Fallout Radiation More Than 24 Hours Old.
By 24 hours after burst, the change in the rate of radiation decay is so low that it is relatively
insignificant. In making estimates of the total dose to be received when the time of entry into the
contaminated area is later than H + 24 hours, you merely multiply the dose rate at the time of entry by
the time of stay (hours). As an example, personnel must enter a fallout area at H + 27 hours and stay for
approximately 4 hours. The dose rate at H + 27 is 3 cGyph. The total dose is then computed by
multiplying 3 x 4 to determine a total dose of 12 cGy.
Nonstandard Decay Rates.
The amount of variation of the decay exponent (n) for residual radiation is expected to be from 0.8 to
2.0. The average value for n is 1.2, which is referred to as standard decay. Any value for n other than
1.2 is considered nonstandard decay. The decay rate should always be considered to be standard,
when it cannot be estimated or determined, but actual determination of the decay rate should be made
Factors that affect the rate of decay may be divided into several broad groups. These groups are:
a. The type of weapon and type of active materials, including the construction and structural
materials, in the weapon.
b. The type and quantity of materials vaporized and drawn up into the fireball.
c. The use of radiological agents designed to produce nonstandard decay.
d. Fallout from a nuclear burst overlapping fallout from an earlier nuclear burst.
Nonstandard decay rate determinations will be accomplished by the NBCC at the tactical operations
centers of major commands. When the NBCC has determined from actual monitoring data that the
decay rate is nonstandard, the information will be disseminated by the NBCC.
Decay rate determinations are based on dose rate readings from one location. With these radiation
readings, the decay exponent (n) can be determined by several methods using either a graphic method or
by mathematical computation. The reliability of decay rate calculations is directly related to the
precision of the dose rate readings, the length of time over which the readings are taken, and the length
of time over which dose calculations