PuBe Sources - Increase in neutron flux

["Ken Smith" <ksmith@cats.ucsc.edu>]

Hi All -



I have a question about PuBe source and the observed increase in neutron

flux over time. A few references I have state that the neutron flux from

a Pu-239/Pu-241 PuBe source will increase over time. A chart (sorry I

don't have a reference for it) shows the increase to be proportional to

the percent of Pu-241 in the source and increases over time. 



I was having difficulty understanding why this is.  Is the Pu-241 being

generated by neutron absorption by the Pu-239 -> Pu-240 -> Pu241? Does

this occur at an appreciable rate?  Or is the Pu-241 just a contaminate

in the processing and isotope separation of the original PuBe source?

Why does the presence of Pu-241 in a source increase the neutron output

over time?  Is the increase in activity caused by the shorter half-life

of Pu-241 (14 yr) and its daughter (Am-241, 433 yr.)? How far in the

future will the output increase stabilize?



Any help pr guidance would be greatly appreciated.



Thanks

Ken



Ken Smith, Radiation Safety Officer

Environmental Health and Safety (EH&S)	voice: (831) 459-3911

University of California			fax: (831) 459-3209

1156 High St.				e-mail: ksmith@cats.ucsc.edu 

Santa Cruz, CA  95064			web: www.ehs.ucsc.edu





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