of 108mAg in Samples from Bikini and
Eniwetok Atolis
Date of
collection
Concentration
of 1°8™Ag
(d.p.m./g
0.20 + 0.04 F
3)
reas)
9)
reas)
it
June 1969
1.140.3
June 1970
0.75+0.15
July 1969
0.09 + 0.05
a
August 1964
*
20) *
reas)
eer i
dry wt)
fies the numberof individuals comprising
sed. Species: Panuflirus sp.
7° confidence level of the count rate
1 was taken from the site of a large
in 1954 (Bravo Crater), the crater of
lagoon and sea water and which is
‘Ag for the biota of the area (71°"Ag
vetween 1954 and 1969). Thermoin the atoll have also produced silver
st at Bikini was in 1958, yet we have
ntration of 5.2+0.06 d.p.m./g of soil
:toH and thus the amount of ?°8™Ag
n and elsewhere as a result of pre-1959
the initial production activity
| may be conservative in its
As we have said, the initial
was assumed to be due only
stable silver. There are other
yr these two radionuclides, the most
yns on stable silver ('°8™Ag) and (n,p)
m (1°8™,110mM Ag) The cadmium reactint because the quantity of stable cadould probably be kept to a minimum
1ermal neutron cross-sections.
The
(n,p) reactions at atomic weights
4tron energies between 10-20 MeV
Ulibarns) by contrast with the excitareactions at atomic weights > 100
‘gies, Which are significantly higher
in rather shallow isolated water masses for several years', then
we ®
about
n ref.
iction.
°8™ Ao
tion of
We haveirradiated stable silver in a fast neutron beam at the
University of Washington cyclotron [*Be(d,n)'°B] and have
produced easily measurable quantities of 1°°™Ag ['°’Ag(n,2n)
166mA5] and 1°°Ag ['°’Ag(n,3n)'°>Ag]; '!°8™Ag has not been
found so far in the irradiated sample. The energy similarities
of the gammarays emitted by '°°™Ag andthe very low specific
activity of '°®™Ag will require several months decay time before
the presence or absence of '!°8™Ag can be confirmed. The
(n,y) cross-sections for '°’Ag and !°°Ag (based on a radiative
capture initial activity ratio of 162) seem, however, to be almost
equal for 11°™Ag/!°8™Ag and so the nuclear states of the two
isotopes seem to be similar. It would be surprising if ?°?"Ag
production by (n,2n) reactions on '°°Ag were inconsequential,
for (n,2n) reactions on 1°’Ag have been shown experimentally
to produce relatively large amounts of 1°°™Ag. Production
pathways are important, because the type and design of a
nuclear device can generate different ratios of fast and thermal
neutron fluxes.
lt is difficult to resolve our findings with the close agreement
previously shown! between silver radionuclide activity ratios
and the time of large scale nuclear testing. If it is possible for
the silver radionuclides produced in 1961 and 1962 to remain
the same possibility must be extended to silver radionuclides
produced before 1959. But the absence of definite knowledge
of the rates at which silver is removed from the surface layers
of the ocean makes it impossible to decide whether or not
198M Ag from pre-1961-62 testing confuses subsequent dating.
The determination of ''°™Ag/'°8™Ag ratios in air filters
from the time period of interest would clarify the
production activity ratios, and the analysis of undisturbed
lichen samples collected annually since 1958 (and earlier)
would elucidate the relative amounts of '°8™Ag caused
by pre-1959 tests and those in 1961-62.
The potential
usefulness of the ''°™Ag/'°8"Ag ratio in dating is apparent
but we feel that some caution should be exercised in their use
for the description of natural processes until the outstanding
problems are resolved.
We thank C. W. Vick for technical assistance and Dr W.
Weitkamp for help with the cyclotron irradiation. The work
was supported by the US Atomic Energy Commission,
T. M. BEASLEY *
FE. E. HELD
Laboratory of Radiation Ecology,
College of Fisheries,
University of Washingten
* Present address: Division of Biology and Medicine, US Atomic
Energy Commission.
Received November 2, 1970.