of
the first half~hour of readings,
since after that time,
the instrument went off
scale at 100 millirads per hour.
Later Surveys
One known factor concerning radioactive materials is the rate at which the
radioactivity "decays", or decreases.
This rate is directly proportional to time.
As an illustration, it is known that the activity will decrease by a factor of ten
after a sevenfold increase in time.
Thus after seven hours, the radioactivity will
be one-tenth of the original, after two days (7 x 7 = 49 hours), it will be oneone-hundredth, after 24 days, one-one-thousandth, and so on.
Consequently, when it
was relatively safe, RadSafe crews returned to Rongelap and other islands and took
measurements of the radioactivity there several days after the people had been
evacuated.
From the readings on their instruments, combined with the approximate
known time when fallout ended,
scientists calculated roughly how intense the
radioactivity had been during the time the people were exposed.
In other words,
since normal decay rates are known, the present level of radioactivity known, and
the rough time of when the fallout stopped was known, what scientists did was to
calculate backwards, so to speak, from the period several days later,
to the time
when the people were still on the island.
One discovery by this later survey was
that
the evacuation were apparently too low
the readings
taken during the time of
by one-half, or 50 percent.
used at
used.
that
(1944,
This was apparently due to the fact that the instrument
time had not been calibrated
(checked for accuracy)
before it had been
p.5)
Another discovery they made concerned the nature of ‘mixed fission products."
The fallout was composed of many kinds of radioactive isotopes giving off gamma
and beta rays of different strengths.
ene) 5010361
The relative strength of these particles is
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