152
HEFT AND KAHN
solely with particulate radioactive debris resulting from nuclear detonations. Two reasons for selecting this problem area are that (1) such
debris
represents
an important fraction of the total burden and (2)
samples suitable for particle analysis are available. These samples
are residual parts of aerial filter papers containing radioactive debris from the initial cloud from a number of nuclear-device tests.
Radiochemical-analysis data on counterparts of the existing filters
provide guidance concerning the representativeness of the sample; i.e.,
if the most volatile and most refractory radionuclides are present in
the correct relative abundance, then the sample may be considered to
be representative of the true particle population.
EXPERIMENTAL PROCEDURE
The overall experimental procedure can be described in terms of
three main divisions: sample processing, particle separation, and
analysis.
Sample Processing
The sample as received consists of a few square inches of IPC
1478 cellulose-fiber filter in which the particles are entrapped. The
experimental objective is to recover the particles from the filter
without altering them in any way. The method used is the so-called
“low-temperature ashing method,’ in which oxygen gas at low pres-
sure is passed through a radiofrequency field and over the filter sample. Under these conditions a gradual oxidation of the cellulose occurs,
during which the temperature of the sample is kept below 100°C. Par-
ticles released during the oxidation of the cellulose are electrostatically attracted to the wall of the glass vial in which the ashing is conducted. Recovery is monitored by gamma counting the sample vial
before and after ashing and is essentially 100%. Comparison of the
gamma spectra before and after ashing indicates no change. Fora
permanent gamma-spectral reference, a disk is cut from the original
filter sample and mounted for counting in the same configuration as
the final particle samples. The remainder of the filter is ashed ina
weighed vial, and the ash weight is determined.
Particle Separation
The particle-separation procedure must provide a series of particle samples in which the particles are divided into groups according
to their sedimentation rates. This may be accomplished by sedimenta-
tion
in
a
liquid
medium. Large
particles
may be separated into