XIS Spaced-row Charge Injection (SCI)
What is spaced-row charge injection?
The XIS has suffered radiation damage, and the energy resolution has degraded. To recover the good resolution, the XIS team has decided to inject charges to selected rows of the CCD (spaced-row charge injection, or SCI) as shown in the figure below. Although this reduces the effective area, the energy resolution becomes much improved.
The radiation damage makes traps in X-ray CCD pixels. These traps obstruct the charge transfer, which causes the degradation. However, if we can put additional charges in front of the X-ray events, these charges work as "sacrifical events" - they fill the traps and the X-ray events are transferred smoothly. Thus we can restore the eneregy resolution.
Results of test observation
The figure below shows the results of employing the SCI method. The energy resolution improved from 200 eV to 150 eV, as expected. The center pulse height of Fe Kalpha line becomes large when we use the SCI. This means X-ray events are transferred with little charge loss.
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