Re: Alpha-Be neutron source construction

["Richard J. Cashwell" <RJC@coefac.engr.wisc.edu>]

> Date sent:      Tue, 12 Sep 95 10:34:20 -0500
> Send reply to:  radsafe@romulus.ehs.uiuc.edu
> From:           "Jeffrey A. Leavey (532-4595 / 5-3466)" <certhp@VNET.IBM.COM>
> To:             Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
> Subject:        Alpha-Be neutron source construction

> Two question on alpha/Be neutron sources: is there any reason
> why I should not expect to see neutrons when a Be foil is exposed
> in contact with Po-210?? The texts say I can expect about 80
> neutrons per 1E6 disintegrations of Po-210. Granted these
> sources have the Po and Be intimately mixed, but is it
> practical to expect SOME neutrons from a foil exposed to
> enough bulk Po-210 on a bench top?  Would normal environmental
> oxide on the foil surface be enough to stop the alphas?? Any one
> know what the threshold energy is for Be(a,n) ???
> 
> For FMY (for my info), what are the details of how Be(a,n)
> sources are made? Any tricks-of-the-trade??
> 
> Thanks for any help!!!
> 
> Jeff
> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
> Jeffrey Leavey                          certhp@vnet.ibm.com
> X-ray Lithography Program                         IBM Corp.
> 914-892-4595                       East Fishkill, NY  12533
> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

As you surmised, (a,n) sources are mixtures.  Pu Be sources of the common 
type made by Mound Labs were made by placing a weighed pellet of Pu into a 
beryllium cup.  This was placed in a tantalum case.  A tapered tantalum plug 
was driven in flush with the case and sealed by TIG welding.  The whole 
assembly was then placed in a vacuum chamber and heated to about 1278 deg C 
to cause the Pu and Be to react and form PuBe-13.  After leak checking, this 
capsule was placed in a stainless steel capsule which was welded closed and 
leak detected. 

Typical Ra-Be sources were made with mixtures of Ra and Be powder.  The 
material is pelletized and double encapsulated.  I have no information on Am-
Be sources, but would assume similar fabrication.

I would expect neutron emission to be extremely low if the materials are not 
in exceedingly intimate contact, preferably in a chemical bond for stability. 
Details on early sources can be found in a Mound Labs Publication MLM-1188 
(TID-4500) published in 1964.





R. J. Cashwell
University of Wisconsin-Madison
141 Mechanical Engineering Building
1513 University Avenue
Madison, WI 53706
Phone 608-262-3392
FAX 608-262-8590