[vsnet-chat 2107] Re: CCD Cosmic ray hits

[aah@nofs.navy.mil]

Lew Cook asked about the origin of 'cosmic rays' seen
by CCDs.  The real cosmic rays, which generate showers
of muons that are absorbed by silicon, create radiation
events at the rate of about 2 events/cm2/min.  Anything
larger than this is locally created, either by low-level
X-rays from glass like UBK7, certain coatings, low-level
radioactivity from the dewar itself, etc.  One generation
of the early RCA CCDs had radiactive ceramic as the support
layer for the thinned CCD and had quite high radiation
event levels.  However, most professional
cameras see rates close to the number given above since
much care has been taken to remove as many radioactive
substances from the camera as possible.  This may not
be the case for amateur cameras.
Arne

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