Contamination

[Paul.Jorden at eev.com]

Posted to CCD-world:
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Chris, & CCD-worlders,

Hi.  This problem sounds familiar!
I refer you to a 1996 discussion, which you can find at Tim Abbotts' 
ccd-world' archive.
go to     http://www.cfht.hawaii.edu/~tmca/CCD-world/ and look at 1996.

For your convenience I repeat a message that I posted then; in the days of 
RGO!!!

Hope it helps.

Best Regards,
Paul
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Re: Grease

Paul Jorden (prj at ast.cam.ac.uk)
Mon, 12 Feb 1996 10:25:50 GMT

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Andrew,
To continue the grease discussion:

> I was rivetted by you statement that:
>
>> Finally, In case you don't know, we have now learnt that silicone 'high
vac.
>> grease' is bad news in a cryostat. It is clear that it can become 
volatile
>> and deposits itself on surfaces. (It's vapour pressure was quoted as ~ 
10-6
>> mbar; this is clearly not low enough!) We no longer use it. We use only a
>> small trace of 10-12 mbar grease (non-silicone) now on o-rings only.
>
> I have been using Apiezon L, M and N with impunity (thermal coupling
> O-rings, etc.) I have, of late, had contamination problems with
> my thin chips.
>
> Tell me more ....
>
We used to use Dow Corning High Vacuum grease; this is silicone, with a 
quoted
vapour pressure of < 10-7 mbar I believe.
We used it as a thin layer to improve thermal contact under our Tek/SITe 1K 
CCDs

and the cold block.
We definitely got contamination within the cryostat from this source- it's a 

long story, but mass spectrometer analysis confirmed it. It is 'well known' 
that
this vacuum grease is actually bad in this respect!

Apiezon 'M' (a mineral-based hydrocarbon grease) is quoted as having a 
vapour
pressure of < 10-7 mbar also.

We have now switched to using Fluorinated Fomblin grease FM090 from Balzers; 

this has a vapour pressure < 10-12 mbar. It costs us #200 per 50 gm, but we
don't use much of it! (Only the minimum on 'o' rings now, never elsewhere..)

For improved thermal contact we now use a thin layer of indium where 
necessary.

Cheers, Paul Jorden
_______________________________________________________________
Dr Paul Jorden,  CCD Applications, EEV
Waterhouse Lane, Chelmsford, Essex, CM1 2QU, UK
Tel:      44 (0) 1245 -453458 (direct), -493493 (switchboard)
Fax:      44 (0) 1245 453224
see also:-     http://www.ccd.eev.com/

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