Chapter |
INTRODUCTION
1.1
OBJECTIVE
This experiment was designed to provide data to aid the formulation of a method
for predicting the gamma-radiation field caused by air bursts. Specific objectives
were to measure the induced activity from a large-yield, thermonuclear air burst and
to exploit the opportunity offered for an investigation of activity induced in some soil
other than the soil of Nevada Test Site (NTS). When a bomb-drop error prevented any
data being obtained on Shot Cherokee, an additional experiment was set up on Shot Yuma
to obtain some data applicable to the second specific objective.
1.2
REASONS FOR EXPERIMENT
Certain military uses of nuclear weapons demand a minimum of residual contamination. For example, it might be desirable to exploit the first shock of a nuclear burst
by having friendly troops occupy or pass through the ground-zero area soon after the
detonation. Although for practical purposes there is no local fallout from an air burst,
a considerable area around ground zero can be rendered radioactive by neutron bombardment, and the gamma-radiation field thus produced can prove dangerous to persons
entering it hours later. The intensity and decay of such a gamma-radiation field will
depend on the kind of weapon, its yield and height of burst, and on the elements which
make up the soil around ground zero. This experiment was expected to provide data
for the evaluation of the military significance of neutron-inducedactivity.
1.3
BACKGROUND
Tests at the NTS have led to empirical methods for predicting neutron-induced dose
rates from air bursts in the kiloton range over Nevada soil (References 1 and 2). These
methods attempt to predict 1-hour dose rates over any area within the test site. However, since no attempts have been madeto correlate dose rates and decay rates with
the chemical composition of a variety of soils, the methods are valueless for any but
Nevada-type soil. Furthermore, since in Nevada soil the amount of sodium, the most
important contributor to induced activity, varies from area to area within the test site
by as much as a factor of four, the accuracy of such methods is severely limited even
for NTS.
A method for the prediction of induced activity which does account for chemical differences in soils has been developed by Canu and Dolan (Reference 3). Before predictions are made for a given soil, the amounts of over twenty elements in the soil must
be determined. The gold-neutron flux expected is also used to help establish intensities
at times of interest. However, this method has the following important shortcomings:
1. No allowance is made for the moderation of high-energy neutrons into the low-