[ RadSafe ] Alpha to Beta Ratio Source Indication

Cindy Bloom radbloom at comcast.net
Tue May 29 09:10:43 CDT 2007


Such ratios depend on analyses of site specific source terms, the sample 
matrix being considered, and the instruments used for analyses (the latter 
having to do with how a wide spectrum of beta energies are interpreted in 
terms of activity for a specific instrument).  A review of the decay scheme 
for uranium, for example, shows that the ratio of alpha to beta is about 1 
through U-234, and about 2 through Ra-226.  (You might want to look at the 
Rn-222 decay scheme, as well as consider Th and K;  I have a vague 
recollection that depending on degrees of equilibrium, etc. the range of 
alpha to beta in U and Th and Rn source terms is about 0.5 to 
3).  Typically, actinides are alpha emitters, and typically fission 
products are beta/gamma emitters, so when ratios are outside the 0.5 to 3 
range, results are attributed to these material categories.  I've mostly 
seen this ratio applied as one consideration in determining whether a 
visitor's contamination is due to radon progeny or whether an air sample 
reflects an increase in radon progeny or longer lived alpha emitters.

At radioactive material processing and waste storage sites, radioactive 
materials can be brought together in a variety in a variety of ways and so 
the rule of thumb regarding the likely origin of contamination should only 
be considered as a piece of the puzzle, not as the whole picture.  Also, 
I'm not sure that U, Th and their progeny would necessarily be considered 
"natural" in the context of a DOE waste site;  it seems that would depend 
on whether these materials originated from the geological formations at the 
location of the site or whether these materials were brought to the site as 


At 06:41 AM 5/29/2007 -0600, Goff, Tom wrote:

>Does anyone know of a basis for determining if the source of radioactivity 
>on a smear or air sample is natural, fission product or transuranic based 
>on the alpha to beta ratio.
>For example, if the ratio is greater than 3 then the source could be 
>Actinide; it the ratio is less than 0.5, the source could be fission 
>product; if the ratio is between then natural radioactivity is indicated.
>Does anyone know of a reference for these ratios?
>Tom Goff
>WIPP Radiological Services
>505 234-8861
>fax 505 234-6062
><mailto:Tom.Goff at WIPP.ws>
>P.O. Box 2078
>Carlsbad, New Mexico 88221
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