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.

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 (*T*p - period

of validity) over a period three times as long as the monitoring time interval. The period of validity (*T*p)

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: *T*p = 3(Tb Ta) + Tb or

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: *T*p = 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: *T*p = 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.

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