(b) Use the following procedure for when the best fit line does not cross both the y- and
x-axes (see location C in Figure 4-5): From any location along the best fit line, draw a line directly down
toward the x-axis. Choose a location down slope from the first location and draw a line directly left
toward the y-axis. Measure the distance downward and leftward to the point where these two lines
intersect. Record the downward measurement as the y value and the leftward measurement as the x
value. See location C in Figure 4-5.
The n used in calculations is determined by an average of the n values determined for each location.
NOTE: See Appendix C for rounding rules.
1.2 1.0 1.1 3.3
For the readings recorded in the three locations in Figure 4-5, the average n value is 1.1. A linear
scale is superimposed on the graph to show the determination of linear measurements.
b. To determination the decay rate using the mathematical method, use the following procedure:
(1) The formula for the mathematical determination of the decay exponent is found by solving
n = decay rate of fallout.
Ra = dose rate (cGyph) measured at time Ta (peak dose rate or later).
Rb = dose rate (cGyph) measured at time Tb (the last dose rate available).
hours after burst) that dose rate Ra was measured.
Ta = the time (H +
hours after burst) that dose rate Rb was measured.
Tb = the time (H +
NOTE: Ra, Rb, Ta, and Tb are determined from the NBC 4 (nuclear) series reports submitted by
units which have been directed by the NBCC to provide dose rate readings (NBC 4 [nuclear]
reports) every half hour for 2 hours followed by hourly reports. These reports begin after the
NBC 4 peak has been determined.