{
Figures 2.10, 2.11, 2.12, and 2.13 snos the prcooe certaminoc.. for the fallout surveys following each shot. The unit of contamination is the in-situ probe re:cing as deduced from the
co™ point source calibration. This is used extensively throughout the report and is always denoted by “mr/hr#” (see Appendix A). The McGinty probe during the Navajo survey (Figure
2.12) was contaminated before the detonation. This was the result of a special preshot survey
of radioactive water flowing westward out of Bikini Lagoon. The Silverstein probe, during all
of the surveys, accumulated little contamination, because this ship was purposely directed
away from water having high radiation levels. This was done in order to have at least one ship
capable of determining low-level boundaries.
9.6.3
Penetration Meter.
The original premise that rates of penetration could be obtained
by jneans of Geiger tubes moored to skiffs and suspended at various levels in the sea has been
proved. Mechanical and electrical difficulties prevented the accumulation of any great quantity
of data; however, one penetration meter unit, triggered by the fallout 18 minutes after Shot
Tewa, provided enough information to predict the usefulness of this type of instrument for other
shots of a similar nature. In general, for the purposes of adding close-in penetration meas-
urements to the Redwing series, the penetration meters were disappointing; however, even the
one measurement is valuable.
The penetration meter was located at skiff station PP (see Figure 4.8) and successfully re-
corded on certain of the probes until 15 hours after triggering, at which time abrasion of the
probe cable against the chine of the skiff caused a general electrical shorting.
The results of this record are shown in Figures 2.14 and 2.15. In Figure 2.14 dose rate is
plotted on log scale and the time in minutes, in order to clearly show the rapid downward penetration of the fallout material in the water. In Figure 2.15, the dose rate is plotted on linear
scale and the time in hours to show the long-term behavior. It is not certain whether the second
peak exhibited by probes 1 and 2 is caused by Secondary fallout or electronics maifunction.
A study of rapid rate of penetration shown in Figure 2.14 indicates the arrival, at the site, of
rather large particles approximately 18 minutes after the nuclear detonation. The recorder was
set to trigger at a radiation level of 15 mr/hr, and the time recorded when the instrument was
recovered indicates that it started at 18 minutes after shot time; during the first cycle, none of
the probes indicated any radiation. On the second cycle, which started 12 minutes‘after triggering, the 1-meter probe read 900 mr/hr; 1.3 minutes later, the 20-meter probe read 120 mr/hr;
1.3 minutes later (14.6 minutes after triggering), the 40-meter probe read 56 mr/hr. The 60meter probe failed to function, but the 80-meter probe, which was read 6.5 minutes after the
Start of the second cycle (18.5 minutes after triggering), read 45 mr/hr. Assumingthat particles
would be falling past the corrected depth of the 80-meter probe at a constant rate of descent and
had reached the water surface at the time the triggering device functioned, a calculation of the
particle size can be made if certain assumptions are made.
These assumptions are: (1) the
Particle has a constant fall rate of 6.05 cm/sec, (2) the particle is spherical and homogeneous
with a density of 2.32 gm/cm’, and (3) the sea water had a density of 1.02 gm/cm’ and a viscosity of 0.0086 dyne sec/em?.
Vv
Where:
Utilizing the formula for Stokes law as:
- 2ga’ (d, — 4.)
9n
y = velocity in cm/sec
a = radius of sphere in cm
dy and d, = density of sphere and medium respectively
,
hn = coefficient of viscosity dynes sec/em?
& = gravity at 981.45 cm/sec?
the Particle
le of this
™ethod,
size can be calculated as 0.02 cm radius or 400 microns in diameter.
A particle
diameter at this range is entirely possible, based on the NRDL fallout-prediction
Because of the high sensitivity of the G~M tubes used in the probes, saturation was reached
sermon ae
+
23