Lesson 2/Learning Event 1
For an example the use of figure 7 on page 35, consider a mission conducted at a 150-meter survey
height in a UH-1. A reading of 10 cGy(rad)/hr is obtained and the air-ground correlation factor for a
UH-1 at a height of 150 meters is 8.2.
Aerial dose rate X Air-ground CF
Unshielded ground dose rate =
10 cGy(rad)/hr X 8.2
=
=
82 cGy(rad)/hr
Decay Rate Determination
The decay rate of the contamination should be determined for making an accurate evaluation of the
hazard. For induced contamination, the soil type, known or assumed (type II), will determine the decay
rate. For fallout, the assumed decay rate (n = 1.2) is used until monitoring reports provide sufficient
data for the valid determination of the actual decay rate.
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 so identified.
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 they cannot be determined until monitoring information is available, or
laboratory facilities are available for analysis of the actual contamination. Analysis will become
available to the NBCE in about 10 hours. It is quite probable that contaminated areas on the nuclear
battlefield will be recontaminated by new contamination; for example, fallout on fallout. Thus, the
NBCE must be prepared to identify combinations and multiples of the various forms of radiological
contamination.
The decay exponent of radiological contamination can be determined by either a mathematical method
or a graphical method. For either method, a series of dose-rate readings from several selected locations
are required. The reliability of the decay-rate calculation is directly related to the accuracy of the dose-
rate readings, the time interval over which the readings were taken, and the length of time over which
dose calculations are to be made. 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. A rule of thumb for decay-rate determination is that reliable dose calculations can be
projected in time (Tp) over a period three times as long as
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