SLIDE 18
Exposure rate measurements.
Mean values for the external exposure rate were
based on many measurements reported at the time of the evacuations.
A Radsafe
Officer plus a 3-man team took readings in the air, throughout the islands and
especially in the village areas with Army, Navy catalogue AN/PDR-27E
instruments.
(0C68).
Readings tended to be the same with and without the beta shield
Readings were interpreted to be taken at waist level, about 1 meter
(3 ft) from the surface (0C68).
Held_{He65) indicated a mean exposure rate at Rongelap Island of 2.9 x
107? C kg s
(40 mR h ") at 26 days post detonation. He reported a storm
with heavy rain two weeks post detonation (He65). This was followed by a
reduction in exposure rate greater than what would have been expected from
decay of BRAVO fallout.
Glasstone (G162) repored a 40% reduction in the
exposure rate, attributed to weathering alone, during the first 25 days post
BRAVO in certain areas of the Marshall Islands.
We estimated the reduction in exposure rate due to weathering at Rongelap
Island based on measurement taken 2.2 days post detonation by the USS PHILIP
radsafe team. It was assumed to be a measurement on unweathered fallout and
to decay with decay at a rate based on the nuclide mixture present at Rongelap
2-2 to 26 days post detonation. This decline was based on the gamma decay of
142 nuclides.
For the estimate of decline, we accounted for 55 contribution
to exposure rate from:
neutron induced nuclides
1} ,the trapsuranic nuclides
S and
Ca,
U and
Np, 2) the
3) the day 26 fission products which
had fractionated according to Japanese results (Ya56, Ts55) and 4) the fission
product and transuranic product precursors.
The day 26 value of the exposure
rate extrapolated from the 2.2 day measurement made by the rqesafs team was 18
per cent over that reported by Held.
Thus, 3.4 x 10°”
C kg
(47 mR h-)
was the mean unweathered exposure rate estimated to have existed on day 26 had
the rain storm not occurred.
EXPOSURE RATE MEASUREMENTS
Rh?
Time Post
Detonation
54 hours
78 hours
62 hours
at
sd
_
vt (ae fe
*
w
Wt
?
26 days
Rongelap Island
Mean
1.5
_
_
0.047
Maximum
1.9
Mean
_
Sifo Island
Mean
Maximum
—
_
—_—
~
0.10
0.13
-
0.41
0.48
—
—
—
_
_
Utirik Island
Maximum
_
—
_
_