stantiated.
As no determinations of initial-gamma dose rate versus time had been made for
nuclear detonations in this yield range, these measurements were necessary to experimentally
substantiate existing scaling methods or provide a basis for obtaining new extrapolation methods.
Much of the effectiveness of the employment of very-low-yield weapons will depend upon the
range at which they can safely be delivered. The position of the aircraft at burst time andits
subsequent maneuvers for the next 20 seconds will largely determine this range from a nuclearradiation standpoint.
1.2.2 Neutron Dose.
Neutron dose has been measured during practically all past nuclear tests;
but for the most part, these have been concerned with dose levels in excess of those of interest
in the delivery of air-to-air missiles. No measurements had been made of the neutron dose from
fractional-kiloton yields below the limits of fission-foil-technique sensitivity, i.e., 10 rep.
Since this dose region corresponds to that of operational interest to the Air Force, measurements
in this range are required.
Several techniques appear to be adaptable to measurements of low neutromdose.
Types A and
B Kodak personal-monitoring films have been successfully used in the past for other applications
(References 7 and 8),
The resonance-threshold foil dosimeter (Reference 9) developed by General
Electric permits an evaluation of the neutron spectrum to be made by determining the activation
of various foils. An energy-corrected total dose can thus be found with this method.
The possibility of using sulfur to determine the neutron dose in the low-dose region had been
under investigation for some time (Reference 10). Measurements during past nuclear tests have
demonstrated that the biological neutron dose closely follows the sulfur neutron flux for fission
weapons:
ly
Remo = ($250.76)
X10?
where I, = the sulfur neutron flux at 1 yard (n/cm?) arid Remy = the neutron dose at 1 yard.
It
can be seen that I/Rem will be constant over the distances of interest if the sulfur mean free
path is equivalent to the neutron dose mean free path, since:
Ibe —DR/A
I Fr
Rem
_ Remy e~DR/A
De
Where: D= distance from the source
R = relative air density, correcting the mean free path back to the atmospheric
conditions existing at shot time
X = mean free path for standard ICAO atmosphere (15 C and 760 mm Hg).
Reference 11 shows that the neutron spectrum does not vary from one to six meanfree paths.
Splenic and thymic weight-loss measurements of mice exposedto fission neutrons have yielded
an average mean free path of 2¥2 + 55 yards. The mean free path for NACA atmosphere of sulfur neutrons is 210 yards. For fractional-kiloton yields, this direct relationship should held
for all doses of operational interest, as these occur well within a range of ten neutron mean free
paths.
1.2.3 Induced-Activity Decay Rate. One source of radioactivity formed during a nuclear detonation is the induced activity in the soil resulting from the activation by neutrons of the soil
10