LN2 cooled Bi and Be crystals

[Neutron]

Dear Collegues
 
We are in the process of designing a collimator/plug for a radial beam tube
on one of our beamlines at the SAFARI-1 research reactor in South Africa.
The intention is to use this beamline for SANS and/or PGNAA work. 
 
The radial beam-tube has an ID of 7 inches, and the intention is to filter
the beam (because of the direct view to the core) using cooled (LN2 temp) Bi
and Be filter of length about 20cm each. To our knowledge, this has been
done frequently at many reactors worldwide, and hence we would like advice
on possible techniques of cooling these materials. We have thought of some
possibilities, and would appreciate criticism or support regarding the
ideas...here are some of them:
 
1) Use a direct cooling system, where crystals are bathed in an insulated
LN2 bath, i.e. direct contact with LN2?
2) Cooling via a heat exchange system, where the crystals are wrapped by
copper tubing with LN2 flowing through the tubing (i.e. indirect contact
with LN2 via copper tubing)?
3) Peltier coolers on the crystals. This option may not attain LN2
temperatures, and may be much more costly than the above 2 techniques- we
are not sure. If anyone has tried this, we would appreciate your input.
 
Many thanks to the mailing list for the opportunity to submit this query...
 
Yours sincerely 
 
 
Jeetesh Keshaw
NECSA - PNI
www.pelindaba.co.za <http://www.pelindaba.co.za> 
Cel: +27 (0) 83 650 0923
Tel: +27 (0) 12 305 5852
Fax: +27 (0) 12 305 5851
email: keshaw at aec.co.za <mailto:keshaw at aec.co.za> 
snailmail: PO Box 582, Pretoria, 0001
South Africa
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