‘Weha Raa iMlNM es
Transuranics in Bikini Lagoon
Table 2.
733
growth band; at least 1 mm of material
Coral section data.
waslost at each cut, so that the coral recsection
from
outer edge
Estimated
Amm
At in years
year
{collection date)
year of
égrowth/
1971-1972
1970
1969
1968
1967
1966
1965
4
7
6
8
9
8
10
1963
1962
1961
1960
1959
1958
10
8
13
12
8
T
9.4
10.4
11.4
12.4
13.4
1h.k
8
5
9
16.4
1T.9
18.4
growth
from Nov 72
(mm)
0-8
8-15
- 15-21
21-29
29-38
38-6
46-56
56-65
65-T5
75-83
83-96
96-108
108-116
116-123
123-129
129-137
137-L47
L47-156
ies (not all necessar‘or radionuclide anal-
wever, is only 2.5
rowth rate during
yr, The annual
‘constant thickness;
oundaries to postthe density variaay film.
:posed X-ray film
reful examination
ind dark bandsin
al exposures were
dlete selection of
” could be made.
1974) concluded
ind (dark X-ray
ith a growth peecember or Januly. In our sample,
tutoradiograph of
1964
1957
1956
1954-1955
1953
9
6
0.9
2.4
3.4
uk
5.4
6.4
7.4
Buh
15.4
the Bikini coral are either associated with
the low-density bands of the X-ray negative
or close to the transition zone between light
and dark bands. By correlation of the
bands shown by both the autoradiograph
and X-ray exposures, the years 1956
through 1958 were identified. The 1954
growth, however, could not be resolved
from the 1955 growth on the X-ray negative. From the 1958 bandto the coral surface (1972 growth), 14 alternating light
and dark areas were identified (Fig. 3).
Table 2 gives the thickness of each annual
section along with the estimated year of
growth.
The varying dimensions of the yearly increments show that growth rates vary from
year to year and also that the dimensions
follow no predictable trend with time. The
mean annual growth from the X-radiograph
is 8.1 +2.2 mm yr, a value in agreement
with the autoradiography results. No activity from the 1946 test was detected because it predated the earliest growth of
this particular coral.
A wedge, containing the area with the
more nearly linear growth record, was removed from the center of the slice (see
wedge outline, Fig. 3). A bandsaw was
used to remove each defined annual
ord between each two sections was lost.
Each section was then ground and homogenized in a ball mill and a known weight was
transferred to a vial for radionuclide analysis by gamma spectrometry. A low-background, Compton-suppressed, Ge(Li) detector system was used for some of the
gamma-emitting radionuclides. The samples were then processed for Pu, *Sr, and
210P9 by chemical separation followed by
radioassay with low-background beta detectors and alpha spectrometry. Selected
samples were also analyzed by mass spec-
trometry to determine 74°Pu,.?3°Pu, and
241Pu. The analytical techniques were essentially those described by Wong (1971)
and Noshkin and Gatrousis (1974). Stable
strontium was determined by atomic absorption. Our discussion here will concentrate primarily on the transuranium elements;
however,
Tables
3-5
give
the
results for all the radionuclides analyzed,
expressed as pCi g! (dry wt) and corrected for decay back to the estimated year
of growth.
Two sources contribute to the ?4Am levels found in the coral. Part is from the radioactive decay of the parent radionuclide
*41Pu, while the remainder is unsupported
2414m. Based on the quantities of 244Am
and 74!Py, the time between separation,
and the age of the coral section, levels of
both supported and unsupported *#4Am
are computed and given in Table 5.
Radionuclide results
General—The
coral
bands
identified
with the 1954, 1956, and 1958 test series,
as would be expected from the autoradiograph, contain the highest concentrations
of radionuclides. A spectrum from 12 g of
the 1954 growth section is shown in Fig. 4
with each prominent gammaray identified.
Severai gamma-emitting radionuclides with
half-lives less than 1-2 years have been
identified in earlier surveys (Welander
1969); these were not detected in any coral
sections with our spectrometer system.
Other radionuclides requiring radiochemi-
4.
Growth