In these expressions, k is a factor converting the number of disintegrations per radiating
particle into Mev, sec, ng is the number of particles per unit volume of cloud, and t, 1s the
time of simulated arrival.
A factor for radioactive decay is also included in the expression
for Ja(t) by assuming the t™'-? approximation applies. These expressions also contain the
tacit assumptions that the average gamma energy per photon remains reasonably constant over
the period under consideration, and that the vertical dimension of the cloud is large enough to
neglect depletion of cloud activity through deposition, i.e., greater than five mean free paths,
or 2,300 feet for a 1-Mev gamma.
Although the radiation due to the airborne material can be
estimated fromthe instartaneous value of Ja (t) , the contribution from deposited material must
be integrated from the simulated time of arrival. Thus,
t
Ip + Iw = 1.54 7 *? vy { Na(t) dt
ty
Although the concentration factor was determined by assuming certain arrival times, the use
of such words in conjunction with a stationary model is misleading; therefore the term ‘cloud
Slope” C, is coined. This term refers to the rate of increase in the concentration factor
Ja (t) , a characteristic which completely describes the particular Situation; thus,
1
Cs = th
Using terminal velocities of 0.3, 3.0, and 30 cm/sec, which bracket those most probable
for base surge (Table 1.2), the expected free-field intensity Tg is calculated for a number of
cloud slopes and expressed as a percentage of Ig. Values for cloud slopes typical of early
and late arrival times are presented in Figures 1.5 and 1.6, and the remainder of the calculations are tabulated in Section A.2. Both these curves and the tabulated data clearly demonstrate
the pronounced decrease in the relative response to free-field intensity as the terminal velocity
and, hence, the particle size increase. This situation becomes progressively worse for cloud
clopes characteristic of late arrival times corresponding to those experienced during Operations
Castle and Redwing. Since arrival of base surge at most of the proposed Hardtack stations was
predicted (Reference 22) prior to 3 minutes, it seemed probable that the cloud slopes greater
than 0.1 would be experienced at all project stations.
.
Since the estimated relative intensity Ty, due to fallout suspended in the surface layer of the
ocean is small, the most obvious means of improving detector response to the cloud radiation
is to reduce the contribution due to deposited material. Any fallout associated with the proposed
underwater detonations was expected to be in the form of liquid droplets; therefore, the most effective approach was to increase runoff from the detector case. In the event of high terminal
failing velocities (or large fallout droplets), most of the material deposited on the detector case
was expected to roll off to the deck, where its relative contribution would be reduced by a factor
of 3. Further improvements in detector response could have been accomplished by shielding;
however, about 500 pounds of lead would have been required to reduce the deck contribution to
1 percent, a weight that was obviously impractical for coracle application. The deck contribution could have been reduced to 50 percentby the addition of a 10-pound lead shield; however,
the same reduction could also have been accomplished without the risk of deposition on the shield
itself by raising the detector 1.7 feet above its present position. The simple deck mounting
shown in Figure 1.2 was finally selected as the best compromise between experimental and operational requirements.
In the hght of Wahoo and Umbrella results, the deposited activity actually observed is charac-
teristic of that predicted for high cloud slopes and small individual droplet sizes; specifically,
the relative response follows the curve for V, = 0.3 cm/sec as shown in Figure 1.5. A similar
response could also have resulted from heavy deposition that immediately ran off the coracle
Surfaces, and it appears that, depending upon station location, the observed low residualactivity
34