[Aavso-photometry] cosmic rays - some progress

[Richard Miles]

Ben,

You look to have made good progress in your quest to elucidate what's going 
on here.

What are other ST-10 owners finding when it comes to long dark exposures? 
If the cover glass on the CCD chip is potassium-rich then they too should be 
experiencing higher backgrounds than normal ambient.  To recap, normal 
ambient is about 1 event per cm2 per minute: you are seeing a rate of more 
than 6 cm-2 min-1

Potassium-40 decay yields beta emission in 90% of the cases and gamma in the 
remaining 10%.  Beta emission is short range, which may explain why it is 
the cover glass that is crucial.  Have you attempted to characterise the 
form of the cosmic-ray/radioactive background signals?  Would beta emission 
give non-straight tracks and gamma give largely point source (or say 2 or 3 
adjacent pixels plus the occasional straight track)?  There is also the 
worry about rare-earth high-dispersion glasses containing radioactive 
Uranium and Thorium.

One other thought is that some SCT scopes use BK7 glass for the front 
corrector plate: maybe gamma emission from the corrector plate could reach 
the CCD chip?

Richard Miles

----- Original Message ----- 
From: "Ben Davies" <ben at davies.net>
To: <aavso-photometry at mira.aavso.org>
Sent: Monday, February 27, 2006 11:02 PM
Subject: [Aavso-photometry] cosmic rays - some progress


> As several of you pointed out, my images are showing a very high number of 
> cosmic ray hits compared to the naturally occurring ambient flux. 
> <http://ben.davies.net/cosmicrays2.htm> Worried that I might have a 
> radioactive house, I went to two other locations and made more test dark 
> images. One location was 4 miles away and the other 140 miles away. 
> Happily, the results were similar to the one obtained at my house – around 
> 7x ambient.
>
> Since I needed to find a source somewhere, I began to look inside the 
> ST-10. I called up SBIG and talked about it with them with them. They said 
> that the issue had been addressed a few years ago and they were aware of 
> no source that they were introducing.
>
> I then called Kodak and learned that the glass window that is glued over 
> the chip is Schott D263. A call to Schott and I learned that D263 glass 
> contains 6.9% potassium, which is a source of beta particles and gamma 
> rays. This makes my 6.9x ambient results consistent with results reported 
> by NOAA. They had Schott BK7 glass containing 11% potassium producing hits 
> at a 10x ambient rate. <http://snap.lbl.gov/ccdweb/ccdrad_talk_spie02.pdf> 
> (page 32). and also: 
> <http://www.ctio.noao.edu/pipermail/ccd-world/2001/000472.html> 
> <%3Chttp://www.ctio.noao.edu/pipermail/ccd-world/2001/000472.html%3E> That 
> whole thread is very interesting.
>
> This of course does not demonstrate that the Kodak windows are 
> contributing the excess radiation that I am finding, but it is suggestive. 
> Someone with more resources than I have should make tests.
>
> I wonder whether the potassium's 1.35 MEV electrons might also introduce 
> spurious voltages into the registers or the analog-to-digital capacitor 
> during readout?
>
> Ben Davies
>
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