E5 Attenuation and decay of radiation over time and distance

Topic

Radiation exposure from a source decreases through two independent processes that should not be confused. The first is spatial: the intensity of a point source drops off with the square of the distance, meaning that moving relatively short distances away from the source reduces exposure far more than intuition might suggest. The second is temporal: the activity of isotopes present in contaminated material decreases over time because each isotope decays according to its own characteristic half-life.

When many different isotopes are mixed together—as occurs with fallout—there is a fairly consistent average behavior that allows for estimating how much the ambient dose rate will drop in the hours and days following an event. However, that average masks widely varying individual behaviors: short-lived isotopes decay much faster than the mixture's average and quickly cease to be significant, whereas others persist far longer than the initial average would suggest.

This expected decay curve determines—as discussed later in this series—how long one should remain sheltered after an event; it is not an arbitrary civil protection decision, but rather a direct consequence of the fact that the ambient dose rate continues to drop predictably as hours and days pass. For this reason, this model—which distinguishes between attenuation due to distance and decay over time—underpins both the instruction to move away from a source and the advice to wait before returning to an affected area.

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