Getters/Molecular Sieves

[Paul Jorden]

Greetings Tim & all,

> What getters/molecular sieves (eg. charcoal, zeolite) do you use in 
> your dewars and why (i.e. what contaminants are they good at adsorbing)?
> 
> Our answer: we use charcoal, but it's not clear to me whether this is best
> for sweeping up water vapour &/or hydrocarbons (plus, it makes this nasty
> black powder...).
>
We use Oxford Instruments cryostats with a built in Sorb. ('usually activated 
charcoal' they say in the manual).
We don't ever examine this, we merely re-activate it occasionally using the 
internal heater. We don't gets bits of nasty black powder around.

I know that water vapour is considered the main vacuum load. This I assume is 
mainly removed by vacuum pumping, and partly by LN2 freezing it out- although a 
large surface area would facilitate this.

I guess the absorber must soak up all the other residuals, and further water 
vapour that outgasses with time.
My understanding is that the getter/sorb will release absorbed gases when they 
warm up- except water vapour. Hence they need heating occasionally to release 
absorbed water.

I believe that 'getter' means an absorber that is chemically active (activated 
charcoal) and that a 'sorb' means something which is basicaly very porous, with 
a large surface area to adsorb gases onto (zeolite). Getters work at room 
temperature (and improve with cooling?). Sorbs certainly work much better when 
cold.

I think that both sorts (especially cooled inside a cryostat) should absorb most 
gases/ contaminants quite well. As usual they will not be so good for light 
gases like He/ H2 etc, but this should'nt matter. However the pumping efficiency 
will depend not only on surface area/ intrinsic efficiency but ALSO on 
mean-free-path considerations. At lowish pressures pumping action is limited by 
path length and I guess that contaminants near the detector might not reach a 
sorb at the other end of the cryostat very quickly. Unfortunately this is 
exactly the arrangement that we have!

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 deposit 
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.

Cheers, Paul
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