant charged particle is the proton, these incident protons can cause displacement damage within the CCD and lead to the formation of stable trap sites. As the trap density increases, the trapping and release of signal electrons can have a major impact on the Charge Transfer Efficiency (CTE) to the detriment of device performance. As the science goals for missions become ever more demanding, such as those for the ESA Euclid and Gaia missions, the problem of radiation damage must be overcome. In order to gain a deeper understanding of the trapping process and the impact on device performance, a Monte Carlo simulation has been developed to model the transfer of charge in a radiation damaged CCD. This study investigates the various difficulties encountered when developing such a model: the incorporation of appropriate clocking mechanisms, the use of suitable trap parameters and their degeneracy, and the development of methods to model the charge storage geometry within a pixel through the use of three-dimensional Silvaco simulations.

Pocket-pumping is an established technique for identifying the locations of charge trapping sites within the transport channels of CCDs. Various parameters of the pumping process can be manipulated to increase the efficiency, or allow characterisation of the trap sites effective during nominal operating modes.A CCD273 was irradiated in a triangular region by protons to a 10 MeV equivalent fluence of 1.2E9 p.cm -2 , ensuring a suitably low trap density for the development of an automated trap recognition algorithm. X-rays of 5,898 eV were incident on the CCD above the region irradiated with the triangle, such that events could be analysed having passed through an increasing length of irradiated silicon and hence number of trapping sites as a function of column number.Here we present the relationship between the number of traps identified by pocket pumping within the parallel transport channels of a CCD273 and the amount of signal that is deferred by the trapping process during readout.