[vsnet-chat 2122] Re: CCD Cosmic ray hits

["Fraser Farrell" <fraserf@dove.net.au>]

G'day Lew,

On Sun, 11 Jul 1999 12:02:34 -0700 (PDT), Lew Cook wrote:
> Back about 20 years ago, I got a bucket of chips (not
> CCD chips - just integrated circuit chips) for gamma
> ray analysis. I found that there was thorium and
> uranium in the chips. Both of these trace elements are
> radioactive and give rise to a chain of radioactive
> decay products. Alpha radiation impinging on a chip
> internally can cause errors. Thorium is also used in
> some optical glass.
[etc]

IBM funded a big study of this problem 15-20 years ago and discovered 
that radioactive trace elements in the plastics and silicon were 
(then) a significant source of random errors in integrated circuits.  
The problem was particularly acute for memory chips.

Consequently, IBM, Intel, Motorola, etc made enormous improvements to
their manufacturing processes to eliminate this contamination.  However, 
the same studies also identified cosmic rays as a source of random
errors.  The only identified cure (at the time) was to shield the
circuits during use, with fifty feet of solid rock.

However, alpha particles within a CCD are not the only source of
local radiation.  You can also worry about gamma rays
(which are highly penetrating) arising from the U & Th in certain rock 
types eg: granites, dolerites, basalts, or from some beach sands (within
minerals such as zircon); and from construction materials made from
these.  There's also the radiation from potassium-40 - which is almost
everywhere - to consider.

The truly paranoid can also worry about soft X-rays emitted by their 
old computer monitors; and nasties from nuclear fallout.


cheers,
Fraser Farrell

http://vsnet.dove.net.au/~fraserf/

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