CHAPTER3
RESULTS
3.1
AIR-BORNE RADIOACTIVITY LEVELS AND GROUND SURFACE CONTAMINATION
Data presented in. Table 3.1 show the levels of beta-gamma radioactivity measured at the
three stations on the 7-mile arc and at the four stations on the 106-mile arc. These measurements were made by Project 37.2. It is apparent that the major exposure of animals located
on the ?-mile arc occurred during a period of about 90 min after the cloud reached thesites,
Likewise, at the 106-mile arc, the major exposure occurred during a period of about 90 min
immediately after the cloud reached these sites. Although the animals remained under fall-out
for about 8 hr, more than 90per cent of the total inhalation exposure occurred during the first
90 min.
From the values shown for air-borne vs soil contamination,it is evident that most of the
radioactivity must have been on large particles which were not drawn into the air samplers.
This is the best explanation for the gross differences between levels of activity in the air and
on the ground surface,
It is important to note that the percentage of radioactivity present in the 0- to 5.0-particle-size range in sofl samples is much greater at the distant stations compared with values
found at the 7-mile-arc stations. This relation is in accord with theoretical predictions and
provides information which clarifies the biological findings.
The values given for mean size of particles found on cascade-impactor samples are
based on measurementsof radioactivity and standardization of the impactors with material of
similar density (2.5) and of known particle-size distribution, Actual measurements of particle
size by microscopic methods were not made, nor were the relative numbers of radioactive and
nonradioactive particles determined, From the work done by others,'® it is likely that the majority of particles in the 0- to 5-p size range found on impactor slides and filters are nonradioactive,
It is important to emphasize that the low levels of radioactive fall-out materiais found are
probably related to two main factors: (1) The bomb was detonated from a 500-ft tower, and
(2) the ground surrounding the tower was paved with asphalt. The combination of these factors
could reduce the amountof fall-out considerably.
3.2
THYROID MEASUREMENTS
Animals exposed at the 7- and 106-mile stations were examined three times in the field
(H + 33, 53, and 73 hr), using external gamma-rayscintillation-counting techniques similar to
those employed clinically for thyroid uptake of I'*! (see Fig. 3.1), Prior to making such measurements, each animal was decontaminated externally, using a vacuum cleaner. The majority
of exposed animals and the restraining boxes were contaminated slightly, to the extent that
gamma-ray activity before decontamination registered by a survey meter was 2 to 3 mr/hr
(background of 0.05 mr/hr) at H+18 to 30 hr. In general, levels of gamma-rayactivity over
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