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also are discussed later. The curve is intended to represent a conservative estimate (for safety considerations) of the stem activity as a
function of height for air bursts.
The rather large scatter in the data may be attributed to several
factors, It appears that stem visibility may be the most importantof
these. Most of the higher activity readings occurred during penetrations when the stem cloud was visible to the pilot. The relatively low
readings were obtained when the cloud was not visible or when it was
not known whether the cloud was visible. It is quite possible that the
aircraft did not actually penetrate the stem on these occasions. The
dose rates measured inside the aircraft may have been due to “shine”
from the stem cloud or to activity from diffuse material outside of the
stem core. In those cases where the stem was not visible and where
several passes were made at the same altitude, only the highest reading has been used.
Unfortunately, for the lower 80% of the stem, virtually all the data
for the larger detonations are questionable because of the stem-visibility problem. Therefore it is impossible to say whether ar not the low
stem activity found for these shots indicates a real decrease in the
fraction of activity in the lower part of the stem with increasing nuclear yield.
The following factors also contribute to the uncertainty in the
results:
1. Stem-volume estimates. For the determination of the total activity present in a 1000-ft-thick layer, the stem diameter at the penetration altitude was estimated. The values used were based on visual
estimates made by obServers on the ground and in the sampling air-
craft or, where necessary, on estimates for other detonations in the
Same yield range. The estimated diameter
could be in error by as
much as a factor of 2 in some cases,
2, Stem-height estimates. The stem was considered to extend
from sea-level to the base of the cloud regardless of the burst height.
The cloud bases used were based on visual observations from the
ground and from the sampling aircraft and verified, where possible,
by radiation readings taken in the sampling aircraft. The uncertainty
in the height of the cloud-base (stem height) is about 10%.
3. Representativeness of dose-rate readings. The measured dose
rates are assumed to represent those in a uniform, infinite cloud. The
assumption
appears to be reasonably valid for those penetrations
where the stem was visible. The aircraft required 20 sec or more to
traverse the cloud at a speed of about 7 miles/min while the mean free
path of gamma radiation in air is on the order of a few hundredfeet.
The navigator reported that the dose rate usually rose sharply on en-
tering the cloud, remained fairly steady (within a factor of 2) during