b. Graphical method. The graphical method of determining the NF is based on the fact that a plot
on log-log graph paper of the NF versus the time after burst at which a reading is taken to desired
reference time is a straight line. This procedure requires that both n and H-hour be known. The graph in
Figure A-2 (Appendix A, page A-9) has been constructed to determine the NF for correction to H + 1. To
use the graph
Read across the bottom to locate the appropriate time after burst.
Follow the line above that point into the graph to the point where it intersects the slanted line for
the appropriate decay exponent.
From that point, follow the line to the left of the scale and read the NF.
c. Tables of values. Tables A-6 and A-7 (Appendix A, pages A-7 and A-8) are tables of NFs of
selected times after a nuclear burst and for various decay exponents. The reference times are H + 1 and H
+ 48. These tables are normally used when H-hour is known for fallout contamination and the collection
effort is initiated immediately. The tables are used after determining the time after burst when the
readings were taken. Locate that time in the left column. Read across the page taking the reading for the
time under the appropriate decay exponent. If the decay exponent is unknown, use n = 1.2.
EXAMPLE: What is the NF if the time of burst is 0900 and the time of reading is H + 4 hours, n = 1.2?
SOLUTION: Locate H + 4 hours on the left column of Table A-6 in Appendix A. Read across the chart
to the H + 4 reading under the 1.2 heading. NF = 5.27.
When the time of reading is between two of the times after burst in the left column, an accurate NF can be
obtained through interpolation between numbers. A rapid interpolation can be made by approximation.
A safe-sided NF may also be determined by going to the next higher time.
5-6. Overall Correction Factor (OCF). The OCF is the product of the CF and the NF. The OCF is
calculated as: OCF = (CF, VCF, or AGCF) x NF. The calculation of this factor greatly reduces the work
required to calculate the final data to be plotted. Instead of multiplying each monitoring and survey
reading by the CF and then the NF, each reading is multiplied by the OCF only.
EXAMPLE: You have received data from an aerial survey. A course leg on the survey had 15 readings.
The AGCF is computed as 4.2. The NF is determined as 2.64. What OCF would each of the 15 readings
be multiplied by to determine the H + 1 ground dose rate?
SOLUTION: OCF = 4.2 (AGCF) X 2.64 (NF) = 11.09, rounded up to 11.1 (see Appendix C for rounding
rules). Figure 5-1, page 5-6, shows a completed survey report using the OCF. Dose rate readings can
also be normalized using the residual radiation decay (fallout) nomogram or the ABC-M1A1 RADIAC
calculator.
(Figure 5-2, page 5-7) allows the user to find the dose rate at any time, if a dose rate at a known time after
the burst is available.
CAUTION
Figure 5-2 is valid only for single explosions.
CM2306
5-4
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