Identifying Cosmic Rays                                                                                         Home

First of all, "cosmic rays" are mostly muons cascading down through the atmosphere as the result of gamma radiation interacting with the upper atmosphere.  But here I take the term (as do most astronomers) to refer to any discrete event which knocks loose electrons in the detector.  "Cosmic ray" then includes local sources such as Compton recoil electrons from direct gamma ray hits and also hits by beta particles given off in radioactive decay, e.g. Potasium 40.  And in fact, Radioactive potassium⁴⁰ occurs naturally in potassium (.01%). It is a primary source of local beta and gamma ray contamination when it is present in glass lenses.²   This potassium occurs in Schott D263 glass that Kodak uses for a cover slip for the CCDs supplied to SBIG.

Concerned about the high incidence of radioactive trash in my SBIG ST-10 ccd images (see below), I tracked my excessive cosmic ray problem down to the cover glass Kodak places over all chips it supplies to SBIG.  SBIG agrees that the glass is "probably" the source, but says that they cannot do anything about it.  This even though Shott makes a similar glass that does not contain potassium.

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This image is a composite of 9 - 30 minute dark frames made with an SBIG ST-10..

The expected number of cosmic ray artifacts is 362 total, or a rate of  .9 events¹ per sq cm of chip per minute. 

However, I find 2,487 total cosmic ray events:  6.2 events per sq cm per minute.

This is 6.9 x the ambient rate.

The test was repeated at 3 sea level locations, all with similar results: (1) at home, (2) 4 miles away, (3) 140 miles away.

     1:1                4:1

      8:1             16:1

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This is a portion of one of the 9 dark images from which the above details were extracted.

A visual inspection reveals one long cosmic ray hit is barely visible at lower center.  In fact however, there are 191 more, only a couple of which are visible without special processing.

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Footnote:

2.  http://snap.lbl.gov/ccdweb/ccdrad_talk_spie02.pdf     page 32