irregular that no straight line could be drawn through the plotted points.
These decay constants
may be compared to the Horizon decay tank values as shown in Figure 2.33. Considering the
difference in the method of collecting and treating the decay sample, the agreement between the
two methods is good.
The late time at which fallout occurred where tank measurements could be made precluded
the possibility of obtaining decay constants for correcting dose rates to H+1. No other decay
measurements at such early times are directly applicable to the probe readings.
In spite of this, an examination of time-intensity records (Reference 7) of gammafield intensities at close-in stations shows that decay constants of dose rate changelittle between H+1
and H+50 hours. Using this evidence, in the absence of actual probe decay measurements, it
will be assumed the decay constants shown in Figure 2.33 are valid for correcting probe readings
to H-1 for each of the indicated shots.
2.6.13 Decay Correction Factor for Dose Rates.
For the purposes of correcting all readings
to a common time, the measured decay constants were used to calculate decay correction factors
(Figure 2.34).
To determine the dose rate at H+1, the radiation level at the time of observation is multiplied
by factors shown on the ordinate, corresponding to the observation time. The effect of the large
decay constants for Shots Navajo and Tewa is clearly shown during the latter days of the surveys,
where the correction for decay becomes very large.
2.6.14 Factor for Determining Accumulated Dose. The dose rate levels at H+1 hour do not
give a realistic picture of the hazards resulting from fallout, because over most of the area,
fallout has not occurred this early.
A realistic presentation of the true radiation hazard from fallout is the total dose that a person in an unshielded position would receive during the first two days following a shot. Using the
measured decay constants and assuming a dose rate of 1 r/hr at H+1, the accumulated dose was
calculated for each hour from H+1 to H+50. These values are presented graphically in Figure
2.35, wherein the abscissa is time of fallout and the ordinate is accumulated dose in roentgens,
assuming 1 r/hr at H+1. To use this figure, the dose rate at H+1 of a given measurement is
multiplied by numerical value of the ordinate corresponding to the fallout time of arrival for
that measurement.
2.6.15 Data Reduction. The large volume of data prohibits the presentation of all measurements in tabular form. A small section of the results, from one ship during one survey, will
be presented here to clarify the steps involved in reducing the measurements. This will also
help to explain the use of various tables and figures which have thus far been introduced.
The ensuing procedure has been used to reduce the data collected during each of the fallout
Surveys.
The columns referred to are found in Table 2.10:
:
Column 1 Date of observation.
Column 2 Time of observation.
Column 3 Instrument number and sensitivity scale (switch position).
Column 4 Current readings in microamperes (probe output) which were recorded on the Leeds and
Northrup recorder.
Column
5
Derived from Column 4 by conversion of the microampere reading to apparent dose rate
tor/hr¢ by use of the Co® point source calibration curves.
Figure A.1 (Appendix A).
Column 6
in Figure 2.13.
An example of these calibrations is shown in
Lists the contamination of the probe in mr/hr# at the time of each observation as shown
Column 7 Derived by subtracting Column 5 from 6. This is the apparent dose rate mr/hr® which
Would have been recorded from an uncontaminated probe in the water.
Column 8 Gives the figures in Column 2 converted to time since detonation in hours. Thue ‘8 re-
Wired for decay corrections.
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