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CCD Radiation Damage (Dark Pixels)

Image name: CCD_DARK_PIXELS.png (click image to enlarge)
Image size: 4.50500 KB (320x352)
Date submitted: 22-Mar-1997


This figure illustrates the affect of accumulated radiation damage on the
SXT CCD.  These mini-images are taken from the same portion of the CCD for
the following two PFIs.

Left:   23-DEC-96  16:41:11  Open/AlMg  Full Norm C  26  7548.0  128x128
Right:  19-MAR-97  20:36:00  Open/AlMg  Full Norm C  26  7548.0  128x128

The lower left corner of each image is at (927,620), i.e., at the east
limb.  It is evident that many of the same the "dark pixel" artifacts are 
evident in both images.

These pixels contain what Jim Janesick would call "traps" where some
charge remains as the CCD is read out -- creating dark pixels.  It appears
that up to 25% of the charge is lost in the dark pixels.  Note that
the 6-day CCD bakeout in Jan-97 apparently had no effect on this radiation
damage.  I have no idea why so many of the dark pixels are 2 or more pixels
wide in the x (E-W) direction.  This would seem to imply that the damage is
in the serial register but, if so, I'd think it would affect every row of
the image.

As far as I can tell today the dark pixels are confined to the limb areas
where the x-ray radiation damage is the worst.  They are not nearly so
evident on the disk.

Earlier I mistakenly identified these horizontal "dashes" as contamination.
That was totally wrong.

Note that this effect is sufficiently severe to affect the response
calibration in the limb regions.  It remains to be seen if the effect
is intensity or spectrally dependent.

					L. Acton  3/22/97

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