PART C: DETERMINE THE DECAY EXPONENT
4-6. General. The decay rate of the contamination should be determined in order to make an accurate
evaluation of the hazard. For induced contamination, the soil type (Type I, II, III, or IV), known or
assumed (Type II), will determine the decay rate. For fallout, the assumed decay rate (n = 1.2) is used
until a series of monitoring reports as described in Part B for determination of H-hour provide sufficient
data for the valid determination of the actual decay rate.
a. After monitoring reports indicate the cessation of fallout, immediate action must be taken to
provide the data required to make a decay rate determination. Until this determination is actually made, it
is emphasized that dose calculations and hazard evaluations are estimates only, and this information
should be identified as such.
b. In the case of fallout from an unknown weapon or an unknown source, neither H-hour nor the
decay rate will be known initially and cannot be determined until monitoring information or laboratory
facilities are available for analysis of the actual contamination. Analysis will become available to the
NBCC in about 10 hours. It is quite probable that contaminated areas on the nuclear battlefield will be
recontaminated, for example, fallout on fallout. Thus, the NBCC must be prepared to identify
combinations and multiples of the various forms of radiological contamination.
4-7. Determine Decay Exponent. To fully understand how to determine the decay exponent of
radiation, you must be able to use both the graphical method and the mathematical method. For either
method, a series of dose rate readings from several selected locations is required. The reliability of the
decay rate calculation is directly related to the precision of the dose rate readings, the length of the time
interval over which the readings were taken, and the length of time over which dose calculations are to be
made. That is, the more reliable the monitoring dose rate readings and the longer the time interval over
which they are taken, the longer the time period over which reliable dose calculations can be made. As a
rule of thumb for decay rate determination, reliable dose calculations can be projected in time (Tp - period
of validity) over a period three times as long as the monitoring time interval. The period of validity (Tp)
is a mathematical calculation that determines how long the decay rate is valid. For example, for a decay
rate determined from monitoring readings taken between H + 4 and H + 8, dose calculations could be
reliably projected from H + 8 to H + 20: Tp = 3(Tb Ta) + Tb or
Tp = 3(8 4) + 8
Tp = H + 20
EXAMPLE: Consider a fallout-producing nuclear burst (H-hour known) with the collection effort
initiated at H + 4 and expected to be completed by H + 6; the target time for preparation of the pattern is
H + 8. By H + 4 to H + 6, a decay estimation can be made and used to process the remainder of the dose
rate information from the collection effort. This procedure will result in a reasonably reliable H + 1
pattern. By H + 6, a decay rate determination can be accomplished which will allow use of the resulting
pattern until about H + 12: Tp = 3(6 4) + 6. By H + 12, a decay rate determination can be accomplished
which will allow use of the pattern until H + 36 hours: Tp = 3(12 4) + 12.
a. The graphical method for determining n is performed by plotting a sequence of dose rates (NBC
4 [nuclear] series reports) from one location on log-log graph paper. The following steps detail how to
use log-log paper to determine the decay rate of fallout. The decay rate of the contamination causes the
line plotted to be a straight line, inclined at a slope n to the axes of the graph. Use the steps below to plot
NBC 4 (nuclear) series reports.
CM2306
4-6
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