[Aavso-photometry] cosmic rays - some progress

[Ben Davies]

Richard,

Actually the D 263 glass is in the whole line of Kodak ccds, not just 
the ST-10.  Bruce Gary got in touch off list to show me a similar 
procedure for exposing cosmic rays that he has been using for some 
time.  <http://brucegary.net/A86279/ISfor6128.html>  If you look at his 
4 min ST-8 image you see 33 hits in 4 minutes over 1.26 sq cm.  About 
the same rate.

No one else has mentioned that they have gone looking for cosmic ray hits.

I haven't tried to identify the type of radiation by geometry of the 
hits, but Laurence Labs did.  They characterize long straight hits as 
muons, and worms and spots as Compton recoil electrons from gamma 
radiation and direct beta hits from the (still presumed) Potassium 90.   
See <http://snap.lbl.gov/ccdweb/ccdradLBNL_spie02.pdf>  If I understand 
correctly, it is only the muons that are cosmic rays proper.  All else 
is local stuff.

I don't know about the BK7 corrector plate.  It would seem to be too far 
away for the betas to reach.  The distance wouldn't be any problem for 
the gammas, but the geometry is not favorable.
.

Ben Davies



> 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