Monitoring and analysis of ACIS trailing pixel fraction by the instrument team. In all cases the focal plane temperature is -120C and the trailing pixels are measured using the Mn-Kα line (5.89 keV). The numbers for each CCD are an average over all nodes.
Updated monthly.
A symptom of radiation damage in CCDs is an increase in the number of charge traps. When charge is transfered across the CCD to the readout, some portion can be captured by the traps and gradually re-emitted. If the time scale of the re-emission is close to the pixel-to-pixel transfer time (40 μsec for ACIS), significant charge may be deposited into the pixel immediately following the original charge packet. A small fraction of the ACIS trap population have time constants in which charge trailing into the following pixel is important.
We monitor two quanities related to charge trailing. The first is the rate of charge trailed into the following pixel (ADU/pixel) and is the linear slope as a function of row number of the pulseheight in the top pixel of the event island for the 5.9 keV spectral line. The plot below shows this quantity as a function of time for all ten CCDs. We expect that the gradual increase in radiation damage and charge loss will cause the charge trailing to increase as well. This is seen in the data below. The rate of change (2-4% / year) is comparable to the rate of change of the charge loss.
If the mix of electron trap time constants remains the same over time, we would expect that the fraction of the charge loss that is trailed into the following pixel would remain constant. The plot shown below shows the ratio of the charge trailing (ADU/pixel) plotted above to the charge loss (also ADU/pixel) as a function of time. The FI CCDs show a small increase in the charge trailing fraction as a function of time. The BI CCDs show a small decrease in the trailing fraction as a function of a few percent. This may be due to the gradual increase in traps due to radiation damage which are likely to have different properties than the traps due to the initial low energy proton damage (FIs) or manufacturing defects (BIs).
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