Re: Dirty bombs- Cs-137 movement into soil question

[Stewart Farber <farbersa@optonline.net>]

2/28/03 6:54:44 AM, BERNARD L COHEN <blc+@PITT.EDU> wrote:





>	--If it seeps down into soil so rapidly, why doesn't it continue

>to seep down until it gets so far down that the soil above becomes an

>effective shield

===========

Hi:



When fallout Cs-137 during the open air testing days was studied over the past 40 

years,  it was found to bind to organic matter in soil by an ion-exchange 

mechanism that varied sharply with organic content and the widely variable 

content of soil in STABLE Cs. Fallout Cs-137 [which came down carrier free] does 

move slowly as time passes at a variable rate based on soil organic content to 

reach a mean depth and distribution that varies widely. The DOE Environmental 

Measurements Lab [formerly HASL] has studied Cs-137 distribution in average soil 

in doing its field gamma spectroscopy studies and technique development since the 

late 1960s. HASL defines a factor in converting gamma flux for any isotope 

measured in a field gamma spec to its presumed depth distribution in soil and 

average soil concentration that HASL refers to as a "relaxation length" that is 

informative in this regard 

[see: www.eml.doe.gov/publications/procman/Sect3/3_3.pdf }



For fresh fallout, sitting on the soil surface one uses a specified factor of 

flux to areal concentration. As time goes on Cs "moves" into the soil and the 

flux for any average areal concentration would go down. However, Cs does not 

reach a depth where it is difficult to measure by field gamma spectroscopy, 

although on a sandy surface [with little if any organic content] with lots of 

rain over many years, Cs might penetrate to a much greater depth.



Stewart Farber, MS Public Health







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