At the time of the Conference, 86 particles had been isolated.
Of
these, 50 had been sent to McClellan Central Laboratory and 36 to Knolls
Atomic Power Laboratory.
The project was completed a short time later
with.the transmittal of the remaining 14 particles to KAPL.
METHODS
The methodology was discussed briefly in a previous paper (Nathans and
Soinski, 1977).
The sequential scheme employed is shown in Figure l.
After receipt of the samples, the -300 mesh fraction was separated by
dry sieving in order to remove the particles that are not appropriate
for alpha autoradiography because of their size.
A density separation
was performed on the small-size fraction with a liquid of density
2.96 g/cm? (1,1,2,2-tetrabromoethane).
The heavy fraction was used for
the isolation of particles, because this fraction generally contains the
greater part of the plutonium activity.
This step introduces a bias in
favor of particles that consist primarily of device debris.
However, in
most of the samples, the plutonium concentrations were expected and also
found to be low, so that this concentration step was designed to reduce
the time necessary to find the required number of particles.
Furthermore,
a similar bias would have been introduced anyway, because, for the
techniques used to locate particles of interest, unreasonably long
exposure times would have been required to find plutonium—-bearing particles that consist primarily of soil components (have densities signifi-
cantly less than 2.96 g/cm).
The heavy fractions were transferred to microscope slides, fixed with a
drop or so of collodion in amyl acetate, and subjected to alpha autoradi-
ography (Nathans et al., 1976)
("hollow-star technique").
Exposure
times varied from 7 days to 1 hour.
The detection limit for a 7-day
exposure is about 5 x 10’ atoms of 239pu in a single particle, or a
239Pu0.-equivalent diameter of 0.15 um.
This detection limit was deemed
adequate for the purposes of the project.
Moreover, longer exposure
times would have increased the difficulties associated with the location
and isolation of the particles.
After scanning the slides and marking of the areas where particles of
interest were located, the isolation of the particles was accomplished
in two or three repetitive steps.
A small section of the collodion
containing the particle of interest, but also other particles, was
removed from the sample slide, and transferred to a clean microscope
slide.
A drop of collodion in amyl acetate was added, dissolving the
collodion holding the particles.
The particles were spread with a pick,
and the collodion was allowed to dry.
The slide was then subjected to a
repeat alpha autoradiography procedure.
Usually the particle of interest
was readily identifiable and sufficiently far away from neighboring
particles to allow isolation.
If not, the procedure was repeated.
The
particle was then transferred to the sample mount.
Autoradiography was
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