[ RadSafe ] Check out Story - Bomb Test Exposed Civiliansto Radiation

[Brennan, Mike (DOH)]

"Around nearby ranches, exposure rates around 15 Roentgen per hour were
measured just three hours after detonation."

>I wonder what isotopes were being measured at the 15 R per hour rate,
and how quickly the rate dropped back to background.

A rule of thumb I came across years ago is that the half-life of fallout
from a nuclear blast is roughly equal to the time since the blast.
Using that, and ignoring other factors (and there would be a number of
other factors), the rate would fall below 1 R/hr in a couple days and
below 2 mR in around 100 days.  

As to "back to background", it depends on your definition.  We still
find Cs137 in almost samples of sediment or soil that we look at.  On
the other hand, the levels that we find are usually lower activity than
one or more naturally occurring isotopes in the same sample.

I personally am not a rad-a-phobe, but if someone asked me if we should
have people stay in the situation described in the article, I'd have
said, "no."