638
FERBER
the mean speed of the aircraft, the time in the cloud, and the average
dose rate are reported, these data can be utilized in the same manner
as the Dominic I stem penetrations to compute the device fraction contained in the cloud at the penetration altitudes. The computed device
fractions are plotted in Fig. 3 for comparison with the Dominic I data.
Several interesting features may be noted. It appears that the
activity in the upper half of the stem is greater for surface bursts
than for air bursts and that the difference increases with altitude. The
largest gradient of activity with altitude appears at about 70 to 80% of
the stem height; this implies that for surface bursts the “radiological
base” lies below the visual cloud base. However, this inference may
not be warranted since the high activities encountered below the base
may be due to the descent of fallout particles.
The values computed for the lower portion of the mushroom indi-
cate about 1 to 2% of the total fission products per 1000 ft. Since the
mushroom portion of the clouds investigated averaged about 30,000 ft
in vertical extent, the average activity in the mushroom must have been
about 3% per 1000 ft. Thus we have some basis for believing that this
admittedly crude method can give at least the right order of magnitude
for the activity at a given altitude, even when the average dose rate recorded curing a single pass through the cloud is used.
We note that the one Redwing data point for an air burst gives
about five times the activity indicated by the curve estimated from the
Dominic ! data. This greater activity might be attributed to the fact that
the detonation took place at a lower scaled height than any of the
Dominic I air bursts. The burst height was somewhat below the minimum altitude for a true air burst according to ourdefinition (burst
height > 180Y°"*); therefore the activity distribution might be expected
to be intermediate between those for air bursts and surface bursts,
Although the close-in fallout measured after this detonation was very
light, it was considerably more than that found after any Dominic I
shot, where shipboard dose rates never exceeded 0.1 mr/hr. However,
Since only one surface vessel was available for fallout measurements
during the Dominic I tests, the very limited number of measurements
obtained does not permit the drawing of firm conclusions.
RESULTS AND CONCLUSIONS
Activity in the Stem Cloud for Air Bursts
Although the Dominic I stem-penetration data leave a good deal to
be desired for defining the distribution of activity in the stem, the curve
in Fig. 3 represents a best estimate based on our interpretation of
these data. A major uncertainty lies in the assumption that the dis-
tribution in the stem does not vary with yield. As indicated previously,