2.3.1.4 Radiation damage

CCDs generally exhibit ionizing radiation tolerance comparable with other CMOS circuits. However, various studies have shown that the charge transfer degradation is a more severe problem than conventional integrated circuit damage mechanisms. Experiments with proton irradiation showed that displacement damage creates measurable loss of charge transfer efficiency at doses as low as 10's of rads, which is well within the anticipated on-orbit radiation dose projected for AXAF over its lifetime. Hence ACIS will require optimized shielding and radiation resistant CCD architectures to minimize degradation of CCD performance.

To maximize the tolerance to radiation effects, the CCDs must be made with very narrow charge transfer channels to minimize the interaction with the population of traps (generated by the damaging effects of radiation). Operation at the lowest possible temperatures also helps to recover response by increasing the trap emission time constants sufficiently to ensure that the traps will tend to remain filled.

Isochronal annealing studies show that the main population of traps (but by no means all) generated by lattice displacement are Phosphorus-Vacancy centers which retain signal charge for many row cycles when the CCD temperature is $\sim200$ K. Cooler operating temperatures prolong the release time constants dramatically so that the traps tend to remain filled between successive readouts.

Additional means of limiting the degradation of energy resolution will be employed by directly measuring the degraded CTE values, which may allow the event pulse heights to be reconstructed by applying a pixel position dependent correction to each measured charge packet. (However energy resolution will still be degraded due to the stochastic loss of charge in the trapping centers.)

Another manifestation of radiation damage is that of dark current generation. The global increase of dark current due to the addition of extra generation sites in every pixel is not expected to be a problem, given the low operating temperatures. However individual dark current spikes with exceptionally high local generation rates are also apparently generated by radiation damage, and these will affect the data by adding a local offset and noise in a handful of pixels. These must be mapped, both spatially and with temporal evolution, so their effects can be minimized in data analysis.