may
rec
tatic.
sulted from either or both suggested causes.
Thus, the gross gamma-intens:ty
" all cases be considered the free-field record without further correction. Presendetailed a theoretical treatment may therefore appear somewhat academic; however,
the approach as given is considered useful both .or interpreting the range of effects observed
during Operation Hardtack and for predicting the relative magnitude of various contributing
components in future underwater detonationS Where larger amounts of deposited radioactive
material may logically be expected.
The radiation intensity due to the upwelling of contaminated water is treated in exactly the
same manner as that already presented for ly, . In this case no mixing factor M is required.
The intensity Iny at the GITR detector, due to an infinitely large body of. such contaminated
water, is computed to be:
low
=
13.5 Jy(t)
If an equivalent source concentration is assumed for both the airborne and the waterborne material, i.e., that Ja(t) = Jy(t), the intensity due to an infinite cloud is roughly a thousand times
that due to an infinite water source. Although significant contributions from such sources were
not considered likely, the intensities due to circular upwellings of various radii were calculated
as a percentage of the intensity from an infinite water source and are presented in Ficure 1.7.
An inspection of this figure indicates that an upwelling 50 feet in radius would be nearly equivalent to an infinite water source. The mathematical model employed implies an absolutely smooth
interface; therefore, the actual intensities could be reduced 20 percent or more by surface rough-
ness (Reference 45). The approximate intensities resulting from the movement of such circular
bodies of radioactive water past a coracle are presented in the following section.
1.3.2 Properties of Moving Fields. Although consideration of stationary radiation fields can
indicate the relative magnitudes of possible contributing Sources, such models are of no use in
deducing cloud dynamics or transport mechanisms. The general solution for the passage of a
radiating cloud would be a powerful tool for the analysis of dose rate histories, but such a general treatment rapidly runs into mathematical difficulties beyond the scope of this project. A
few simple cases are investigated, however, and are used later in this report for interpretation
of the GITR records.
The approach of an infinite rectangular radioactive cloud may be treated as a special case of
the intensity above an infinite radiating slap developed in Reference 39. The approximate expression for the radiation intensity L at a point on a nonradioactive plane and at distance x
from the forward boundary of such a rectangular cloud of radioactivity is:
Jalt)
lap = toa
{a0 e~ PAX _ wax [- Ei (- nan}
where Jait) remains the source intensity per unit volume of cloud, yu, is the linear attenuation
coefficient for air, K is a constant approximating the buildup factor in an expression of the form
(1+K wax), and pax is the mean-free-path iength in air for gamma rays of a stated energy.
Since radiation from sources approaching from a distance are to be considered, the errors inherent in the buildup approximation must be carefully inspected. Ignoring contributions from
Scattered photons with ultimate energies less than 0.068 Mev, the linear buildup approximation
used is sood to within +16 percent for a gamma source energy of 1 Mev up to distances of 10
mean free paths. When the distance to the approaching cloud becomes zero, the intensity is
given by the expression:
Jatt)
=
I,
= TA
(1-K)
37
a
lap)x— 9