°”
y
or south-westerly direction, so the heavy fallout areas were
island complexes, located on a coral rim surrounding 6
lagoon twenty-two miles long and thirteen miles wide
primarily the islands of the south-western reef. There
were exceptions to the normal wind pattern, however,
(Fig. 1). Total land area of the atoll is 2-32 square miles.
More than half the area is included in the three largest
islands, Bikini. Eneu and Nam. The largest island,
most notably for shot Bravo in 1954 when unexpected high
altitude winds carried fallout eastward over Bikin: Island
and on to the Marshallese natives of Rongelap Atoll.
Bikini. is 2-5 miles long and 0-5 mile wide.
The survey techniques utilized were largely those
developed by the Health and Safety Laboratory for detailed investigations of the properties of the external
Thus non-blast, low fallout areas, which include the islands
Eneman Complex, experienced lesser but not insignificant
amounts of local fallout. Eneu on the south-eastern mm
of the atoll was the most favourably situated to avoid
local fallout and exhibited some of the lowest exposure
rates measured (3—7 ur./h).
radiation environment in the United States’. The
instrumentation included a high-pressure ionization
chamber, an Nal(TI) field spectrometer system for in situ
y-ray spectrometry, and a number of hand-held survey
instruments (Geiger-Miller counters and scintillation
detectors).
In addition to this general pattern of radiation levels
around the atoll, we found considerable variation on
Because of the number of islands to be sur-
individual islands, similar to that shown in Fig. 2 of a
typical radiation profile across the middle of Bikini
veyed andthe difficult logistical problems and environ-
mental conditions (difficult access, dense vegetation, high
temperatures, humidity and so on), we restricted spectrometer and ionization chamber measurements to representa-
Island. The lowest levels were measured near the shores
where the vegetation was sparse and the soil very sandy,
conducive to weathering and deeper penetration of
tive locations on the main islands of Bikini, Eneu and Nam.
120
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2,700
Distance along transect (feet)
Fig. 2. Ex
ure rate profile of a survey transect across central Bikini Island from the lagoon shore to the ocean beach. The dotted lines
enclose the various measurementa with the scintillation detector (@), the Getger-MQUer counter ( x ) and the ionization chamber().
The hand-held survey meters were used to extend the
survey throughout the atoll so that variations in radiation
levels from island to island and on individual islands could
be studied in some detail.
Pattern of Radiation Distribution
The external y-radiation levels were found to vary
considerably from island to island around the atoll.
Wecan, however, roughly classify most islands into three
general areas, characterized by the relative exposure
rates and also the composition of the radiation fields:
blast areas immediately surrounding the ground zeros of
testa where the highest exposure rates were measured,
heavy fallout areas down-wind from the blast areas with
intermediate exposure rates, and non-blast, low fallout
areas with the lowest exposure rates.
The location, code name and year of each announced
nuclear weapons test‘ are indicated on the map in Fig. 1.
It can be seen that the blast areas include the western tip
of Eneman, Lomilik near the centre of the Aomen-Iroij
Complex, and the north-western reef near Nam. The
prevailing winds in the area are generally in a westerly
fallout.
Other significantly low exposure rate i-vels
could be associated with weathered areas, such as {1 mer
roadways. Higher levels were recorded in the central
parts of the islands, where the vegetation was much more
dense and where increased amounts of organic matt--r in
the soil apparently influenced the retention of fallout
near the surface of the ground.
Exposure Rates and Isotopic Contributors
Representative exposure rates in air 1 m abave the
ground from y-ray emitters in the soil obtained from the
analysis of the field spectra and ionization chamberiieas-
urementa on Bikini, Eneu and Nam are given (Table 1).
Exposure rates from cosmic radiation (3-4 «ar./h) are not
included. More detailed data on the measurement «1 the
radiation fields on Bikini Atoll are given in ref. 5.
The exposure rates for Bikini Island were in general
20-40 ur./h near the share, 50-80 ur./h in the interior. and
up to 120 ur./h at scattered hot spots. The field spectrometer measurements showed the exposure rate levels on
Bikini Island to be due primarily to three radionuclides,
with about 75 per cent of the exposure rate at a viven
tn. me an
of Bikini, Eneu and the eastern half of the Aeroko)j-