collected
In a very few cases, where the fallout particulates have a sufficiently high
plutonium content, special methods have been employed that take advantage of
the special properties of plutonium nuclides (Nathans et al., 1976).
By
autoradiographic registration of alpha particles ("hollow-star technique"),
the plutonium content of each particle is registered as distinct tracks on a
photographic emulsion.
These tracks form a star and the number of tracks is
converted to a “PuOj-equivalent" particle diameter.
This method has been
applied to soils from the fallout field of a safety shot (Schulz et al., 1975)
and to a few resuspension samples.
The lower-size limit depends on the isotopic
jalf-life and on the exposure time, but with reasonable exposure times extends
down to below 0.1 um Pu0:-equivalent diameter for plutonium-239.
Particles
above 1 or 2 um may be identified, measured, and isolated for additional
measurements. These special methods also make it possible, in many cases, to
obtain information from old debris for which normal autoradiographic techniques
are no longer applicable.
Density-gradient separations have also been applied in a few cases (Tamura,
1976; Soinski and Nathans, 1976),
This method fs particularly useful for the
gross separation of undiluted debris particles from particles that consist
primarily of soil components and is based on the observation that such particles
have specific gravities well in excess of 3.0 or thereabout.
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Most of the size distributions reported before the mid-60s show lognormality.
These results are probably correct for the fallout samples,
Lognormal size
distributions of cloud samples, particularly early samples, are probably
biased against the larger-particle end of the distribution,
Later work has
shown that many cloud samples have a particle size distribution that may be
described as lognormal below a few micrometers, but obey approximately a
r -law, with m having a value between 3 and 4, above the range of lognormality (Russell, 1965; Nathans et aZ., 1970) (Figure 2),
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OTHER PHYSICAL PROPERTIES
1.
Shape.
Particle shape was measured to determine if the radioactive
debris possesses any special characteristics that distinguish these
particles from background, entrained inactive material, and contamination.
The radioactive particles were generally identified by autoradiography,
occasionally by micromanipulation, and the use of a beta-gamma meter or
phosphors.
Usually a sufficient number of particles were viewed to
encompass the various shapes (and colors) that were present.
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samples as derived from the particle
Particle size distributions of three Johnie Boy
size distributions of size fractions
the measurements were made on fallout samples
Figure 2.
In quite a few cases,
throughout the close-in fallout field.
This may make it possible to derive
estimates of the size distributions of debris on the soil anywhere in the
fallout field.
Such work would be quite complicated, however, in areas where
fallout from more than one shot is present.