are to be made. That is, the more reliable the dose rate readings and the longer the time interval over
which they are taken, the longer the time period during which reliable dose calculations can be made.
As a rule, reliable dose calculations can be made over a period three times the length of the time interval
between the first and last dose rate reading.
a. Graphic Method for Determination of n.
When a series of dose rates from one location is plotted on log-log graph paper, the decay rate of the
contamination will cause the line plotted to be a straight line, inclined at a slope (n) to the axis of the
graph.
EXAMPLE PROBLEM: Suppose the set of readings shown in Figure 6-1 is received for decay-rate
determination; H-hour is known or has been determined to be 0930 hours.
Figure 6-1. Dose rate readings
When these data are plotted on log-log graph paper, using the time as the number of hours past H-hour,
and the best possible set of three lines is drawn through the points, the graph shown in Figure 6-2 is
obtained. The slope of these three parallel lines is n, the decay exponent.
6-3
CM2308
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