4%
9
areas is generally sparse and the organic content of the
soils quite low, so weathering may accelerate the reduction
Table 1. BXTERNAL y-RBAY EXPOSURE RATES AND MAJOR CONTRIBUTORS
TO THE RADIATION FIELDS AT REPRESENTATIVE LOCATIONS ON BIKINI, ENEU
AND NAM
18105
Bikini
1 Near iagoon shore
19:0(77)
(iu brush)
18-9(78)
2 50 feet along transect 17-8 (73)
$ 50 fect along transect
5 300 feet
8 400 feet
22-8 (61)
27-2 (62)
7 1,800 feet
8 1,410 feet
3:0(12)
28(11)
24-8
24-0
21 (0)
$38(14)
248
25-0
113-4
36-8
103-2
36-1
2-4 (10)
11-3 (30)
12-5 (20)
2-7 (12)
35 (9)
40 (9)
88-8 (74)
28-:1(76)
10-5 (17)
4-913)
10-3 (9)
3-8(10)
island
8-1(78)
2 1,200 feet north of 1-3-0 (63)
@5 (12)
=—-1-5 (31)
0-4 (10)
0-3 (6)
Eneu
1 $00 feet inland—midNam
1 Near lagoon shore
18-1 (50)
2 Island centre
$ Near NE corner
17-2(48)
25-8 (39) 304(50)
60-6 (33) 119-5 (66)
22°9
06
(2)
ize the composition of the radiation field on islands where
we were unable to obtain field spectra, a large numberof
soil samples were collected from throughout the atoil.
These samples, usually obtained in several depth incre-
41-2
475
400
48°
35-9
ments, were analysed quantitatively by laboratory Nal(T!}
-spectrometry and also qualitatively by Ge(Li) spectrometry. We found that in high activity areas most of the
activity (two-thirds or more) was usually in the top
2 or 3 inches of soil. Because of large local variations in
41
5-1
34-1
66:3
1821
soil activity on all the islands, we could not calculate
accurate exposure rates in air from the one or two ‘soil
755
2040
The sums of the component exposure rates, obtained with the field spectrotmeter, are compared with the total exposure rates obtained with the jonization chamber (percentages within parentheses).
samples obtained per site. The soil samples proved very
useful, however, for identifying and determining relative
activities of the isotopes present, which were then used to
estimate the relative contributions of these isotopes to
site due to 187Cs, 15 per cent to Co and 10 per cent to
5
128b.
sium)
the exposure rates at the various sites*. The relative
exposure rate values obtained for the samesites with the
Natural emitters (uranium, thorium and potas-
were
almost
Also the greater proportional!
contribution of the shorter-lived *°Co and '**8b relative
to "Cs to the exposure rates in these areas will also cause
these radiation levels to decrease more rapidly with time.
To complement our field spectrometry and to character-
22-8
376
43-7
11 (2)
2-0 (1)
densely vegetated ureas.
entirely undetectable in the field
spectra. The composition of the radiation field on Eneu
spectrometer-ionization chamber system and from soi!
87Cs with some "Co and '**Sb. even though the exposure
sample analysis agreed quite well. Because the field
spectrometer and ionization chamber “‘see’’ large arcas
of blast areas, including high exposure rates and increased
amounts of "Co and !"Sb in the soil relative to 1*’Cs.
The composition of the radiation field on Lukoj, a
densely vegetated heavy fallout area, where exposure
was quito similar to that of Bikini, that is, prodominantly
rate levela were much lower. Nam, however, because of
its proximity to several test sites, had several properties
(approximately 30 feet in diameter), local variations are
averaged out and these measurements are very reliable.
rates varied from 60 to 200 ur./h, is indicated by the
The maximum exposure rates measured on Bikini
Atoll in 1967 were in blast areas very near the ground | Ge(Li) spectrum (Fig. 3). Approximately 60 per cent of
the exposure rate at the soil sampling site in the high
activity interior of the island was from *Co, 30 per cent
from '*5Sb and ™Rh and the remainder principally from
zeros wf tests. At one isolated area on West Eneman
near the ground zero for two surface tests we measured an
exposure rate just over 500 ur./h. Vegetation in the blast,
yr
r
t
:
Mey
865
aeigm 108 6
ean
ao?
3M
a
lf
a
379
1235p
1764
JM ce
ws
Orgy
ays
VY
/
|
ya fi
j
zm,
Rn
4m
|
250]
22:
462
633
|
\'
‘C3
666
wm"Rh
767
“
seo Sb
ia
1
;t
3
rR,
~\
toz ™Rh
10aT
nos
H
H
|
'
= 10"!
&
®Co
W7d~
*2n
i366
|
&
“Co
(32
j;
i
yl
1
i
10% tr
i
(I
I
|
10 J
0
Fig. 3,
LUKOJ
4
500
i
1,000
a
1,500
Channel
i
2,000
\
2,600
Ge(Li) spectrumiwith’peak identification and approximate energies In keV of a soil sample taken from Lukoj Island, a heavy fallout
area on the south-western rim of the atoll.
.
8,000
cee one
L: cation
of radiation levels in these areas compared with the more
Exposure rates (ur.fh)
Components
Field
—_Toniz“Co
‘Sb
spectro- ation
meter chamber