PuBe Sources

[Janzow1@aol.com]

     I have seen several questions and comments on RADSAFE relating to PuBe 
neutron sources, and there appears to be a near-vacuum of information about 
them.  This is an effort to provide such information as I can related to 
specific issues, to provide provide information to partially fill the vacuum, 
and to provide cautionary information which may be of use to those who still 
have  PuBe sources.
      First, there are two general species of PuBe sources, and several 
sub-types of each.  The older sources were Pu-239/Be, and the later ones were 
Pu-238/Be.  Production methods, encapsulation materials and methods, neutron 
output variations, and potential problems are significantly different for the 
two isotopic types.
      THE MATERIAL WHICH FOLLOWS APPLIES ONLY TO Pu-239/Be SOURCES.
       Some sources of this type were made in the early 1950s.  Exactly when 
the first ones were made is not known to me.  The only maker prior to 1956 
that I am aware of  was Los Alamos Scientific Lab.  The general method was to 
mix Pu and Be metal in an atomic ratio of approx. one Pu to thirteen Be atoms 
in a BeO crucible, then to heat the mix in a vacuum furnace  until the 
exothermic reaction forming the intermetallic compound PuBe-13 initiated.  
After the reaction went to extinction and the products solidified (melting 
point of alloy, ~1750 C ),  the crucible was broken off the alloy slug, and 
the slug  was doubly-encapsulated to make a source  (Ref. 1, p. 3).  No 
additional details of the LANL sources are known to me.
     Mound Laboratory developed a similar but less costly technique,  and 
made sources using this technique at least throuh 1960 (ref.1, p.3).   The 
Mound process (ref. 1, p. 3 and ref. 2)  involved putting a weighed pellet of 
Pu-239 metal into a Be metal cup, placing both into a tantalum capsule and 
TIG-welding the capsule shut, then placing the sealed capsule into a vacuum 
and heating it by RF until the reaction forming the PuBe13 intermetallic 
started.  Ref. 1 notes that while Pu melts below 650 C, the reaction did not 
strart until the temperature approached 1278 C, the melting point of Be.  
Also that the temperature during the reaction reached approx. 2000 C.  After 
cooling, the  item was welded into a 304 stainless-steel outer capsule.

_______
References:
1.  M.R. Hertz, "Inspection And Recanning Program Of PuBe Neutron Sources", 
January 7, 1964, issued May 1, 1964 as MLM 1188, TID-4500(27th ed.).*   
2.  J.L. Richmond and C.E. Wells, US Patent 3,073,768, Jan. 15, 1963.
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