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PELLETIER, WHIPPLE, AND WEDLICK
through the milk food chain. We are concentrating our efforts ona
single farm in Tecumseh, Mich. We have divided the program into two
parts. The first part is a routine sampling program in which we mea-
sure the radioactivity in every medium that might enter into the milk
food chain. The second part of the program is the developmentof a
mathematical model. In developing the model, we are relying heavily
on theoretical considerations and on the experimental results of other
investigators.
In certain cases,
however,
we have designed special
experiments for the sole purpose of providing information for the
model. The results of the routine sampling program will be used to
check and refine the model.
In our climate a greater proportion of radioactive contamination
is deposited by precipitation than by dry processes. Therefore the first
exchange of radioactivity in the milk food chain is between air and preCipitation. To keep the development of the model separate from the
data collected on the farm, we haveused data from an environmental
survey around the Enrico Fermi Atomic Power Plant* to develop the
model.
BACKGROUND
Chamberlain! has used the term washout factor, W,, to equate the
radioactivity in rain to the radioactivity in air. He defines the term as
the ratio of the rain concentration to the air concentration measured at
1200 m, both concentrations being computed on a weight basis. Peirson
and Keane’ have also used the washout factor but have used the air con-
centration at ground level rather than at 1200 m.
Table 1 shows the washout factors measured by various investi-
gators. As far as this paper is concerned, the important thing to note
is that the washout factors can vary considerably when computed over
periods of less than a few months.
One cannot discuss the deposition from rain without mentioning the
deposition from dry fallout. This is because, unless they are covered
when there is no precipitation, all precipitation collectors will collect
some dry fallout. A useful term in measuring dry fallout’ is the depo-
Sition velocity, V,, which is defined as the ratio of the deposition rate
onto a surface to the air concentration measured above that surface.
Surface characteristics are important in dry-fallout measurements. Megaw and Chadwick? have reported a fivefoldincrease in depoSition velocity measured on real grass relative to that measured on
horizontal sheets of filter paper. The aerosol used in this case was
*This survey was made by the National Sanitation Foundation, Ann Arbor,
Mich., under contract with the Power Reactor Development Company, Detroit,
Mich.