Hot Pixel
The hotpixel maps are made over temperatures ranging from
-40 c to -120 c. The meanbiasclip tool is applied to 10 dark frame readouts
to eliminate background x-ray events. This will also reject weakly flickering
pixels for which there is as yet no evidence. Hotpixels can be marked by
measuring relative to the rms deviation of the background, in this example
a 10 sigma excess is flagged. This example taken at -75 c shows only a
small number of hotpixels - most of which are not due to the radiation
damage. As the temperature is decreased these remaining hotpixels vanish.
The
intensity of the hotpixels decreases as the "glow" of the radiated
spot decreases so that the hotpixels may be considered the most extreme
examples of an enhanced dark current which affects all pixels in the radiated
area.Looking in more detail at the hotpixels it is easily seen that they
are all "single events".
Here
is a closer look at the same hotpixel map. Notice that a number of pixels
lie slightly below the 10 sigma threshold level. This is the same situation
which affects the shuttle device hotpixel maps. The sacrificial charge pram
can be used to help locate particularly strong traps, but at the present
time none have been found. It should be noted that immediately following
irradiation there is a much higher number of truly hot and flickering
pixels but that after a period of time at room temperature these go
away. At some point we should try to separate the radiation damage effects
into the short term and those that persist.