Due to the space radiation environment at L2, ESA’s Euclid mission will be subject to a large amount of highly energetic particles over its lifetime. These particles can cause damage to the detectors by creating defects in the silicon lattice. These defects degrade the returned image in several ways, one example being a degradation of the Charge Transfer Efficiency, which appears as readout trails in the image data. This can be problematic for the Euclid VIS instrument, which aims to measure the shapes of galaxies to a very high degree of accuracy. Using a special clocking technique called trap pumping, the single defects in the CCDs can be detected and characterised. Being the first instrument in space with this capability, it will provide novel insights into the creation and evolution of radiation-induced defects and give input to the radiation damage correction of the scientific data. We present the status of the radiation damage of the Euclid VIS CCDs and how it has evolved over the first year in space.
The trap pumping technique has proven to be a powerful method of detecting and characterising single defects in the silicon lattice of radiation damaged Charge-Coupled Devices (CCDs). With the ever-increasing demand for improved accuracy of data from space telescopes, radiation damage calibration will play a big role for future space emissions. While current charge transfer inefficiency (CTI) correction algorithms are based on fitting parameters to EPER charge tails, trap pumping can provide more knowledge about the single defects creating these tails, and thus be used to improve the accuracy of the CTI calibration. Using data from an irradiated CCD monitored over a long period of time, we show which information can be gained from the trap pumping data and how the trap densities of different trap species can be used to inform the CTI calibration routines.
Over the course of a mission, Irradiation damage will compromise images obtained through CCDs as the Charge Transfer Inefficiency is altered by traps induced in such devices. To ensure that the evolution of CTI is understood throughout the lifetime of a mission a long-term study of an irradiated CCD273 device has been undertaken at the Open University (OU) Centre for Electronic Imaging. Irradiations were performed both in room temperature and cryo (153K) conditions at a variety of irradiation doses from 6x109 protons to 1.5 x109 protons at 10MeV equivalent. Following irradiation, the device was kept under Cryo-mission-like conditions for 12 months, over which the major imaging figures of merit were measured (EPER, FPR, X-ray and dark current) to track the changes through the lifetime of a detector with focus on the evolving CTI. Over the course of the study this has given insight into the temperature dependence and long-term changes in CTI that can be expected from a CCD in orbit.
The Euclid mission aims to image the far galaxies of our universe to better understand and characterise the structure of dark matter and energy. This galactic survey requires highly accurate images to document the effects of dark matter via weak lensing on the shapes of galaxies. Over the course of the mission, radiation damage to the CCDs will cause an increase in detector Charge Transfer Inefficiency (CTI) reducing the accuracy of measurements [2]. To analyse this, we undertook a long-term study on the evolution of CTI on the CCDs in the Euclid VIS focal plane, after a cryogenic irradiation campaign at 153 K. Following the irradiation, the device has been monitored for 12 months of operation under mission representative conditions. We present the CTI evolution as measured by Extended Pixel Edge Response (EPER) over this time.
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