also larger than nor:..1. The possibility exists that this detector was double pulsing (a malfunction that would cause a dose rate about twice the actual dose rate}; however, the instrument
was checked after each shot and found to be operating perfectly.
Furthermore, the record
from this station agrees with those obtained from other GITR’s aboard the same ship after
passage of the base surge. The records are, therefore, considered reliable. Although no
completely satisfactory explanation for the discrepancy has yet been advanced, the mostlikely
possibility is that the bow records for the DD-593 are the result of an extreme case of ship
retardation (Sections 3.3.2 and 3.4.3).
In many instances during Umbrella, the film pack dose is understandably higher than the
cumulative GITR dose, since the GITR was saturated during peak dose rate (Table 3.27). This
difference, however, occurs at the forward station aboard the EC-2 during Wahoo where there
is no evidence of GITR saturation. The EC-2 was so oriented for Wahoo that the forward end
was engulfed by the base surge slightly ahead of the rest of the ship. Although this difference
might account for a higher total dose at the forward station, it does not account for the difference between the film pack dose and the total cumulative GITR dose. Superstructure shielding
effects (Section 3.4.2), ship retardation (Sections 3.3.2 and 3.4.3), or variation in deposition at
the two locations can be postulated but cannot be conclusively demonstrated.
Like those from the coracles, the shipboard records (Figures 3.183 through 3.194) may be
divided into characteristic types (Table 3.11). The general discussion of all shipboard records
has been incorporated with that of the coracle records (Section 3.3.2). The shipboard records
are presented in this section, uSing the same format and key previously described for the coracles. The records are individually discussed here to indicate possible modifications due to
the superstructure or ship movement. In general the shipboard and coracle records are so
Similar that the effects of the ship’s superstructure on the free-field gammaradiation is not
immediately apparent.
An analysis of shipboard film pack doses, however, gives definite evi-
dence of superstructure effects and may be used in conjunction with cumulative GITR doses to
estimate the magnitude of such effects (Sections 3.4.2 and 3.4.3).
For Wahoo, the shipboard records from the DD-474 and DD-592 are limited to film pack
and meter survey information, because a power failure aboard both ships prevented receipt of
the radio signals that started the project instruments (Section 2.1.1). When this information is
considered, the fact that both ships moved after the detonation should be borne in mind (Figure
2.1). At 20 minutes after Wahoo, the DD-474 had moved about 900 feet farther downwind, and
the DD-592 had moved about 500 feet farther downwind.
During Wahoo, the EC-2 was anchored halfway between a crosswind and an upwind position
at 2,300 feet from surface zero with the starboard side facing surface zero obliquely. At 20
minutes after Wahoo, the EC-2 had moved an additional 600 feet crosswind (Figure 2.1); however, the estimated maximum movement during base surge transit is 200 feet. The EC-2 was
So oriented that the bow was first engulfed by the base surge as evidenced by the slight differ-
ence in time of peak between the forward and after stations.
The abrupt decrease in dose rate
(Figures 3.183 and 3.184) immediately after the first peak is probably due to the passage of the
upwind surge beyond the ship.
The gently sloping plateau from 2 to 4 minutes represents an
inner edge transit of the upwind surge with some additional contributions from white water,
which reached the ship at approximately 2 minutes. The gamma record persists about 4 min-
utes after the final transit of the primary surge photo-bpoundary P, (Appendix F).
Using the
reported surface wind this delay in final transit could indicate a tail (Appendix F) of approxi-.
mately 6,200 feet, but a more probable cause is the temporary retention of surge remnants in
turbulent eddies associated with the superstructure (ship retardation). The fact that all shipboard detectors were installed in positions that are both distant and shielded from waterborne
activity probably accounts for the absence of dose rate spikes caused by these sources.
The records for the DD-593 during Umbrella (Figures 3.185 and 3.186) representa nearly
central cross section of the base surge, clearly showing an intersurge decrement. The ship
Was positioned with the stern into the wind; thus, the starboard side faced the hot line (Appendix F). The records from the starboard and port GITR’s do not, however, show any significant
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