Radiation damage in the Chandra X-ray CCDs

摘要

Front side illuminated CCDs comprising focal plane of Chandra X-ray telescope have suffered some radiation damage in the beginning of hte mission., Measurments of CTI and dark current at differetn temperatures led us to conclusion that hte type of damage is inconsistent with the much studied type of damage created by protons with energies higher than 10 MeV. Intensive ground based investigation shwoed that irradiation of CCD with low energy protons (about 100 keV) results in the device characteristics very similar to the ones of the flight chips (very lo dark current, the shape of hte CTI temperature dependence). We were able to reliably determien that only image section of the flight chips was damaged and therefore only fast transfer from image to frame store section was affected. We have developed several techniques in order to determine the parameters of the electron traps introduced into hte transfer channel of the irradiated device. One of them is based on the analysis of the amplitude of the signal in the pixels trailing the pixel that absorbed an X-ray phton of known energy. Averaging over large number of photons allowed us to get high signal/noise ratio even for pixels with extremely low signal far behind the X-ray event. Performing this anlaysis at different temperatures we were able to measure trap density, emission time constant, and trap cross section. Another technique is based on the analysis of the tail behind the events of very high amplitude, such as cosmic ray hits.